I Know, I Know... Not another P0300 Question

[calsdad]

Hi everyone. I’m new to the forum and was hoping to find help in resolving an issue that I’m having with my Silverado (I also own a 2004 Tahoe with the same engine). If you have time to read this post and have any suggestions, I'd greatly appreciate your help.

I’ve got a 2002 Silverado with a 5.3L Vin-T that I purchased new. The truck’s been well maintained, despite the deterioration resulting from the NE Ohio salty roads. It has 350k miles on it and still runs smooth and strong. It starts right up every time, idles smooth, drives and shifts like new. Two years ago it abruptly lost power and acceleration on the freeway and I opened the catalytic converters, emptied the contents and re-welded them shut. I also installed O2 sensor extensions on the post cat bungs to ward off any unsuspecting emission DTC’s. In the end (after I purchased an OBDII scanner), it actually turned out to be the knock sensors, so I replaced them and the harness with genuine GM parts. The intake received new Fel-Pro gaskets. Ever since, no CEL’s and things have been great for the last 35k+ miles. It has an AC Delco fuel pump that I installed about 100K miles ago.

Problem: While driving the other day, I began to feel a single-cylinder miss while stopped at a red light. Continuing on, it began to lose power and shutter whenever I would push the accelerator beyond 2k RPM. That’s when the CEL began to flash intermittently and would then go away. But the acceleration issue remained. I limped home on backroads at 45 mph top speed, since I couldn’t get the rpm’s up enough to accelerate to highway speeds. The OBDII scanner showed a P0300 (Random Misfire) code. The voltage of the post-cat O2 sensors fluctuate uniformly as if they’re working properly. I’ve since cleared the P0300 code, and during my test runs it has yet to reappear.

Since then, the miss at idle has resolved itself (sea foam & injector cleaner added to fuel) and it runs perfectly smooth as before. I checked all of my AC Delco Iridium plugs and all eight were tan-colored, dry and still gapped at 0.040”. I pulled each plug wire and all eight coils showed good spark. Out of curiosity, I swapped the MAF sensor from my other truck and nothing changed. I hooked-up a fuel gauge to the rail and it showed 50 psi with the key on and 48 psi at idle. To experiment, I ran the fuel gauge up through my hood so that I could observe the gauge through my windshield while driving. That’s when things became fishy. The gauge stayed at 50 psi while cruising at around 1,500 RPM, would drop to 45 psi under gentle acceleration to just over 2k RPM and then recover right back to 50 psi. However, once the accelerator is pushed down to a certain position (~ 2,500 RPM) the engine begins to shudder, RPM’s drop, it pops through the intake occasionally and then the fuel pressure drops suddenly from 50 psi down to 5-10 psi. If I floor it, the same thing happens (fuel pressure drops to 5-10 psi). But what’s odd, is that if I let up on the accelerator slowly, there’s a particular position in the accelerator’s travel where the engine immediately smooths out and the fuel pressure shoots right back up to 50 psi. I don’t know if the immediate pressure drop is something that occurs electronically as part of a “limp mode” process or something. It doesn’t seem that a dirty fuel filter or partially plugged fuel pump pick-up screen, or even a faulty fuel pump would cause such a drastic and immediate change in pressure like this. I did change the vacuum operated fuel pressure regulator on the fuel rail, but this had no effect.

I’ve been reading forums and watching Youtube videos for the last three days, hoping to find someone who experienced something similar with respect to the fuel pressure drop relative to the throttle position in their 5.3 with the P0300 DTC. I’ve heard of people performing a crankshaft position re-learn procedure. However, I’m not sure if this would help, since the problem had an immediate onset and wasn’t something that started off mild and progressed over time.

I don’t mean to beat this topic to death. But if anyone has any experience with the conditions described and then found a resolution, I’d sure appreciate your input. I’d sure like to get this old horse through one more winter without dumping a whole lot of money into it.

Check the wiring.

Have you EVER had any issues with mice where you park the truck? Because it's possible they may have gotten in there and chewed up wiring. And it might not be totally obvious where it is until you start digging.

