Idle Hunting in Gear with a 1990 3.1L MPFI

[Kaline]

I’m having a frustrating problem with my 1990 Oldsmobile Cutlass Supreme SL (3.1 V6/auto), and I’m looking to all of you for advice. The car runs great when the engine is cold, but once it is warm and in gear with the brake on (like at a traffic light or stop sign), the idle will hunt up and down and eventually stall. It is fine in park or neutral, but in drive or reverse it has this problem. It just starting doing this last week. The car has 115K miles on it, uses NO fluids, does not set the Check Engine light and otherwise runs and drives great. In the last 5K miles, I have replaced:

 

Spark plugs and wires

Ignition Control Module

Oxygen sensor

Fuel pump and filter

Air filter

Transmission fluid and filter

Run Chevron Techron thru the fuel (twice)

Cleaned throttle body and IAC

Performed the idle re-learn procedure religiously after every engine service

 

The rest of the car is very original. The car started doing this w/ the old Ignition Control Module, so I removed it, had my local auto parts store test it (it tested bad), so I replaced it. However, that was not the idle problem. Any ideas what might be the problem? I just want it to run right!

[Robby1870]

 

cleaned throttle body and IAC

Performed the idle re-learn procedure religiously after every engine service

 

 

Does this mean you just cleaned the IAC pintle?

[Kaline]

Yes, I cleaned the pintle. I'm wondering if the IAC is going bad....

[1990lumina]

Did you reset the IAC properly?? I know when I cleaned my IAC it took a bit for everything to return to normal (I put the IAC back in with the pintle totally messed)...

 

I think the reset procedure is turn the key to ON, but don't start.....hold it there for a few seconds, turn it off, wait 30seconds or so, turn the key back to ON, and then start the engine....I know this procedure DID NOT work for me, but this is how it is outlined in my Haynes manual.

[Robby1870]

Yes, I cleaned the pintle. I'm wondering if the IAC is going bad....

 

Thats what Im thinking. Ive never had to "reset" the IAC. Ive heard of it working before, but never in my experience. When I replaced the one on my STE, I just went out to one of my parts cars and grabbed its IAC, bolted it on and the car ran fine.

[Kaline]

UPDATE - went to a friend's house today who has a GM Tech II tool with adapters to read OBD I cars like mine. We tried to communicate to the ECM to see what the IAC was doing but the tool could not communicate with my ECM. The tool is OK as my friend uses it for work all the time and we could communicate to the other controllers in my car just fine. My friend had a spare used IAC, so we installed it, performed the idle re-learn procedure, and the car still acted the same way (stalling when hot in gear with the brakes on).

 

I'm wondering if the ECM might be acting up. I'm going to try to read codes again tomorrow and see what is going on. Any other ideas? Maybe the EGR valve going bad?

[Brian P]

this sounds more like a vacuum issue. Either a broken/split plastic vacuum line, crack in the brake booster vac line, or possibly (less likely) even a leak in the upper or lower intake gaskets. I would look around for anything like that, especially at the "rear" of the engine where the MAP sensor is.

[Kaline]

UPDATE: I checked codes this morning, but I forgot that all codes were reset when I did the idle re-learn procedure. No info there.

 

I listened for vacuum leaks while the engine was warming up, and everything sounded normal. I even sprayed the upper and lower intake mating surface with carb cleaner, and sprayed lube on the EGR (incase it was sticking) However, once the engine was warm, I heard the idle shift slightly and a miss could be heard in the exhaust. When I shifted into gear, it started hunting. It does not do this when the engine is cold. Wondering if it a closed-loop problem the ECM or some other sensor. No codes are set. Any ideas?

[Supreme Cutlass]

Just to make sure, you're getting a code 12, correct?

[Kaline]

Yes, code 12 the only code my ECM displays.[br]Posted on: March 27, 2006, 12:44:22 PM_________________________________________________UPDATE: I checked the wiring harness from my ECM to the ALDL port to see if the serial wire was damaged. This wiring checked out to be OK. So, I decided to replace the ECM, as I figured someone was wrong with it since the Tech II could not communicate with it. I replaced that (remanufactured unit from Murray's Auto Parts, $77), did the idle re-learn, and the car runs much better. No stalling, but it still have a miss at warm idle. When my friend is free, I'm going to go to his house and see if the Tech II will communicate to the new ECM. I found some oil near the throttle body n the intake tube, so I think my PCV is stuck (hence causing a vacuum "leak"). The rubber tube is hard as a rock, so I am going to pull the intake plenium and replace the PCV valve and rubber tube later this week.

