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Rough idle and stalling NOT FUEL related.

Customer Question
Rough idle and stalling NOT FUEL related.
Submitted: 2 years ago.Category: Chevy
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Answered in 3 minutes by:
10/28/2015
Chevy Mechanic: Dan, Technician replied 2 years ago
Dan
Dan, Technician
Category: Chevy
Satisfied Customers: 24,754
Experience: Independent Shop Experience, 2 Year Automotive Diploma, & Access to Information Database
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Hello, my name is***** the check engine light on? Does it run rough when going down the highway? Thanks

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Customer reply replied 2 years 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.
Chevy Mechanic: Dan, Technician replied 2 years 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.

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Customer reply replied 2 years ago
OK, I dont have a working logic probe can I put my voltmeter on it to ground and tone it out ?
Chevy Mechanic: Dan, Technician replied 2 years 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.

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Customer reply replied 2 years ago
OK, got an led probe on each of the 4 wires on the IAC and got a pulse.
Chevy Mechanic: Dan, Technician replied 2 years ago

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

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Customer reply replied 2 years ago
I am trying to find one but no one near me carries that IAC by ACELCO
Chevy Mechanic: Dan, Technician replied 2 years ago

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

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Customer reply replied 2 years 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.
Chevy Mechanic: Dan, Technician replied 2 years ago

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

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Customer reply replied 2 years ago
Thank you sir, btw what is the little flapper valve on top of the plenum directly above the spider electrical connection ?
Chevy Mechanic: Dan, Technician replied 2 years ago

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

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Customer reply replied 2 years 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 ?
Chevy Mechanic: Dan, Technician replied 2 years 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.

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Customer reply replied 2 years ago
Dan, I feel bad for wasting your time, I though this was done and over with with :/
Chevy Mechanic: Gary, Chevy Technician replied 2 years ago
Gary
Gary, Chevy Technician
Category: Chevy
Satisfied Customers: 17,283
Experience: 15 years GM master tech
Verified

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 ?

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Customer reply replied 2 years 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.
Chevy Mechanic: Gary, Chevy Technician replied 2 years ago

see if it still does this with the EGR unplugged

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

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Customer reply replied 2 years ago
No codes before. It just threw a code when I ran it with EGR unplugged, ran rough but did not stall
Customer reply replied 2 years ago
If I could literally bump the RPM's by about 150 at idle I would be golden
Chevy Mechanic: Gary, Chevy Technician replied 2 years 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 ?

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Customer reply replied 2 years ago
Where is this screw located ?
Chevy Mechanic: Gary, Chevy Technician replied 2 years 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 ?

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Chevy Mechanic: Gary, Chevy Technician replied 2 years 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

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Customer reply replied 2 years 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.
Chevy Mechanic: Gary, Chevy Technician replied 2 years ago

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

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Customer reply replied 2 years 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
Chevy Mechanic: Gary, Chevy Technician replied 2 years ago

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

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Customer reply replied 2 years ago
sounds like a plan
Chevy Mechanic: Gary, Chevy Technician replied 2 years ago

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

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Customer reply replied 2 years ago
yup, dont suppose there is an "at home" way to do the smoke ?
Chevy Mechanic: Gary, Chevy Technician replied 2 years ago

not that i am aware of

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Customer reply replied 2 years ago
going to get it smoked today, tried replacing the PCM - idled smooth longer but still stalled
Chevy Mechanic: Gary, Chevy Technician replied 2 years ago

ok let me know

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Customer reply replied 2 years ago
smoked, no leaks ... I am losing hair over this thing
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Gary
Gary
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