I had some similar "strange" issues on my 2003 Suburban 2500 a few years back. To make a very long story short, first it started quitting while driving on the highway randomly. I checked the codes - it had a fuel pump code. So I pulled the tank and replaced the pump. Then it still threw fuel trim and lean codes. So I replaced the filter. Then I got a camshaft sensor code. So I pulled the manifold and replaced that . Then it threw a crankshaft sensor code - so I replaced that.

At that point somebody suggested checking the wiring , especially under the fusebox. So I pulled the fusebox off - and sure enough the harness under the fusebox had been seriously chewed up by mice. There were probably 10 wires that were chewed right thru - and another ten or so that had the copper exposed. Took me a few hours on a Sunday morning but I carefully repaired all the wiring under there. That *seemed* to make things better. But didn't cleanly fix the problem.

Then it started acting like it had misfires. So I replaced the coils , plugs & wires. I found some plugs and wires that might have been bad (broken wire and broken ceramic) But it still kept quitting - in fact it just kept getting progressively worse. Had a friend who was a GMC service manager bring over his diag tools - and we pulled the wiring harness apart - and found nothing wrong. The truck was so bad at this point that if I drove it a mile down the street it would quit and one time I had to sit by the side of the road for a half hour and wait for it to cool down before it would even start. Even after all of that - when I was starting to suspect the ECU was bad - everybody insisted "that never happens!". At this point I was seriously considering seeing if somebody could steal it and burn it for me.

By now the weather was getting colder , and I did notice the truck would start up and run ok when it was cold - but as soon as it warmed up - even a little - it would quit and run like complete crap.

So one cool November morning I went out really early and started the truck up cold. It started and ran fine. Then I left it sit with the hood open so everything would cool off for a couple of hours. Went back out - and then put a heat gun on the ECU and heated it up to the point where I couldn't touch it any more. Tried to start the truck - and it started but ran like complete crapola. Ok then - now I had a symptom that seemed to point to where the problem was. I ordered a replacement ECU from one of those places that programs the VIN and all that into the unit and sends it out to you. Got it within a week - put it in , and the truck *seemed* to run fine. I was still suspicious because the thing had been quitting on me for so long and so unpredictably that I wasn't really sure the problems had gone away until I drove it for a while.

Long story short - the ECU *finally* fixed the problems. In fact the truck ran awesome after that , but I suspect it had ignition issues even before because of the faulty plugs and wires I found. I think in the end what happened - was that the mice chewing the harness totally forked the ECU - which then just got progressively worse as it lost it's mind. That's what lead to the seemingly random failures.

So check the wiring and don't discount that the ECU might be bad. If I remember correctly it was only like $150 or so to exchange my ECU for the refurbed one. Actually cheap at the price once you consider what the hourly labor rate would be at a shop for them to analyze and then replace a part.

 

[jwth]

maybe the throttle position sensor

 

[iamdub]

Hi everyone. I’m new to the forum and was hoping to find help in resolving an issue that I’m having with my Silverado (I also own a 2004 Tahoe with the same engine). If you have time to read this post and have any suggestions, I'd greatly appreciate your help.

I’ve got a 2002 Silverado with a 5.3L Vin-T that I purchased new. The truck’s been well maintained, despite the deterioration resulting from the NE Ohio salty roads. It has 350k miles on it and still runs smooth and strong. It starts right up every time, idles smooth, drives and shifts like new. Two years ago it abruptly lost power and acceleration on the freeway and I opened the catalytic converters, emptied the contents and re-welded them shut. I also installed O2 sensor extensions on the post cat bungs to ward off any unsuspecting emission DTC’s. In the end (after I purchased an OBDII scanner), it actually turned out to be the knock sensors, so I replaced them and the harness with genuine GM parts. The intake received new Fel-Pro gaskets. Ever since, no CEL’s and things have been great for the last 35k+ miles. It has an AC Delco fuel pump that I installed about 100K miles ago.

Problem: While driving the other day, I began to feel a single-cylinder miss while stopped at a red light. Continuing on, it began to lose power and shutter whenever I would push the accelerator beyond 2k RPM. That’s when the CEL began to flash intermittently and would then go away. But the acceleration issue remained. I limped home on backroads at 45 mph top speed, since I couldn’t get the rpm’s up enough to accelerate to highway speeds. The OBDII scanner showed a P0300 (Random Misfire) code. The voltage of the post-cat O2 sensors fluctuate uniformly as if they’re working properly. I’ve since cleared the P0300 code, and during my test runs it has yet to reappear.

