Low pressure all of a sudden

[Scottydoggs]

I really appreciate all the advice. As suggested I got a ride home after work and drove back with all my oil change equipment. Drain all the oil in the place the filter. I let it run for about 5 minutes and oil pressure came up to about 20 psi. That's about half what it normally is. Drive it home (2 miles) and it seemed to drive okay despite having low pressure, and a few times that it seemed to choke up for an instant and then pick back up. I let it sit for last 30 minutes and just went out to see, and now I'm back at it 10psi it won't idle without me pushing a little gas. Uggg...

So my question is this, would the oil pressure sensor cause the engine problems? When I first got this Tahoe it was pegged at 80 psi. Last month I changed it and it went went back to normal. I didn't buy an oem, so I wonder if it went bad already and it reading low oil pressure is causing the engine to not run correctly.

I also ran codes on it, and I'm seeing P0101 and P0640 which say the mass airflow sensor needs to be replaced. I've never seen that error before. Would that sensor just go out all of a sudden then engine to sputter? I don't know if the sensors actually affect the engine as a safeguard, or they just come up as warnings.

the po101 code, open the fuse box under the hood and check all fuses marked IGN, one of em powers the maf. you sure about that other code? looks like its a code for diesel trucks.

 

[HiHoeSilver]

The oil looked fairly dirty but I didn't see any chunks come out of it and I watched it completely drain. It is the first time that it's been changed since I purchased it in December and I'd been meaning to do it. As a matter of fact, that's why I used the Seafoam yesterday. I planned on just running it on my short drives to work (less than 5 miles round trip) and change the oil Saturday day morning. So much for that. Before I screwed it up it had been running like a top. :/

I pulled off the MFS tonight and thought I could swap it with my 2004 Yukon just to see if that was the problem, but the Yukon is a FlexFuel and this Tahoe is not so the connector would not fit. Should I just go buy a new one? It doesn't look that dirty to me, but what do I know? Even the air filter looks pretty clean. I'll probably just replace them all just to be on the safe side.

My only other idea is to unplug the battery and maybe that'll reset the PCM? As Ron put it though, it's a WAG. lol

Also, I went and looked and I'm not crazy! Even the Seafoam website says to put it in your crack case and it won't hurt the engine. I'm guilty of putting more than the recommended, however. I'm starting to think the Seafoam incident is a coincidence and something just happened to go out...hopefully, the MFS.

https://seafoamsales.com/how-to-add-sea-foam-motor-treatment-to-gas-and-diesel-crankcase-oil/

No, you're not crazy. Maybe just a little overzealous and unlucky. You mentioned any sounds. Is it ticking, knocking, or blowing smoke anywhere? I agree with dub on cleaning or even replacing the MAF. If you clean it, use the correct cleaner. Them suckers are sensitive.

I don't see how the MAF going bad could affect the oil pressure, though. Also, this is an NBS truck, right? I don't think there's a screen for the sensor. I also agree with PNW on replacing with AC Delco. It's a $20 part thai takes 20 minutes to change.

Did you chop the filter open for a peek? I think not seeing lumps come out is a good sign, at least.

the po101 code, open the fuse box under the hood and check all fuses marked IGN, one of em powers the maf. you sure about that other code? looks like its a code for diesel trucks.

I saw that, too. Intake air heater circuit? Nope. Check that second code again.

 

[SnowDrifter]

Seafoam is thin as hell too. And admittedly not incredibly lubricative. You're adding a half a quart of something that's literally thinner than water to your oil. It'll dilute it down. Far more than intended. More is not better.

It could also be off-gassing through your PCV system, leading to that stumble. Though that's purely speculation.

Any improvements after doing your oil change?

What was the reason for adding seafoam? Have you removed your oil fill and taken a peek inside the valve cover? If you have that much sludge, frankly, a mechanical cleaning is preferred.


I'm wondering if it torched the o-ring in the pickup tube. One of the ingredients in seafoam is isopropyl alcohol, which has a less than perfect interaction with the buna-n gasket.

 

[RaiderRodney]

I doubt you've done any long-term damage. Right before an oil change I periodically run 1/3 bottle of seafoam through a vacuum line then cut the motor for about 15 min. Then pour 1/3 bottle into the engine, and then pour the remaining amount in the gas. After the 15 minutes, take a nice "spirited" drive for about 10-15 minutes, come back and do an oil change immediately. I've never had a problem with this method.

 

[iamdub]

Yes, it does say to add it to the crankcase, but not as much as you did. Also, they can't generalize for the internal condition of every engine. Yours may have had some gunk ready to fall off and the sudden, strong hit of cleaner released it. A faulty MAF won't directly cause no oil pressure. It can cause a low idle, which can make the oil pressure low, but not zero. Your MAF may not be bad, it could be the harness. This is somewhat common, almost as common as the harness going to the throttle body. You could let it idle while you slowly move and gently but slowly tug on the harness going to the MAF. Move slowly is necessary so the PCM has time to respond if the circuit opens. If you move too fast, such as giving the harness a quick shake, the loss of connection could be so short that it makes almost no perceivable difference. If it doesn't act up, then you can start looking at the MAF itself. Of course, try cleaning it with MAF cleaner, first. Also, clearing the codes is necessary cuz the PCM may still be reacting to the intermittent problem even though it has been resolved. It may need a full-on reset rather than just clearing the codes, but try that first.

