Have you found fuel trims in your scanner? A test if you want to gamble with it is clamp the return fuel flex line and see if fuel pressure goes up, misfires goes away. A long hose on fuel pressure guage allows you to watch it while you drive.
Chasing a P0300
- Thread starter Cornelius
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ReReading your posts, what were the misfire counts before after coil swap? My bet is a ten second test, not a drive on the three mile loop. Same thing applies to the fuel trim/fuel pump tests. That ten second diagnosis is parts cannon fuel!
For the fuel trims, must be in closed loop (engine warmed up).
On the coil swap, you used coil from a no misfire cylinder? and it still misfired after swap to misfire cylinder? With engine at operating temp? Details matter- you have them in your head, write them for us. Alot of knowlegeable guys here, and a shop diagnostic time for this is 1.5 hours average. Many shops don't care if they bill you $700. for a fuel pump and diagnostics even if it doesn't fix it. We are trying to save you from buying 8 coils and a fuel pump. More info= better diagnosis.
Leaky check valve in tank module could be your culprit, pressure should hold for 1/2 hour minumum, or at least close as its testing injectors too.
For the fuel trims, must be in closed loop (engine warmed up).
On the coil swap, you used coil from a no misfire cylinder? and it still misfired after swap to misfire cylinder? With engine at operating temp? Details matter- you have them in your head, write them for us. Alot of knowlegeable guys here, and a shop diagnostic time for this is 1.5 hours average. Many shops don't care if they bill you $700. for a fuel pump and diagnostics even if it doesn't fix it. We are trying to save you from buying 8 coils and a fuel pump. More info= better diagnosis.
Leaky check valve in tank module could be your culprit, pressure should hold for 1/2 hour minumum, or at least close as its testing injectors too.
OP
OP
Cornelius
TYF Newbie
@exp500....Thank you for your help to help me help you help me lol
I'm really not that bad at diagnostics in general, but I am only just learning best practices regarding using OBD data and interpreting results.
So today I fired the parts cannon again and replaced the fuel pump. the fuel pressure improved and held pressure after stopping. P0300 with misfires on 1 5 6 8 persist, and STFT goes from around 0 at idle to -30 at 2500rpm, then the CEL flashed and set p0300. This is in my driveway, not on the road, so if real world driving gives better data then I'll find out right after i hit enter on this post.
When I swapped coils it was from a non misfire cylinder. Engine at operating temp. I will report back when I get back from some errands, with scan tool connected. Thank you for your help and attention I appreciate it!
I'm really not that bad at diagnostics in general, but I am only just learning best practices regarding using OBD data and interpreting results.
So today I fired the parts cannon again and replaced the fuel pump. the fuel pressure improved and held pressure after stopping. P0300 with misfires on 1 5 6 8 persist, and STFT goes from around 0 at idle to -30 at 2500rpm, then the CEL flashed and set p0300. This is in my driveway, not on the road, so if real world driving gives better data then I'll find out right after i hit enter on this post.
When I swapped coils it was from a non misfire cylinder. Engine at operating temp. I will report back when I get back from some errands, with scan tool connected. Thank you for your help and attention I appreciate it!
FrankU
Full Access Member
My 2 cents FWIW check your battery.
Be sure to get ST and LT fuel trims for both banks, at at least a couple of RPM data points (idle and 2000 or 2500, for example).
OP
OP
Cornelius
TYF Newbie
I took a ride and gathered some data:
STFT seemed normal this time mostly staying -1 to 3 percent. This was at all RPMs, all live data. I don't know what was going on earlier right after i installed the fuel pump where it went right to -30 once it hot 2500rpm.
LTFT hovered around 0 to about 3%, and that all seemed to be normal since I disconnected the battery to clear the PCM memory. The scan tool I have indicated that learning mode was engaged so maybe thats why the readings were very similar to STFT?
upstream o2 sensors fluctuated from around 100-ish mV to about 700mV, and bank 1 and 2 had consistently similar readings
Downstream o2 sensors were both pretty steady around 600-700mV. They didn't seem to go back and forth like the upstream ones
However, the misfires for 1 5 6 8 were still coming. One thing that was different this time is that the CEL didn't go solid, it would just flash then go out. Always at steady throttle, between 1500-2500 rpm.
