2000 Suburban- Occasional P0171 Problem

[MrFleming007]

Hey everyone, throwing this out there for the folks that are a litle more knowledgeable in this type of stuff.

The issue started out as an occasional p0171 and p0174 code that would throw if the vehicle was under heavy load or after a few hours of driving. Chased that down to leaky intake manifold gaskets and finally got around to changing them yesterday. Used the correct new style teal green GM gaskets. Still threw the codes, but after some digging around I found that the EGR hose inlet gasket had slipped and was letting some air in. Prior to this I had used Torque for Android to capture the freeze frame data from the time the code flipped. My Fuel trims were very high- STFT1 averaged 12%, STFT2 averaged 13%, LTFT1 averaged 24.219 and LTFT2 averaged 24.219.

After repairing the EGR inlet gasket and cleaning the MAF, I reconnected Torque and took a drive. STFTs for both banks are now between -3% and 3% on average, but the LTFT numbers are still between 16% and 18%, and although the code hasn't flipped, Torque is indicating a pending p0171 code.

A freeze frame of the pending code gives the following numbers: Engine Load: 2.745%, STFT1: -1.562%, LTFT1: 17.188%, STFT2: 1.562%, LTFT2: 17.188%, Intake Manifold Pressure: 4.931 PSI, Mass Air Flow Rate: 7.48 g/s, Throttle Position (Manifold): .0392%, Engine RPM: 724, Speed: 14.913mph.

Truck runs great with maybe a slightly rough idle, and thats a big maybe and could be me imagining it. Fuel filter is new within the last month. I havn't tested the fuel pressure yet, but imagine it is good as there is no throttle hesitation at all. Tried leak testing the new gaskets with carb cleaner to see if there was a difference in engine rpm and got nothing. PCV and vacuum hoses all seem to be in good condition as well. What other components`could be causing this, and what should my baseline fuel trims be at?

Thanks in advance for everyones help and advice!

 

[bkboatnsleds]

Are you sure you put all the vac lines on- there are a few that are easy to miss. also check ALL the bolts front one is hidden on the intake and easy to miss... I'd check your work first..

Also not to burst your bubble.. but the teal gaskets are not the latest update...

 

[MrFleming007]

Not bursting my bubble at all. Gaskets were bought from the local Gm dealer. I've got the post number around here somewhere. Double checked all connections again, all good. All bolts were originally installed and torqued in the order and to the specs specified in the Chilton manual. Just rechecked and nothing has backed off.

 

[bkboatnsleds]

Were they the style that sits on the block - that look like this:

or the ones that "Snap on " the intake ... like this?

 

[MrFleming007]

Snap on style. Is the other style the newst type and does it seal better? If so, what makes it a better design?

 

[bkboatnsleds]

Yes - the other style is the newer style. You lay it on the motor before you drop the intake on. In comparing the 2 - the newer style is all metal frame / support area and the gasket area is MUCH thicker and seems much more stable - if you have the 2 of them in your hands one seems like a 'walmart' brand and the other a top end. Thats the best way I can explain it.

Having said that - even the old style gasket should still seal up, and work properly given it's installed right.

 

[MrFleming007]

Great. I have a feeling I'll be pulling this manifold one more time as my LTFT numbers are sitting really high still, according to what I've read. Nobody has mentioned yet though what the LTFT numbers should be on a stock 5.3 with no mods. I'm not convinced the problem lays in the gasket though, as this isn't the first one I've put in, and the process is so simple that anyone with any mechanical ability and some patience could probably do it. What else, if anything could be causing this?

 

[afpj]

O2 sensors can cause excess fuel trims IIRC. Usually the precat ones, but there's where my knowledge wanes...that info fell out of my brain when I had to make room for other nonessential facts.