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Bad PCM?


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#1 rich_e777

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Posted 03 August 2017 - 05:41 PM

So I`m stumped on this one, 1994 3100 CS coupe. Cranks, but wont run unless the gas pedal is depressed but its rough then dies when let off. Was driving around fine one day and stopped at a store then it all started. Replaced 3x sensor harness and confirmed spark at 1-4 coil and fuel pressure at shrader valve. The only thing I can think of is the fuel injectors not spitting out gas or getting the command. Any ideas?



#2 Imp558

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Posted 03 August 2017 - 06:13 PM

Vacuum leak or a ground. Bad PCMs happen but not frequently. I just replaced the BCM in her MC only to find one of the ground bolts under the hood was stripped and making a poor ground from body to battery.
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#3 Schurkey

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Posted 03 August 2017 - 06:52 PM

So I`m stumped on this one, 1994 3100 CS coupe. Cranks, but wont run unless the gas pedal is depressed but its rough then dies when let off. Was driving around fine one day and stopped at a store then it all started. Replaced 3x sensor harness and confirmed spark at 1-4 coil and fuel pressure at shrader valve. The only thing I can think of is the fuel injectors not spitting out gas or getting the command. Any ideas?

Schurkey's Perpetual Advice:  The answer is as close as the nearest scan tool.  The data stream rarely lies.

 

You can make guesses, replace parts, visually inspect all sorts of items including the wire harness.  Sometimes guys get lucky.

 

Might be stuck EGR.  Might be faulty coolant sensor.  Could be defective TPS.  Could be a vacuum leak.  Could be this, could be that.  It does run when the gas pedal is depressed.  Therefore it probably has fuel, spark, compression, and adequate exhaust.  Doesn't mean that those items are blameless, just that it's somewhat unlikely that they're at fault.  If you were to verify compression pressure in ALL SIX, and it's good, that also verifies cam timing.  A running compression test could verify cam lobes, lifters, pushrods, rocker arms.  You did verify fuel pressure, you've gone a long way to assuring that the fuel pump and fuel filter are OK, and that the injectors are receiving adequate input pressure.  You crammed a spark-tester on each of the plug wires , you've verified the coils, plug wires, primary wiring, some sensors, etc.

 

Put a scan tool on it, and in fifteen minutes you'll learn more than a day's worth of dicking around.  Even if the engine doesn't run, you can test the intake air temp sensor, coolant sensor, MAP sensor, TPS, knock sensor (have a helper rap on the engine with a brass hammer), crank sensor (crank engine, look for RPM on the scan tool) and others in minutes not hours.


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#4 rich_e777

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Posted 03 August 2017 - 09:45 PM

Really cannot argue that logic, I`ll bite the bullet and get a real scan tool tomorrow. I was trying to solve it on my own but someone called codes on me for having it and other cars being worked on in my driveway so I need to get it started and at least turned around so it appears to be functional.



#5 Galaxie500XL

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Posted 03 August 2017 - 09:52 PM

Going to be hard to find an OBD1 scantool these days new...


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#6 rich_e777

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Posted 03 August 2017 - 10:32 PM

I ordered one that does OBD 1 and 2 for about $150 off ebay. Equuis brand IIRC. If it doesnt work then I`ll track down an actual Tech1 or MT2500 several have mentioned using before with good results. I want one I can use with the FSMs.



#7 Galaxie500XL

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Posted 03 August 2017 - 11:56 PM

If you lived closer, I'd have let you borrow my OTC 4000E, or my Actron CP-9110, or my laptop with TunerPro, and a Saar built ALDL cable.  They work great on our cars...I even have the cable and cartridge for the Actron for the LeBaron.

 

I seem to be collecting scantools.... :think:


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galaxie500xl

1988 Camaro IROC-Z 5.7 TPI 5-speed

2011 Chevrolet Equinox LT

1994 LeBaron GTC Convertible

GONE to a new home... 1995 Cutlass DOHC Convertible, 3rd Quarter 2012 W-Body.com COTQ

1993 Buick Century station wagon...sold to our son, kept in the family since 2003


#8 Grandprix1

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Posted 04 August 2017 - 12:45 AM

X2 on the vacuum leak. My GP did the same when I forgot to put the vacuum line back on the TB. no fuel pressure regulation. Wouldn't start unless pedal was pushed and would rev to 3k then stall out Sent from my iPhone using Tapatalk

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