93 Jimmy 4.3 Vortec idle problem

I'm posting for a buddy who has a 93 Jimmy 4.3 vortec. The car will fire sometimes, other times requires ether to get it going. Will run OK at elevated RPM's (no tach on this one), I'm guessing over 1K RPM. If you drop the idle down to under that, it will die. He has replaced the following:

EGR
All top mounted (intake) sensors
Cap, rotor, wires, plugs
fuel pump, screen, and filter
Ignition module was tested - OK
CPU replaced

Checked the fuel pressure and it snaps to 60 at start up and drops to about 56 while running (pressure holds OK). Every once in awhile while we had the idle cranked up a bit, it would idle a lil faster then drop down to what it was doing before. We put some seafoam in the gas (man does that stink coming outta the exhaust! :) ). I think it would throw a code 36 every once in awhile too, then go away. This is driving him NUTS! I know this is the place to check...any suggestions?

I'm wondering if it might be running rich or lean. Does the exhaust smell like gas when it is running? If so, that's an indication of a run-rich condition. I'd check the injectors for leaks (I'm presuming you have TBI because I had a '92 S-10 4.3L with that...two big injectors just under the throttle plate). Don't know the Code 36 offhand... Hope this helps some!

Exhaust don't smell like fuel. I will check to see if his is the CMFI type - maybe his is different.

I can't find a Code 36 on the trouble code charts for that truck at all nor any other similar trucks. Next time you see it, have someone read it with a code scanner. But I'm guessing that since you replaced many sensors, it may or may not reappear.
My thoughts turn to plug fouling, but I'm also guessing that when you replaced them they appeared okay.
My last thought is to check for vacuum leaks around the throttle body as well as all the vacuum lines. Keep on keepin' on....

OK, I just talked to my buddy - I was wrong about the code - it is code 35. I have one of those books with the codes and it says 1 of 2 things. IAS or ISC module (?). His engine is like mine, CMFI type with the spider injector. We took the IAC module out & cleaned the port & the IAC thoroghly. BTW, this is a new module. The code will only show up at idle RPM's. When you kick it in a the arse a lil, the check engine lite will go out. Let it settle again, and it pops on. Kick it up, goes off again. Obviously something to to with idle control. We read the codes by shorting 2 pins on the interface connector for the diagnostics. What the heck is a ISC? New plugs, wires, rotor, checked all vacuum lines. He found one bad one awhile back & replaced it. He (and I) are stumped.

Any ideas?

THANKS! :)
Jon

Code 35 is a faulty IAC (Idle Air Control) valve or circuit. If the valve is new and/or functional, then you should check the connector and wires for breaks.
The valve also needs to be adjusted to the proper length by using thumb pressure on the valve end and a slight back-and-forth motion. It should be no longer than 1-1/8" as per manual instructions below:

From Haynes manual for CPI intake:
Measure the valve pintle extension. To retract the pintle on a NEW valve, use firm thumb pressure and, if necessary, rock the pintle with a slight side-to-side motion. BUT, if reinstalling a used valve on which the pintle is extended further than specification, an IAC tester MUST be used to electrically retract the pintle.
Do not attempt to physically retract a pintle on an IAC valve that has been in service; the force may damage the pintle threads. The force required to retract the pintle is only safe on NEW IAC valves.

The valve should not be soaked in cleaner since it is an electrical component.

Hope this helps you. Good luck!

NOW THAT is good info! Thanks Jdmccright! We just cleaned the plunger part, not the whole assy. as it is new. Cleaned some carbon out of the intake when the thing mounts. We did retract it though (manually)...damn I hope we didn't kill the thing.

THANKS! :)