Chevy C1500 P0171 and P0174 after I/M Gasket job

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First off, I’d like to thank you Eric for all the videos and this site and forums. Above and beyond, buddy, and for that I thank you.

1998 Chevy C1500 5.0l Vortec. I did the intake manifold gasket job using Felpro gaskets. Right before I did this job I got a P0420 code. I went and did the gasket job and all went well. Now I get a P0171 and P0174 codes. The truck doesn’t hesitate at all except upon cold first startup if I start her up and try to go. She does usually start right up quickly. I smoked the engine and also did the propane vacuum leaks test and couldn’t find anything. I cleaned the MAF sensor twice using MAF sensor cleaner. I put the scanner on the truck and see the LT fuel trim for both banks is at 25%. Revving the engine makes the LTFT drop to about 10% with STFT going to also dropping slightly. I did the propane test again but now with the scanner running. Still no vacuum leaks apparent via STFT reading. Where should I go next? Thank you.

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[Richard Kirshy]

I’m a little confused…
Did you do the intake manifold gasket because you had a P0420???

Now after the repair you get P0171, P0174
Common problems that trigger the P0171 and P0174 Code
Vacuum leaks (Intake Manifold Gaskets, vacuum hoses, PCV hoses, etc.)
Mass Airflow Sensor (MAF)
Plugged Fuel Filter or weak Fuel Pump
Plugged or dirty Fuel Injectors
If you didn’t have the lean condition prior to doing the intake gasket.
My guess would be something you did during the repair.
Could be as simple as forgetting to hook up a vacuum line or as bad as the intake not seating properly on the gasket

[Splat]

Hey Nightflyr. I was losing antifreeze and not seeing any leaks so from what I read it was most likely the intake mani gasket, which is a common issue. I smoked the engine twice and didn’t see any leaks, also did the propane vacuum test and didn’t find anything. I noticed today that there’s a sound coming from the rear underneath of the truck. It’s like a vacuum, almost whiny, sound. I’m wondering if that’s the fuel pump going. I’m gonna go underneath her tomorrow. PS, I’m also gonna get a fuel pressure tester and see what the pressure is.
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[Richard Kirshy]

Good luck.. keep us posted

[EricTheCarGuy]

Thanks very much for your comments on my work. I really appreciate it.

25% means it’s adding a lot of fuel. The codes you have mean it’s reading lean. Can you also see the live data for the O2 sensors? It could be that you have a bad O2 that’s reading lean and causing it to run rich. If you see one that’s pegged lean, that could be a bad O2.

I know you say you checked for vacuum leaks, but since you just replaced the intake gasket, there could have been an issue with the installation somewhere causing a leak as well. I’d put that toward the bottom of the list for now since you’ve already checked, but anytime you do work and have a problem after that that wasn’t there before, you become the most likely suspect.

More info on issues of this type here.

http://www.ericthecarguy.com/faq/solving-automotive-performance-issues

Please keep us updated on your progress.

[Splat]

Well, this goes to show you, kids. Always do your smoke tests inside, or in as wind-free a place as possible. The past times I did the smoke tests I did them with a tarp over the engine bay out in the driveway. My garage is so full of ….stuff… but today I cleared enough out to get the front of the truck in there. Hey, garages are where you store all your stuff…no? 😛 So did the smoke test in the garage and this time I saw smoke emanating from where the air plenum meets the throttle body/intake. The clip that secures the air plenum to the throttle intake broke. I had a small clamp on there but it was leaking from the opposite of where the clamp was. I got some Ultra Black silicone (I love that stuff!) and put it all around where the plenum meets the throttle body. Gotta let it sit overnight and we’ll see tomorrow. That’s the only place I saw any smoke leaking. I also sent smoke up the tailpipe to check for exhaust leaks but nothing there, either.

[EricTheCarGuy]

Sounds like you may have found the problem. Keep us posted.