Back and forth on the highway I drove, trying to (as safely as possible and in very light traffic) try to observe certain sensor data when the flashing CEL was happening. Other than the misfires at each cylinder, all numbers were in normal ranges. But I did notice the MAF sensor didn't seem to be changing that much so I unplugged it, the idle didn't really change at all (AHA?) and when I got back on the highway didn't see any CEL, flashing or solid. Live data didn't show any misfires either. Am I on to something? Is there a different way to verify MAF functionality? I have cleaned it but I am sure it's the original. I got the truck at 94k and it now has 230k on it. When I shut it off then back on the code for MAF came up (of course) and also a p0300, even though the CEL didn't light while I was driving (with MAF unplugged).
Thanks in advance again
Tempted to drag the parts cannon out and shoot a MAF sensor at it but I will wait for y'all to guide me.
STFT seemed normal this time mostly staying -1 to 3 percent. This was at all RPMs, all live data. I don't know what was going on earlier right after i installed the fuel pump where it went right to -30 once it hot 2500rpm.
LTFT hovered around 0 to about 3%, and that all seemed to be normal since I disconnected the battery to clear the PCM memory. The scan tool I have indicated that learning mode was engaged so maybe thats why the readings were very similar to STFT?
upstream o2 sensors fluctuated from around 100-ish mV to about 700mV, and bank 1 and 2 had consistently similar readings
Downstream o2 sensors were both pretty steady around 600-700mV. They didn't seem to go back and forth like the upstream ones
However, the misfires for 1 5 6 8 were still coming. One thing that was different this time is that the CEL didn't go solid, it would just flash then go out. Always at steady throttle, between 1500-2500 rpm.
Back and forth on the highway I drove, trying to (as safely as possible and in very light traffic) try to observe certain sensor data when the flashing CEL was happening. Other than the misfires at each cylinder, all numbers were in normal ranges. But I did notice the MAF sensor didn't seem to be changing that much so I unplugged it, the idle didn't really change at all (AHA?) and when I got back on the highway didn't see any CEL, flashing or solid. Live data didn't show any misfires either. Am I on to something? Is there a different way to verify MAF functionality? I have cleaned it but I am sure it's the original. I got the truck at 94k and it now has 230k on it. When I shut it off then back on the code for MAF came up (of course) and also a p0300, even though the CEL didn't light while I was driving (with MAF unplugged).
Thanks in advance again
Tempted to drag the parts cannon out and shoot a MAF sensor at it but I will wait for y'all to guide me.
Just a check and see, How about swapping 2 spark plugs for me? A few times now I have had plugs that employees at local stores were learning how to juggle with.
Also- does the misfire come and go or is it reasonably steady always there?(worse under load)
Also- does the misfire come and go or is it reasonably steady always there?(worse under load)
ks03
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I know you say you replaced the fuel pressure regulator, but I’m suspect of it still.
one possibility for misfires on 1 and 8 is liquids in the intake, weather it’s caused by gas, oil or something crazy.
5 and 6 are near where the vacuum line for the regulator connects.
occasionally pulling lots of fuel, then evening out, especially after a hot soak, would indicate a regulator.
im not saying replace it, but just check it a couple more times. Just pull the vacuum hose off, checking for fuel. If it’s dry, twist the regulator to point the port down and make sure everything is dry.
one possibility for misfires on 1 and 8 is liquids in the intake, weather it’s caused by gas, oil or something crazy.
5 and 6 are near where the vacuum line for the regulator connects.
occasionally pulling lots of fuel, then evening out, especially after a hot soak, would indicate a regulator.
im not saying replace it, but just check it a couple more times. Just pull the vacuum hose off, checking for fuel. If it’s dry, twist the regulator to point the port down and make sure everything is dry.
ks03
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at wide open throttle, just before it up shifts, what is the highest the MAF reads in grams / sec?
Do not disconnect battery to clear codes, use scanner. Every time you disconnect battery it goes into Re-learn mode. Needs a 50 mile drive to adjust fuel spark and trans. Don't do anything except clear codes and monitor for a few days.