 

I checked all the other rubber vacuum lines and they look OK.[br]Posted on: April 03, 2006, 10:52:13 AM_________________________________________________UPDATE: This past weekend I replaced the PCV valve and upper intake plenum gaskets, as well as cleaned the upper and lower intake with Seafoam Top Engine Cleaner and replaced the vacuum line from the fuel pressure regulator to the MAP sensor (because I accidentally broke it when reassembling the intake). The car runs MUCH better now with minimal stalling. Another friend loaned me an old Tech I he has laying around his shop and everything looks OK when I run thru the tests. The car runs smoothly off-idle, other than a slight miss at idle when hot. It has had this miss ever since I bought it. I'm still looking for the root cause of the stalling. I plan to use a bottle of the Seafoam aerosal cleaner in Monday to see if that clears it up, as well as check the fuel pressure and see if the Tech I will do an injector balance test. Any other suggestions are appreciated![br]Posted on: April 12, 2006, 09:15:54 PM_________________________________________________UPDATE: Tech Is cannot do an injector balance test : ( I guess that is for Sequential Port Fuel Injection. Anyway, I cleaned the throttle body again and took the IAC plate off to throughly clean it. The car runs well but still has an occasional miss at idle, in gear, when hot. I'm puzzled. Any ideas are appreciated!

[Crazy K]

I fix that hot miss at idle by turning on the AC!

that makes the engine rev slightly higher and makes the fan turn on at a lower temperature.

Fix it right with 216k on 'Frankenstein' the chevy that won't die? well i should, but throwing nuts and bolts down the intake will do it wonders i have read....

 

But seriously, The idle adjustment screw looks like it has worn down slightly by the butterfly in those 200k miles, and may be running the engine a little slow.

 

other observation of the phenominom....

 

On my 91 Euro, it had the hot miss, it was corrected by replacing the temperature sensor for the ecm/fans (screws into the thermostat housing)

 

on the same car, I replaced the fuel pump when it started stalling warm at stop lights.

 

on a POS corsica 2.8 that I'm embarressed to have had... it would stall warm due to a kink in the fuel line, due to some #^&* who fooled with the fuel filter.

 

I imagine my car would benefit from service!

[Kaline]

UPDATE: I checked fuel pressure tonight. Using the procedure in the GM Service Manual (key on, engine off) the fuel pressure at the rail was 290-300 KPa (within spec). With the engine running (both cold and hot), the running pressure at idle was 245-250 KPa. My fuel pump is brand new AC Delco unit. When I scan it with a Tech I, the coolant temperature values were accurate but the fuel integrator and block learn cycle values are between 94-120 (both 128 at key-up). Any thoughts on what might be wrong are appreciated!

[tdaye]

My 3.1l had a miss when the motor was warm/hot too, it was a bad fuel injector. After a few months of this problem the car started shutting down when hot because the injector was shorting.

[Kaline]

Hmm, I can see how the car would run rough with a bad injector. I spoke with a friend who used to be an engine calibrator for GM and he stated a bad injector is probably the reason why my fuel integrator values are so low. How were you able to diagnose which one was bad? I know there is a resistance test, but that only tests the electrical portion of the injector. I'd hate to replace all 6 injectors, since they run $53 each (Murrays Auto Parts). However, that might be the best way to do it so I know they are all new. Did you replace all of them? Any help is appreciated.

[tdaye]

Yes, I took them all out and had them tested and they found the bad one, aparently a bad injector can show itself differently under different temps. It seems to be common according to the garages I deal with. My car ran good when cold, when the injector got hot/warm it would act up.

[Kaline]

FINAL UPDATE: After thinking about the fuel integrator values were reading very low (they should be 120-160), I replaced the fuel injectors this weekend. 3 of the 6 tested low resistance with my VOM meter, so I replaced all 6. The car runs GREAT now! No hesitation, no miss at idle, no hunting at stoplights. I'm very happy with it, even if my wallet is $300 lighter...

 

FYI, watch out for remanufactured fuel injectors from United Auto Remanufacturing. I bought a set part number URF 7996 for the car for$44 each from my local Auto Value parts store, and after running it for 20 minutes, one injectors stuck in the ON position and made the car run terrible! I was able to diagnose it by checking the fuel pressure (pressure bled off after the 2-second key-on prime), then pulling the fuel rail and watching it spray fuel when I keyed-on. I got a replacement injector, pulled the flooded cylinder's spark plug to pump the raw fuel out, put everything back together and it is OK now. However, to do it again, I'd probably spend the $$$ and buy new injectors so I did not have to do the job twice!