[Splat]

[quote=”EricTheCarGuy” post=158022]Sounds like you may have found the problem. Keep us posted.[/quote]

One thing I haven’t been able to find anywhere is how long does it take for the LTFTrims to go back to normal? I drove the car for a few hours yesterday and the SES light is still on. I have to get the scanner on the truck and see if still have the 171/174 codes. I thought the LTFT would go down over time due to the STFT seeing less fuel is needed now the leak has been fixed. Or do I have to reset the computer?

[Emmanuel]

Hi there. No you don’t have to reset it. The O2 sensor will read the fix and send the correct signal to the pcm so the fuel trim normalize. It would be good if you provide us with the O2 readings as well. Make sure that the sealant you used did the job right. Use water as well to check for vacuum leaks and just for the heck of it, pinch the brake booster’s vacuum hose also. Keep us posted.

[Splat]

Thanks iManuEl. I used Permatex Ultra Black, which may be overkill for this application, but it’s damn good stuff. I didn’t a smoke test again and now no leaks can I or my brother see. I took the truck for a ride and then smoked the engine again thinking since it was then hot any small cracks or expansion my be detected with the smoke. Still nothing. I have to figure out which o2 sensors on my truck or the ones I want to read since there appear to be quite a few in the listing of the scanner s/w. The STFT now is down between 0 and 4.5, yet the LTFT is still at 25 for both banks. I may not have done enough driving yet to clear the P0171/P0174 codes yet though. More tinkering tomorrow as I have to go somewhere tonight. I’ll let you guys know tomorrow what I find.

[EricTheCarGuy]

It does take a while for the LTFT to reflect the change. That said, you might want to pull the negative battery cable to clear the codes you have and see if they come back. That will do a complete reset of the computer over just clearing the codes. If they don’t come back, you’re good. If they do, you have more work to do.

Please keep us updated.

[Splat]

[quote=”EricTheCarGuy” post=158663]It does take a while for the LTFT to reflect the change. That said, you might want to pull the negative battery cable to clear the codes you have and see if they come back. That will do a complete reset of the computer over just clearing the codes. If they don’t come back, you’re good. If they do, you have more work to do. Please keep us updated.[/quote]

No kidding? I thought clearing the codes via scanner or removing battery accomplished the same thing. The LTFT values get wiped when removing the battery for a while? I’m gonna do that today then. I’m heading over to auto parts store to get a new PCV valve. Mine looks like the rubber around the tubing is degrading so I may have a slight leak that way. It’s only $10 with a new rubber tubing and elbow so I figure what the heck. I’ll let you know. Thanks brother!

[Emmanuel]

If you find that u still have an issue, pour (not spray) some water right at the intake manifold gasket while the engine is COLD and see if u get something. If not, you might consider a fuel delivery issue. Good luck.

[Splat]

[quote=”iManuEl” post=158745]If you find that u still have an issue, pour (not spray) some water right at the intake manifold gasket while the engine is COLD and see if u get something. If not, you might consider a fuel delivery issue. Good luck.[/quote]

I assume I need to have the engine running (but cold) and I’ll hear some type of rpm difference if the water gets in? I’ll do that. I picked up a new PCV valve and the rubber connectors. Gonna go do that right now. Thanks.

[Emmanuel]

Correct, engine running. Also check around injectors’ orings with water (sprayed) u might have a vacuum leak there too. I really hope you fix this today, good luck.

[Splat]

[quote=”iManuEl” post=158788]Correct, engine running. Also check around injectors’ orings with water (sprayed) u might have a vacuum leak there too. I really hope you fix this today, good luck.[/quote]

OK, I replaced the PCV valve and rubber connectors, which were pretty brittle, and did the water test. I dumped water on the intake manifold area (none got on the alternator, amazing!) and then sprayed water here and there. My brother and I noticed NO change in RPM or overall engine sound. I was just tinkering around with my scanner while at work and found what I think are the correct O2 sensors so I’m going to check them out tomorrow morning since I’m at work now. I can’t see it being an O2 sensor though because the truck is running great. No hesitations, no hard starts, no stalls, etc.. so I’m really banging my head on this one. Why’d they have to put computers in cars?! Worst thing they ever did, IMO. :angry:

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