Since then, the miss at idle has resolved itself (sea foam & injector cleaner added to fuel) and it runs perfectly smooth as before. I checked all of my AC Delco Iridium plugs and all eight were tan-colored, dry and still gapped at 0.040”. I pulled each plug wire and all eight coils showed good spark. Out of curiosity, I swapped the MAF sensor from my other truck and nothing changed. I hooked-up a fuel gauge to the rail and it showed 50 psi with the key on and 48 psi at idle. To experiment, I ran the fuel gauge up through my hood so that I could observe the gauge through my windshield while driving. That’s when things became fishy. The gauge stayed at 50 psi while cruising at around 1,500 RPM, would drop to 45 psi under gentle acceleration to just over 2k RPM and then recover right back to 50 psi. However, once the accelerator is pushed down to a certain position (~ 2,500 RPM) the engine begins to shudder, RPM’s drop, it pops through the intake occasionally and then the fuel pressure drops suddenly from 50 psi down to 5-10 psi. If I floor it, the same thing happens (fuel pressure drops to 5-10 psi). But what’s odd, is that if I let up on the accelerator slowly, there’s a particular position in the accelerator’s travel where the engine immediately smooths out and the fuel pressure shoots right back up to 50 psi. I don’t know if the immediate pressure drop is something that occurs electronically as part of a “limp mode” process or something. It doesn’t seem that a dirty fuel filter or partially plugged fuel pump pick-up screen, or even a faulty fuel pump would cause such a drastic and immediate change in pressure like this. I did change the vacuum operated fuel pressure regulator on the fuel rail, but this had no effect.

I’ve been reading forums and watching Youtube videos for the last three days, hoping to find someone who experienced something similar with respect to the fuel pressure drop relative to the throttle position in their 5.3 with the P0300 DTC. I’ve heard of people performing a crankshaft position re-learn procedure. However, I’m not sure if this would help, since the problem had an immediate onset and wasn’t something that started off mild and progressed over time.

I don’t mean to beat this topic to death. But if anyone has any experience with the conditions described and then found a resolution, I’d sure appreciate your input. I’d sure like to get this old horse through one more winter without dumping a whole lot of money into it.

Does your scan tool show live data? Get one that does and watch the throttle angle (might be logged as TPS percent, degrees, voltage, etc.). Slowly push on the pedal and watch for a smooth, linear change in the scale. I'm curious if it gets to a point where it suddenly maxes out. Also, watch the data for injector duty cycle (IDC). I don't see how all eight injectors being at 100% would cause that much fuel pressure loss, but it's worth looking at. Monitor commanded fuel pump voltage, too, if this is an option.

If you don't have access to such a scan tool, as a start, you can use a DVOM to test the TPS. Check this out: https://easyautodiagnostics.com/gm/4.8L-5.3L-6.0L/tps-multimeter-tests-1

If I'm not mistaken, an '02 would have a cable throttle body and no fuel pump control module. I don't think it would even have variable voltage control for the pump in the PCM.

 

[hushko]

Check the wiring.

Have you EVER had any issues with mice where you park the truck? Because it's possible they may have gotten in there and chewed up wiring. And it might not be totally obvious where it is until you start digging.

I had some similar "strange" issues on my 2003 Suburban 2500 a few years back. To make a very long story short, first it started quitting while driving on the highway randomly. I checked the codes - it had a fuel pump code. So I pulled the tank and replaced the pump. Then it still threw fuel trim and lean codes. So I replaced the filter. Then I got a camshaft sensor code. So I pulled the manifold and replaced that . Then it threw a crankshaft sensor code - so I replaced that.

At that point somebody suggested checking the wiring , especially under the fusebox. So I pulled the fusebox off - and sure enough the harness under the fusebox had been seriously chewed up by mice. There were probably 10 wires that were chewed right thru - and another ten or so that had the copper exposed. Took me a few hours on a Sunday morning but I carefully repaired all the wiring under there. That *seemed* to make things better. But didn't cleanly fix the problem.

