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Gary
Gary, Chevy Technician
Category: Chevy
Satisfied Customers: 15702
Experience:  15 years GM master tech
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Rough idle and stalling NOT FUEL related.

Customer Question

Rough idle and stalling NOT FUEL related.
Submitted: 1 year ago.
Category: Chevy
Expert:  Dan replied 1 year ago.

Hello, my name is***** the check engine light on? Does it run rough when going down the highway? Thanks

Customer: replied 1 year ago.
OK, sorry I should have mentioned. Recently replaced:- CPI assembly
- TPS
- MAP sensor
- IAC
- New Intake Plenum Gasket
- New plugs and wires
- Cleaned EGRThere are no apparent vacuum leaks, fuel pressure is bouncing around 52-55PSI at idle and 60+ at throttle, no leaks under the plenum - confirmed with a borescope. it starts at half a crank and runs like a champ for about 20 seconds then starts to stumble - with IAC compensating. It gets progressively worse until it stalls. If any throttle is given it runs like a champ, no issues accelerating or driving.
Expert:  Dan replied 1 year ago.

It sounds like problem with iac wiring or PCM not operating IAC correctly. If OK, check the IAC wiring from the Powertrain Control Module (PCM). Verify there are no shorts or opens. Disconnect the IAC motor. For this specific test, a logic probe works the best. Connect the logic probe from ground to each IAC harness wire one at a time. Check for blinking Red and Green lights at each circuit while manually revving the engine up and down. If the circuits are OK, replace the IAC. If one or more circuits are dead, check the wiring/connections, then replace the PCM. Let me know how it goes or if you have more questions, thanks.

Customer: replied 1 year ago.
OK, I dont have a working logic probe can I put my voltmeter on it to ground and tone it out ?
Expert:  Dan replied 1 year ago.

You can but it will not work as well because the blinks happen faster then meter can keep up. Next best option would be to use a LED tesetlight. Let me know how it goes or if you have more questions, thanks.

Customer: replied 1 year ago.
OK, got an led probe on each of the 4 wires on the IAC and got a pulse.
Expert:  Dan replied 1 year ago.

Replace the iac again with a ac delco brand. Let me know how it goes, thanks.

Customer: replied 1 year ago.
I am trying to find one but no one near me carries that IAC by ACELCO
Expert:  Dan replied 1 year ago.

Then just try a different brand. Based on testing and symptoms it definitely sounds like IAC is the problem. Let me know, thanks.

Customer: replied 1 year ago.
Sry took me a while to try and find another valve - went to pick it up and it is the same darn brand. Well, started the van with the iac still in but disconnected from power - the engine surged and ran steady at higher RPM's. I plugged the IAC in and it normalized and ran smooth for 30 mins. Started it again today after sitting over night and ran with stumbles, disconnected the iac - ran smoother. I guess I will be buying a new one just need to find a better brand that is in stock.
Expert:  Dan replied 1 year ago.

I agree, maybe try a different parts store. Let me know how it turns out. Thanks.

Customer: replied 1 year ago.
Thank you sir, btw what is the little flapper valve on top of the plenum directly above the spider electrical connection ?
Expert:  Dan replied 1 year ago.

Not sure. If you send a picture I could tell you for sure.

Customer: replied 1 year ago.
Ok, replaced the IAC with an Elfin unit from Napa - first start and drive no problem. Restarted and same stumbling idle. What is going on with this thing ?
Expert:  Dan replied 1 year ago.

I'm not sure, we have done just about everything I can think of. I'll open question back up to see if someone else has some new ideas. Thanks.

Customer: replied 1 year ago.
Dan, I feel bad for wasting your time, I though this was done and over with with :/
Expert:  Gary replied 1 year ago.

new expert here

any access to a scan tool to see live computer data ?

if you unplug the EGR valve will it still do this ?

Customer: replied 1 year ago.
No live scan tool that I can get my hands on for OBD1, I can only pull codes. Let me unplug the EGR and see whats up.
Expert:  Gary replied 1 year ago.

see if it still does this with the EGR unplugged

and i am assuming there are NO codes set in the computer ?

Customer: replied 1 year ago.
No codes before. It just threw a code when I ran it with EGR unplugged, ran rough but did not stall
Customer: replied 1 year ago.
If I could literally bump the RPM's by about 150 at idle I would be golden
Expert:  Gary replied 1 year ago.

if there are NO vacuum leaks and NO codes set ( not the EGR one it just set ) then this issue with 2 new IAC valves being replaced would really lead up right to the computer being bad

now may i ask does it look like anyone adjusted the throttle screw at all ?

Customer: replied 1 year ago.
Where is this screw located ?
Expert:  Gary replied 1 year ago.

if you look at the throttle on the throttle body there should be a stop that the throttle rests on when at idle is that screw been moved ?

Expert:  Gary replied 1 year ago.

the odds are it has not been moved if you did not touch it

this is the direction i would go next i would remove the upper intake manifold and be sure the lower intake where the injection unit is not filled with fuel and also see if the upper intake gasket is broken ot all between any ports

if a lot of fuel is seen replace the injector and a new upper intake gasket

if no fuel is seen just replace the upper intake gasket

if it still does this the only thing let would be the computer itself

Customer: replied 1 year ago.
No fuel leaks under the plenum, no air leaks from the plenum gasket itself - confirmed meticulously. I do hear an airy high pitch whistle but cannot locate the source, appears to come from the driver side area.
Expert:  Gary replied 1 year ago.

try running it spraying some carb cleaner or listen with a stethoscope to pin point that area

Customer: replied 1 year ago.
Thats the frustrating part - I spent a full bottle doing just that to no avail. Cannot get a good sense with stethoscope due to the crappy layout of this plenum/engine
Expert:  Gary replied 1 year ago.

then see if you can have it hooked up to a smoke machine and look for the smoke leaking

Customer: replied 1 year ago.
sounds like a plan
Expert:  Gary replied 1 year ago.

that way we can at least vacuum leak as being the issue

Customer: replied 1 year ago.
yup, dont suppose there is an "at home" way to do the smoke ?
Expert:  Gary replied 1 year ago.

not that i am aware of

Customer: replied 1 year ago.
going to get it smoked today, tried replacing the PCM - idled smooth longer but still stalled
Expert:  Gary replied 1 year ago.

ok let me know

Customer: replied 1 year ago.
smoked, no leaks ... I am losing hair over this thing