lean on both banks...1999 C2500 Suburban

[travis]

I’m out of ideas and out of patience on this damn thing!

L31 350 vortec, bone stock other than a cat back exhaust. 3/4 ton suburban, 4L80E, 3.73’s.

So far I have replaced the fuel pump, fuel filter, injector assembly, O2 sensors, intake gaskets, cap, wires, plugs, and cam and crank position sensors. Fuel pressure is perfect, TPS sensor tests perfect. Runs great at heavier throttle opening, light throttle it surges. It’s throwing bank 1 and bank 2 lean codes, and the fuel trims are maxing out again at light throttle. LT fuel trims are pegged out at 25% just sitting there at idle. Cranking compression is 180 in most cylinders, with a couple down around 150-155.

At this point, I suspect either a valve seal issue or a heavily worn timing chain. Does this sound like I am on the right track? These things appear to be highly sensitive to miss fires. Or is there something else I am overlooking?

[In-Tech]

It sounds as though your EGR valve is inactive. When the computer "thinks" the EGR is active it pulls fuel accordingly.

[bobalattie]

Is it a map sensor speed density system or mass air flow?

With cranking compression at 180 the cam timing is prolly ok..

How is the idle quality with fuel trims that lean??

Possible stuck open egr valve?

O2 sensors don’t know Rich or Lean, they just know if oxygen is present or not.. misfires can be lean, overly rich misfire can also look lean, they can’t detect fuel..

Vacuum leaks are usually ruled out pretty easy if you raise the rpm up to 2500-3000 and the long term fuel trim returns to an acceptable value- you know you have a vacuum leak, if it stays high then you have a metering problem.

With mass air systems you can typically (not always) unplug the sensor and the ecu will return to base values and if fuel trims are still high, you have a fuel delivery or combustion problem.

No exhaust leaks, correct?

[In-Tech]

Code: Select all

	EGR Spark Correction Vs. RPM Vs. MAP
 
  Kpa                              RPM
        400  800 1200 1600 2000 2400 2800 3200 3600 4000 4400 4800 
    40  0.0  0.0  3.0  5.0  5.0  5.0  4.0  3.0  2.0  2.0  0.0  0.0 
    50  0.0  0.0  5.0  6.0  6.0  6.0  5.0  4.0  3.0  2.0  0.0  0.0 
    60  0.0  0.0  6.0  6.0  6.0  5.5  5.0  4.0  3.0  2.0  0.0  0.0 
    70  0.0  0.0  6.0  6.0  6.0  5.0  4.5  4.0  2.5  1.5  0.0  0.0 
    80  0.0  0.0  5.0  5.0  5.0  5.0  4.0  3.0  2.0  1.5  0.0  0.0 
  82.5  0.0  0.0  5.0  5.0  5.0  5.0  3.0  2.0  2.0  1.0  0.0  0.0 
    85  0.0  0.0  5.0  5.0  5.0  5.0  3.0  2.0  2.0  1.0  0.0  0.0 
  87.5  0.0  0.0  5.0  5.0  4.0  4.0  3.0  2.0  1.5  1.0  0.0  0.0 
    90  0.0  0.0  4.5  4.5  4.0  4.0  2.0  2.0  1.5  1.0  0.0  0.0 
  92.5  0.0  0.0  4.0  3.5  3.5  3.0  2.0  1.5  1.0  1.0  0.0  0.0 
    95  0.0  0.0  3.0  2.0  2.0  2.0  2.0  1.0  1.0  0.5  0.0  0.0 
  97.5  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0 

The computer also adds timing when the EGR is active, you could be in detonation realm even with good GM knock detection system so don't be surprised if that code shows up too until you fix it. I can't remember if that year has an electronic EGR(stepper) or just a vacuum controlled via a solenoid.
Your year shows this but also this is an example since I don't have your calibration in front of me.

Code: Select all

Emissions Control Diagnostics
 
		X    EGR Performance (P0401)                               MIL on 1st Error (Type A)
		X    EGR Pintle Position Error (P0404)                     MIL on 2nd Error (Type B)
		X    EGR Position Sensor Low Volt (P0405)                  MIL on 2nd Error (Type B)

It's been a while since I worked on one of those Vortec's but I seem to remember it is external and not on the intake manifold. Check this system and google I suppose to help.

[In-Tech]

Sorry, I just noticed you said you are maxing out the trims at idle. The EGR shouldn't be active at idle. Check the exhaust system for leaks probably around the external EGR valve.

[travis]

The idle quality is very good...holds around 660 rpms and will idle all day long with no issues. The exhaust doesn't smell rich or lean either like it did when I was chasing the misfire a few weeks ago.

The EGR is all electrical and is mounted on the front of the intake manifold. The inside of the manifold was pretty nasty when I opened it up...lots of gooey tar like stuff in there that I cleaned out the best I could when I replaced the intake gaskets. I haven't messed with the EGR at all...left it attached to the intake when I pulled it apart. If the EGR valve was an issue "shouldn't" it throw some sort of code? It may not...it never threw any sort of misfire code even with a plug wire disconnected. I'm not sure exactly what all the computer in this thing will do.

I have no indication of an exhaust leak at all, and I have listened closely at various points on the system looking for leaks.

I'll pull the EGR tomorrow and see what I can see.

[Tuner]

If it has this type of distributor cap replace it if you haven't already, maybe even if you have.

[JoePorting]

Sounds like you replaced just about everything I was thinking about. I'd go back to the stock cats.

[travis]

If it has this type of distributor cap replace it if you haven't already, maybe even if you have.

It has been replaced...twice in the last few weeks.

[travis]

Sounds like you replaced just about everything I was thinking about. I'd go back to the stock cats.

The cats are stock, and back pressure testing shows they are not clogged. Could be dead I guess...I believe they are original and have a lot of miles on them. However, lots of people just remove them and don’t have the light throttle surge.

[JoePorting]

Maybe not a fuel issue but an electrical issue. Check voltage and consider replacing the alternator and/or battery.

[ozyfordman]

What are your plug gaps? Too tight a plug gap will give misfires. I battled with this issue because of old school thinking and using .032” plug gaps. Problem went away after I opened the gaps to factory specs. A long shot but worth checking, and it’s free.

[Tuner]

Sensor signal ground jacked someway with a voltage offset?

[naukkis79]

What is map reading at idle if speed density system, mass airflow if maf-based system?

[BigBro74]

Have you checked the distributor gear? I put intake gaskets in ones of these, And it ran fine. Few months later it comes back acting similar to what you describe, and the gear on the distributor was all worn sharp. New one of those and all done (after much head banging trying to figure it out)