Then it started acting like it had misfires. So I replaced the coils , plugs & wires. I found some plugs and wires that might have been bad (broken wire and broken ceramic) But it still kept quitting - in fact it just kept getting progressively worse. Had a friend who was a GMC service manager bring over his diag tools - and we pulled the wiring harness apart - and found nothing wrong. The truck was so bad at this point that if I drove it a mile down the street it would quit and one time I had to sit by the side of the road for a half hour and wait for it to cool down before it would even start. Even after all of that - when I was starting to suspect the ECU was bad - everybody insisted "that never happens!". At this point I was seriously considering seeing if somebody could steal it and burn it for me.

By now the weather was getting colder , and I did notice the truck would start up and run ok when it was cold - but as soon as it warmed up - even a little - it would quit and run like complete crap.

So one cool November morning I went out really early and started the truck up cold. It started and ran fine. Then I left it sit with the hood open so everything would cool off for a couple of hours. Went back out - and then put a heat gun on the ECU and heated it up to the point where I couldn't touch it any more. Tried to start the truck - and it started but ran like complete crapola. Ok then - now I had a symptom that seemed to point to where the problem was. I ordered a replacement ECU from one of those places that programs the VIN and all that into the unit and sends it out to you. Got it within a week - put it in , and the truck *seemed* to run fine. I was still suspicious because the thing had been quitting on me for so long and so unpredictably that I wasn't really sure the problems had gone away until I drove it for a while.

Long story short - the ECU *finally* fixed the problems. In fact the truck ran awesome after that , but I suspect it had ignition issues even before because of the faulty plugs and wires I found. I think in the end what happened - was that the mice chewing the harness totally forked the ECU - which then just got progressively worse as it lost it's mind. That's what lead to the seemingly random failures.

So check the wiring and don't discount that the ECU might be bad. If I remember correctly it was only like $150 or so to exchange my ECU for the refurbed one. Actually cheap at the price once you consider what the hourly labor rate would be at a shop for them to analyze and then replace a part.

Thank you for your detailed response. It’s actually funny, because I live in a rural area and actually do have a mouse hanging around. He’s been seen near my truck, in fact. After reading your post, I removed my front fender support and pulled the bussed electrical center to check the wiring underneath. I was actually hoping to find a nest so that I could at least have a tangible clue as to why this truck is running so bad. It turns out, there is no nest or chewed wires visible. In fact, I exposed the ECU to check its wiring and found nothing unusual there either. I suppose there’s always a possibility that the ECU can still be bad.

This whole problem began when driving in a heavy rain at freeway speeds. A few weeks ago, I discovered that my fuel filler neck had been rusted clear through, leaving the fuel tank exposed to the elements. I thought ah ha! Found it! Especially since my wheel wells are so perforated from rust, it only seemed feesible that there was water likely being slung into the tank. So I replaced the filler neck and inline fuel filter, knowing that there’s likely an unknown amount of rust scale that entered the tank from the rusted out filler neck. So I thought I’d try adding a few bottles of Iso Heet dry gas to the tank. It seemed to start running better at first after about 20-30 miles, but then backslid to where it was.

The performance is so erratic, making it even more difficult to diagnose. One day I can reach 75 mph on the freeway if I accelerate slowly and carefully up to speed. Other days, I find that I’m limping around at 35 mph just trying to make it home. If I floor the throttle while parked in my driveway, the rpm’s will surge at the 3,000 rpm range. The same thing occurs while driving. I suppose my next step may be to pull the tank to see if perhaps the screen is plugged from possible rust infiltration. If all else fails, the ECU may be the next step.

 

[hushko]

maybe the throttle position sensor

Thanks for the reply. I actually just changed the throttle position sensor this afternoon with an AC Delco part. No effect on the performance, unfortunately.

 

[hushko]

Does your scan tool show live data? Get one that does and watch the throttle angle (might be logged as TPS percent, degrees, voltage, etc.). Slowly push on the pedal and watch for a smooth, linear change in the scale. I'm curious if it gets to a point where it suddenly maxes out. Also, watch the data for injector duty cycle (IDC). I don't see how all eight injectors being at 100% would cause that much fuel pressure loss, but it's worth looking at. Monitor commanded fuel pump voltage, too, if this is an option.

