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Car runs perfect but will randomly stall and only idle

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Old 08-01-2017, 11:15 AM   #1
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Car runs perfect but will randomly stall and only idle

This one is stumping me. Car runs like a champ, but randomly will stall when depressing the gas pedal. If I shut it off and restart the car, it will fire right back up and run fine until it happens again. I can also let the car idle for about 10-15 mins and when I press on the pedal or operate the throttle body linkage can sometimes reproduce the condition...but then at other times I can stab the throttle WOT and the car doesn't miss a beat. The car was well taken care of but this is really a confusing issue.

I have a Chilton and have done some troubleshooting but also have thrown some sensors at the car with hopes to fix it. Obviously it has not.

No codes registered...used a code reader all flashes on dash are 12, 12, 12,

Here is the list of parts I have replaced and not replaced...yet.

Replaced
Fuel Injectors
Fuel Pump/Filter (dropped the tank/axle and replaced correctly)
Fuel Pump Relay (was good but replaced anyway...also checked fuse)
Checked vacuum lines (they look good from what I can see)
Throttle Position Sensor (damn stupid aluminum bolts GM used)
Oil Pressure Sender (oil pressure was pegged).
Distributor Cap/Rotor (very corroded)
Ignition Control Module (tested good but replaced anyway)
IGR Valve (new)
IAC Valve
Coolant Temp Senosr
Fuel Pressure Regulator
ECM Computer


Also can VATS kick in after the car has been running and disable the car or is it only operational on startup?

Last edited by Shinobi'sZ; 08-18-2017 at 09:07 AM.
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Old 08-01-2017, 06:54 PM   #2
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Re: Car runs perfect but will randomly stall and only idle

Sorry I made an error. I replaced the fuel injectors in my 91' Firebird Formula 350 not this car as I thought. Years ago I replaced one of the Multecs that went bad with a spare that tested good but not a complete aftermarket set. I got home today and noticed this car has all OEM Multec injectors which explains everything now . Too many cars, kids, and getting old I guess. Time to order some fuel injectors from Southbay Fuel Injector. I guess I won't have to worry about all the other parts I replaced going out for awhile.
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Old 08-01-2017, 07:09 PM   #3
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Re: Car runs perfect but will randomly stall and only idle

Take a real hard look at the distributor and any connected components or wiring. Seems like distributor issues are common in these old V6 cars.

While you are in there, consider replacing the O-ring oil seal. Notorious source of oil leak.
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Old 08-01-2017, 10:10 PM   #4
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Re: Car runs perfect but will randomly stall and only idle

Quote:
Originally Posted by Shinobi'sZ View Post
Also can VATS kick in after the car has been running and disable the car or is it only operational on startup?
Vats will not kill a running engine , and is only operational at startup .
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Old 08-06-2017, 06:01 PM   #5
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Re: Car runs perfect but will randomly stall and only idle

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Originally Posted by W.E.G. View Post
Take a real hard look at the distributor and any connected components or wiring. Seems like distributor issues are common in these old V6 cars.

While you are in there, consider replacing the O-ring oil seal. Notorious source of oil leak.

Well It ends up that I had replaced the fuel injectors I looked to quickly and the stock replacements from SouthBay Fuel injectors are metal barrel looking so I thiught I still had the Multechs in there. Anyway you clued me in and I pulled the Dizzy out and the Ignition Pickup Coil was bad. Put the new one in and it fired right up and didn't quit.

However time for the bad news. When I was pulling it out some of the gasket around the back of the intake in front of the distributor base plate came off as I pulled the Dizzy out of the motor. Now I have oil streaming out of this location and pouring onto the ground. I didn't replace the O ring on the distributor as it looked good and was in tact. What is the gasket that tore off....I'm assuming it is the intake manifold gasket but there must be an oil passage near the rear as I did t think it's coming up from the distributor unless the O ring got torn or pushed out of place when reinstalling. But the Dizzy dropped in easy when re-installing so I don't think they is it but maybe.

Question is whether or not the manifold intake gasket has an oil passage in the intake or is it getting past the O ring on the distributor?
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Old 08-18-2017, 09:19 AM   #6
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Re: Car runs perfect but will randomly stall and only idle

I updated the list of parts that I replaced and wanted to report back that although the parts in the distributor and plugs looked bad and needed replacing none of it fixed the issue.

What fixed the issue is the last part that I replaced which was the ECM.

I noticed that when the car would start that within a minute or two the fan would kick on and not cycle off. I was performing the IAC relearn process when I noticed this. I disconnected the Engine Coolant Temp Sensor and the fan would kick off, but I knew my CTS was new.

I pulled the lower panel on the passenger side to access the ECM and when I pulled the panel off the ECM fell and was hanging unsecured. I have know idea when this occurred as the car only as 39K miles on it and was my grandmothers who drove it to for 20 years 1/2 mile round trip. When I reset the connectors on the ECM the car started and I drove it around with no issue. So i secured the ECM back into it's plastic mount up in the dash so that it was not hanging and then the car wouldn't start again.

I determined that some damage must have occurred to the ECM as it was probably hanging and caused some separation of a solder joint due to bouncing or vibration by not being properly secured.

I purchased a new ECM from Autozone for about $100. Swapped in the PROM/MEMCAL from the old ECM. I have now been driving the car for days with no issue and the car runs like new.

If you experience the problem that I had. If it is not the Fuel Injectors/Fuel Pump/Ignition Control Module or Pickup Coil. I suggest you try your ECM and save yourself money.

Note: I had no trouble codes other than 12, 12, 12. Which I believe does mean no communication with ECM. But everybody always says that is normal.

Just wanted to report back what FIXED my car because so many of these threads don't have a report back on what fixed the issue.

The oil leak that I reported above after replacing the Ignition Pickup Coil was in fact due to the flattened hard O ring. I replaced it and the oil leak is gone. I regret that I forgot that it was mentioned in the post above to make sure I checked that why I was replacing the ignition coil.....cost me an hour of my time (rework) for a $1.00 O ring.

Last edited by Shinobi'sZ; 08-18-2017 at 09:26 AM.
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Old 08-18-2017, 10:20 AM   #7
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Re: Car runs perfect but will randomly stall and only idle

Thanks for the follow up report!

Drives me nuts too when people don't post the post-mortem to the forum.

Mine is running ok since replacing the distributor. But can't get the timing mark to be still. Timing chain maybe. Plan on looking at that when the heat index comes down.

Will be having a look at that ECM too.
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