Jump to content
  • Sign up for FREE! Become a GM-Trucks.com Member Today!

    In 20 seconds you can become part of the worlds largest and oldest community discussing General Motors, Chevrolet and GMC branded pickups, crossovers, and SUVs. From buying research to owner support, join 1.5 MILLION GM Truck Enthusiasts every month who use GM-Trucks.com as a daily part of their ownership experience. 

2003 5.3L Express persistent high LTFTs at idle

Recommended Posts

I'm usually pretty good at finding these things, but this one has me entirely-stumped.  This is a cable throttle unit, not the newer electronic throttle body, so it has the IAC valve.


The truck had a problem with the IAC wiring, which is pretty common on these and led to all manner of misbehavior with the requested idle not matching actual including both a stumble/stall and racing idle depending on what sort of mood it was in.  That's fixed and the idle is now stable.  The MAF was also questionable and not producing the sort of numbers it should under hard acceleration, so that got replaced too.  MAP is good; shows 14.1 engine off, ~4psi at hot idle, normal and plausible while driving.  Idle is stable at 550, plugs are new laser iridium as are wires as the plugs in there were generating a few misfires and, when removed were slightly out of spec on gap.  Swapped one bank's O2 sensors, no change (so obviously the originals were good); they LOOKED ok but if they're slow I've seen this before.  Nope, not this time.


Go drive it and fuel trims are normal.  When cold they're wildly high; LTFTs +25 on both banks.  After it warms up they'll come down to about +15 at idle in park, which is still too high.  STFTs behave normally.  Come off idle a bit (e.g. 1200 RPMs in park) and they come down, but not to zero.  O2 sensors sweep normally once the engine goes into closed-loop.


This implies a vacuum leak, but I'm pretty darn sure there isn't one as the MAP is normal and MAF is mid-4s at hot idle, which is also normal -- and a vacuum leak will cause MAP to read high.  No damage on any of the hoses to the manifold and detaching and plugging the ports manually does not resolve it (e.g. if there was an open somewhere into the crankcase thus the PCV was basically open all the time to atmosphere, etc.)  No code for evap fault so its not the purge into the manifold.  Fuel rail pressure is also good; 53psi at idle, right around 60 when the key is on before you start; the vacuum modulator for the fuel rail appears to be good and pressure is fine.  I ain't buying a manifold gasket leak given that; if that was the cause I should see it in the MAP sensor and I don't.


Misfires are also normal (zero other than one or two counts during start); there were a few recorded when I first started looking here and there and the wires looked original so I changed both plugs and wires (the wires disintegrated trying to remove them); plugs are the newer iridium ones as what was in there were the older-style platinum.  There was no evidence of misfires or fouling on any of the 8; all looked good other than normal wear and slightly out of spec on gap implying there's not an injector problem; there is definitely not a cold or fouled cylinder involved, and besides, both banks are out of range so its not logical its a cylinder-specific issue nor a bad/plugged catalyst.


In cold weather (below freezing) it goes far enough out of range on a cold start to trip bank lean codes (which the fuel trims say it is) but in warmer weather generates no codes.


I'm rather stumped; any ideas on where to look next?

Link to comment
Share on other sites

Solved -- leaking intake gaskets.  The OE ones had ZERO relief off the plastic carriers when removed and there were clear tracks where they had been leaking at idle.


Its a Jenga-style puzzle getting the intake out on these vans; it comes out the back via the doghouse once you figure out what has to be unclipped and otherwise gotten out of the way.  Of particular amusement is the transmission dipstick tube which is actually two pieces vertically and that is not at all obvious -- but it is, and it does separate.  Without that you have no way to gain clearance for it to come out.


I used the FelPro replacements; it looks like a better solution.  Now that I know what has to be moved and in what order I could probably get it out again in an hour or two.  Not nearly as bad as I thought it might be -- it LOOKED impossible.

  • Like 1
Link to comment
Share on other sites

That will do it.  I was about to suggest you look at the coolant temp. sensor voltage and the ambient air temp. voltage, I have seen where one failed but within range and they didn't agree (30 degrees coolant temp., ambient 100 at startup), caused all sorts of wacky problems until warmed up and in closed loop. 

Link to comment
Share on other sites

  • 3 weeks later...

Yeah I checked those and the MAF as well; the IAT is in the MAF and both were reading ok, along with coolant temp.  I'll look for the ambient sensor if I can find the PID for it; I know there is one somewhere as that also gates the A/C off when it gets too cold so below-freezing temps don't slug the compressor.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...

Important Information

By using this site, you agree to our Terms of Use.