If you don't have access to such a scan tool, as a start, you can use a DVOM to test the TPS. Check this out: https://easyautodiagnostics.com/gm/4.8L-5.3L-6.0L/tps-multimeter-tests-1

If I'm not mistaken, an '02 would have a cable throttle body and no fuel pump control module. I don't think it would even have variable voltage control for the pump in the PCM.

Thanks for your input. My 2002 does have the throttle cable, but it doesn’t have an EGR valve as the 2003’s. So it’s more of an 02/03 hybrid I suppose. My OBD does have live data, so I’ll try to get those readings in the next day or so. I did just change the TPS this afternoon with an AC Delco part, with no change. The problem is very erratic, allowing me to carefully achieve freeway speeds one day, and barely limp home at 35 mph the next. In fact, while in park, I can floor the throttle and it will just surge in the 3,000 rpm range. Same thing while driving. I also just posted above with regard to just discovering recently that my fuel tank filler neck was completely rusted out/off, thinking perhaps water and rust flakes entered the tank. I’m trying to run the fuel out of the tank so that I can pull it and check it for debris. Even so, I’m not sure that this would be related to the performance issue or not.

 

[jwth]

if water entered the tank thru the rusted inlet you may be fighting that, water at the fuel pick up screen on the pump, the water will stop gas flowing thru the filter screen- personal expierience here.

the ecu controls fuel and spark as well as other functions. i replaced mine after not finding any reason for a loping idle and surging, fixed the problem immediately. again been there done that.

230k now and the last 100k have been flawless.

 

[iamdub]

Thanks for your input. My 2002 does have the throttle cable, but it doesn’t have an EGR valve as the 2003’s. So it’s more of an 02/03 hybrid I suppose. My OBD does have live data, so I’ll try to get those readings in the next day or so. I did just change the TPS this afternoon with an AC Delco part, with no change. The problem is very erratic, allowing me to carefully achieve freeway speeds one day, and barely limp home at 35 mph the next. In fact, while in park, I can floor the throttle and it will just surge in the 3,000 rpm range. Same thing while driving. I also just posted above with regard to just discovering recently that my fuel tank filler neck was completely rusted out/off, thinking perhaps water and rust flakes entered the tank. I’m trying to run the fuel out of the tank so that I can pull it and check it for debris. Even so, I’m not sure that this would be related to the performance issue or not.

Water intrusion and rust flakes could be an intermittent problem. Gasoline floats on water, so the water would sink to the bottom, which is where the pump picks up from. The larger rust flakes might clog the inlet sock of the pump module when it's running, then fall off of it when it's parked, also causing an intermittent issue.

Regardless, it needs to be repaired. If it's not the cause for your current problems, it will be the cause for later, possibly worse problems.

Speaking of rust, check all the grounds in the area. It's been years, but I think the ground for the fuel pump is on the filler neck. If you have rust/corrosion, it might just be intermittently losing its ground.

 

[calsdad]

Thank you for your detailed response. It’s actually funny, because I live in a rural area and actually do have a mouse hanging around. He’s been seen near my truck, in fact. After reading your post, I removed my front fender support and pulled the bussed electrical center to check the wiring underneath. I was actually hoping to find a nest so that I could at least have a tangible clue as to why this truck is running so bad. It turns out, there is no nest or chewed wires visible. In fact, I exposed the ECU to check its wiring and found nothing unusual there either. I suppose there’s always a possibility that the ECU can still be bad.

This whole problem began when driving in a heavy rain at freeway speeds. A few weeks ago, I discovered that my fuel filler neck had been rusted clear through, leaving the fuel tank exposed to the elements. I thought ah ha! Found it! Especially since my wheel wells are so perforated from rust, it only seemed feesible that there was water likely being slung into the tank. So I replaced the filler neck and inline fuel filter, knowing that there’s likely an unknown amount of rust scale that entered the tank from the rusted out filler neck. So I thought I’d try adding a few bottles of Iso Heet dry gas to the tank. It seemed to start running better at first after about 20-30 miles, but then backslid to where it was.

