I can't answer the question about the EGR valve being fully seated... how would I test that?

I tested the TPS (KOEO) as suggested in my Chiltons Repair manual... it showed the proper amount of reference voltage, the correct amount of voltage at closed throttle (.95V) and also the voltage increased smoothly as the throttle was moved through it's full range, with a maximum of 4.24V at full throttle, which is within specs.

I thought I had narrowed it down to the IAC valve... Even though it tested fine, it did nothing when I unplugged it, so I assumed maybe it was stuck open. So I made a cardboard gasket to completely cover the 2 ports, and then put it back on. The idle was dramatically lower (like 600-700 rpms), and while driving it around that way the RPM's dropped when I pushed the clutch in, instead of rising like they used to do. So I thought, YES! I finally found the problem. Wrong. Replaced the IAC valve with a new one and it's acting almost exactly the same way it did before.

Now I am getting 3 codes. P1506, P1131, P1151. They probably all indicate the same thing, that damn vacuum leak that I haven't found and fixed yet.

So my plan (once it warms up) is to take the intake plenums off, replace the gaskets and the isolator bolts (clean/vacuum the egr ports while the intake is off) and maybe replace the bushings on the IMRC and verify the butterfly valves are clean and operate smoothly.

I'll probably also remove the fuel injectors and clean them, and install new o-rings.

I would assume that if the PCV elbow would qualify as a vacuum leak, then valve cover gaskets would too, right? If so, I bet I should replace those while I am at it too.

Does all this sound like the right thing to do, or am I going overboard trying to rule things out? It just seems that if I've got everything removed anyway, that would be the time to do this sort of maintenance.

 

Did you reset the KAM when you installed the new IAC?

The P1506 is saying that the IAC can't slow the engine down to the target rpm. And the P1131/P1151 is saying both banks are reading rich as they are trying to correct for an overly lean condiction. (The lean condiction is prolly caused by the vacuum leak that makes it difficult for the IAC to slow the idle down.)

That sounds like an awful lot of work with no assurance that you will hit pay dirt. Might be better off to have it smoke tested to see if there are any vacuum leaks in the intake.

Another thought is you *might* have a leak in the EVAP canister purge valve. You can remove the vacuum hose from the bottom of the canister purge valve and plug it and also remove the smaller vapor line coming from the intake manifold and plug it. That will take the EVAP system out of the equation. See if it will now idle correctly.

If not, you may have a wiring issue in the harness that controls the IAC (especially since nothing changed when you unplugged the IAC). The PCM commands the IAC by varying its duty cycle. Might try and unplug the IAC (since it seemed to have no effect when you unplugged it) and plug in your other IAC and watch to see if it moves when you start the engine.

 

Yes, I erased codes with my OBD-II scanner, let it idle for about 15 minutes, drove it about 5 minutes up to 45 mph, stopped twice, shut it off. Restarted twice, drove 30 minutes @ 65 mph.

But the engine was already warm when I reset the codes, if that makes a big difference. I don't know how important it is to have a cold engine when you reset the codes.


I assumed as much.



It does, but it sounds like this engine is pretty notorious for having the leaking isolator bolts, and I'd like to rule it out. My fuel filter was also filthy so I'd like to inspect the condition of the injectors out of pure curiosity.

That actually is the next thing I'd like to check. However I've removed the EVAP vacuum line at the intake manifold and plugged it with my finger and it made no difference in the idle. I've actually done this with each of the 3 intake manifold ports and nothing has changed the idle. That's kind of why I suspect the isolator bolts in the plenum.

I never have any residual air pressure when I stop to get gas and remove the gas cap. Is that normal?

EDIT: I combed over the EVAP system and found nothing to report. One short piece of hose is kind of collapsed where it turns a tight corner, but no vacuum leaks. Also plugged the EVAP port on the intake and plugged the purge valve vacuum line with a golf tee and it made no difference in the idle speed.

The truck seems to idle fairly nicely when it switches to idle on OL-Fault. Pretty smooth and around 800-850 rpms. But when it's on CL it idles closer to 1100. And when I'm decelerating and push the clutch in (like when I'm going to downshift), it revs to like 1600 rpms. Course maybe its anticipating a downshift?

That's a great idea. I did that tonight when I got home and the IAC barely cracks open when I'm parked @ idle and I manually snap the throttle. So the air that is causing the engine to race doesn't seem like it would be coming from a wide open IAC valve.

 

Ended up working on it this weekend. Wanted to cover all my bases so here's what I did:

replaced intake plenum gasket set (upper/middle)
replaced throttle gasket
replaced PCV (on throttle body) cuz o-ring was hard and baked
replaced valve cover gaskets & new bolt grommets
replaced middle plenum isolator bolts (8)
also removed fuel rail and injectors, bench tested ohms, then cleaned w/carb cleaner and homemade spray setup
replaced all fuel injector o-rings and lubed
cleaned oily gunk from the orifices of all 6 EGR ports and blew out EGR passages w/ compressed air

Started it up and it runs like a top. Normal idle (around 840 rpms instead of 1100) better SHRTFT's (hover around 0% at idle instead of sticking at 32.8%) and better front O2 readings (nice fluctuating wave .1 - .75 V at idle instead of a flatline .03 V at idle). Then engine also no longer surges when I push in the clutch, either when accelerating or decelerating.

Thanks to all who posted. I'm no mechanic, but this was pretty easy stuff... and took maybe $150 in parts, rags, solvents and a few tools I didn't have before.

 

Good to hear.
Apparently there was more than one source of air leakage.
Good luck.