Since you just changed the oil, it should be still be really translucent. Pull that oil pressure sensor and see if it has black junk in the hole. Otherwise it'd only have the fresh oil on it. Even though you drained the dirty/diluted oil from the pan, you still may have particles circulating through the system that have yet to get out. Watch the color of the new oil on the dipstick to see if it turns dark really quickly. If so, do another oil change to get that crud out. I hope you're using a good filter.

 

[avalonandl]

Is it possible the PCV line came disconnected? Sounds like a vacuum leak to me....

 

[mijohnst]

I can't think you all enough for taking the time to help me and taking the time to write up all this up. Great group of people here and I appreciate all of you.

So this is my plan for tonight.

1. Check all the IGE fuses. I noticed this morning several of them but I didn't pull them. I did disconnect the battery whoever so it could sit all day. I'll check all those fuses before I put connect the battery. This car is an NBS and I don't hear any funny sounds except that the engine is struggling to stay at idle.

2. I have cleared the codes but I'll that again.

3. I'm just going to go ahead and buy a new MFS today to replace the one I currently have. OEM only. I'm also going to try and run it and slowly move through the harness and watch for a reaction. My only issue is I can't keep the engine going running me pushing on the gas a little.

If none this works, I'm thinking I need to pull the oil pan and check the o-ring in the pickup for the oil pump. Should I just go ahead and replace the entire pump while I'm in there? I've seen a few YouTubes on getting the pan off of an RWD but none with a 4WD. I'm thinking it's going to be much more complicated because of that. Any tips for that?

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This past summer our 2004 Yukon was acting a very similar way in that it ran really rough and it was hard to keep the engine running at idle. I don't know exactly what I did to fix it but I replaced the fuel pump, flex-fuel module, and the ignition switch and one of those fixed it. This is hauntingly similar. I'm going to go rent a fuel pressure gauge while I'm at the parts store today and check the fuel pressure. Does that seem reasonable?

And since several of you brought up harnesses, would replacing the stereo cause issues with the electrical to cause this? I just put in a stereo this past weekend and I've had issues with it continually resetting. I didn't cut any wires in the car harness but did have to solder the stereo wiring to a control box for my steering wheel controls and that box connects into the car harness. All my wires are clean and have heat shrink at the connections and I've double-checked them several times. Just another thought...

Sorry, this is so long but thank you all for the help!

 

[Joseph Garcia]

I'm confident that you will get through this issue, with the assistance from all of the great folks on this Forum.

Just consider, in less that 24 hours of your post for assistance, the folks here have generated 3 pages of posts offering their suggestions and perspectives. Try getting that from any GMC dealer!! And, all of this sage advice for FREE!!

 

[SnowDrifter]

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Forgive me if this was already mentioned, I'm horrible at scanning text.

Did you check codes? Fuel trims in particular will let you know if there's something amiss on the fuel pressure end or a vacuum leak.

And honestly.... Yeah I'd do the oil pump if it won't be a pain for you. Those things aren't immune to wear, it's cheap insurance if you're in there. Let me know if you have any questions on doing it. There are a few, not pitfalls, but things to be aware of during install. But it's not a bad job all in all. Just messy because everything will be dripping on you

 

[iamdub]

I can't think you all enough for taking the time to help me and taking the time to write up all this up. Great group of people here and I appreciate all of you.

So this is my plan for tonight.

1. Check all the IGE fuses. I noticed this morning several of them but I didn't pull them. I did disconnect the battery whoever so it could sit all day. I'll check all those fuses before I put connect the battery. This car is an NBS and I don't hear any funny sounds except that the engine is struggling to stay at idle.

2. I have cleared the codes but I'll that again.

3. I'm just going to go ahead and buy a new MFS today to replace the one I currently have. OEM only. I'm also going to try and run it and slowly move through the harness and watch for a reaction. My only issue is I can't keep the engine going running me pushing on the gas a little.

If none this works, I'm thinking I need to pull the oil pan and check the o-ring in the pickup for the oil pump. Should I just go ahead and replace the entire pump while I'm in there? I've seen a few YouTubes on getting the pan off of an RWD but none with a 4WD. I'm thinking it's going to be much more complicated because of that. Any tips for that?

-----

This past summer our 2004 Yukon was acting a very similar way in that it ran really rough and it was hard to keep the engine running at idle. I don't know exactly what I did to fix it but I replaced the fuel pump, flex-fuel module, and the ignition switch and one of those fixed it. This is hauntingly similar. I'm going to go rent a fuel pressure gauge while I'm at the parts store today and check the fuel pressure. Does that seem reasonable?

And since several of you brought up harnesses, would replacing the stereo cause issues with the electrical to cause this? I just put in a stereo this past weekend and I've had issues with it continually resetting. I didn't cut any wires in the car harness but did have to solder the stereo wiring to a control box for my steering wheel controls and that box connects into the car harness. All my wires are clean and have heat shrink at the connections and I've double-checked them several times. Just another thought...

Sorry, this is so long but thank you all for the help!

Low fuel pressure can certainly cause these issues, but what are the chances the pump started failing right after pouring Seafoam into the engine? Also, low fuel pressure wouldn't cause zero oil pressure. Knowing what your fuel pressure is wil certainly be good info, especially when it'll only cost you a little time.

I'm wondering something: Does any one know if the PCM will make the gauge drop to "0" if the actual pressure drops to a certain threshold to promote a sense of urgency? I know this is or was a Jeep thing. The oil gauge on my XJ would display actual pressure but then drop to "0" once it was down to ~10psi. This would make the dash warning lights and all come on even though it still had oil pressure. I doubt it, but am curious if the GMT800 had the same or similar function.