The performance is so erratic, making it even more difficult to diagnose. One day I can reach 75 mph on the freeway if I accelerate slowly and carefully up to speed. Other days, I find that I’m limping around at 35 mph just trying to make it home. If I floor the throttle while parked in my driveway, the rpm’s will surge at the 3,000 rpm range. The same thing occurs while driving. I suppose my next step may be to pull the tank to see if perhaps the screen is plugged from possible rust infiltration. If all else fails, the ECU may be the next step.

Hmm. The way your truck is acting - sounds very similar to what my truck was doing. When you get into a problem like this - and this is my opinion only - it's not a bad strategy to start replacing parts. You found that the fuel filler neck was rusted out - so replace it. It's a problem anyway - so replacing it at least eliminates one potential cause of the issue.

When my 2003 Suburban started running like crap - one of the first codes it threw was fuel related. I knew I had an issue with the fuel pump - because it had been leaking fuel for a couple of years. I had taken it to a dealer - and they wanted like $900 to replace it - I said screw that - and just drove the truck for a couple of years never filling the tank past halfway so it didn't leak. When it started throwing the codes I figured it was time to bite the bullet and pull the tank. Turned out - it was pretty straightforward and not bad of a job at all. And sure enough - the top of the fuel pump assembly was completely rusted out. The tank inside though - was cherry clean. So I put the new fuel pump in - made sure to zinc paint and undercoat the top of the new pump assembly so it wouldn't rust out again - and installed the new one. I had run the tank down to like maybe 4 or 5 gallons of gas left in it - so it wasn't too bad to handle when I was taking it out. Even though the new pump didn't fix the running problem - it did fix the rusted out and leaking gas problem.

To try and diagnose my issues - I also picked up a copy of AutoEnginuity's ODB code reading software for my laptop. That really let me see all the codes it was throwing. The cheapo code readers don't really give you much insight. This was a few years back - now I think I'd probably get a good Autel code reader.

 

[hushko]

Hmm. The way your truck is acting - sounds very similar to what my truck was doing. When you get into a problem like this - and this is my opinion only - it's not a bad strategy to start replacing parts. You found that the fuel filler neck was rusted out - so replace it. It's a problem anyway - so replacing it at least eliminates one potential cause of the issue.

When my 2003 Suburban started running like crap - one of the first codes it threw was fuel related. I knew I had an issue with the fuel pump - because it had been leaking fuel for a couple of years. I had taken it to a dealer - and they wanted like $900 to replace it - I said screw that - and just drove the truck for a couple of years never filling the tank past halfway so it didn't leak. When it started throwing the codes I figured it was time to bite the bullet and pull the tank. Turned out - it was pretty straightforward and not bad of a job at all. And sure enough - the top of the fuel pump assembly was completely rusted out. The tank inside though - was cherry clean. So I put the new fuel pump in - made sure to zinc paint and undercoat the top of the new pump assembly so it wouldn't rust out again - and installed the new one. I had run the tank down to like maybe 4 or 5 gallons of gas left in it - so it wasn't too bad to handle when I was taking it out. Even though the new pump didn't fix the running problem - it did fix the rusted out and leaking gas problem.

To try and diagnose my issues - I also picked up a copy of AutoEnginuity's ODB code reading software for my laptop. That really let me see all the codes it was throwing. The cheapo code readers don't really give you much insight. This was a few years back - now I think I'd probably get a good Autel code reader.

Hi calsdad. Thanks for the input. I did change the rusted out fuel filler neck as soon as I realized that it was perforated. This was the second time that I changed it, actually. The salt and brine in my region is extremely destructive to vehicles. I’ve had the tank off of this truck a couple of times to replace the fuel pump. The 1st time the pump went, the truck just died on the freeway. The second time I changed the pump was because the fuel level sending unit went bad, but the pump worked great. I’m just trying to use up the gas to get the level low enough to manage The tank removal. But the truck is running so erratic, that I’ll probably end up trying to syphon it out instead. The funny thing about the codes is, that there haven’t been any codes since I cleared the P0300 code weeks ago. Ocassionally I’ll get a flashing CEL that simply goes away, leaving no code. The only pending codes that I get are the 02 sensors, but I believe that’s from the lean condition that occurs when the truck stumbles under acceleration and surges at the 3,000 rpm range, going nowhere.