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Old 11-15-2009, 10:22 AM   #1
jarl
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'94 3800: dies while driving...

Ok... reporting back
I followed some good advice, and decided to check what I could before posting again.

The problem: '94 TS 3800 starts, runs for several minutes, then the RPMs start going down and back to the original value. It does this either at idle or while running. Since I bought the car you could feel the car "missed" sometimes at highway speeds (never long enough to notice anything more than a slight pull), and died from time to time when starting uphill. Now -after spending hours updating the interior, replacing the intake gaskets and buying a new battery- the engine not only misses but actually dies and it's becoming harder and harder to start after it does. Fuel pump sounds like it's whining more than normal, but I'm not sure.

Already done: changed the fuel filter, built an OBD cable, pulled the codes (QDM1 and QDM4 errors), checked G200 and a couple others. The cable shows some weird values sometimes, but it may be due to it being a makeshift version before I build "the real thing". The problem seemed to start when the car warmed to the "closed loop mode", but can't be sure.

My guesses at this point are a bad fuel pump, a bad ignition module, a bad ground I have not checked yet, or something like a small hole somewhere in the fuel pump intake making it lose suction or something weird like that.

My question: is there something obvious I may be missing? Is there any test I could do to verify the ignition module is not working ok?

My resources are limited so I would really try to avoid the "change and then test again" approach. If I were to choose between buying the fuel pump or the ignition module first, which one should it be?

Thanks!
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Last edited by jarl; 11-24-2009 at 01:07 PM. Reason: Performed new tests
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Old 11-24-2009, 01:09 PM   #2
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Re: '94 3800: dies while driving...

(bumping my post after modifying the original message... I didn't think it would not do it automatically)
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Old 11-25-2009, 06:39 AM   #3
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Re: '94 3800: dies while driving...

Our 95 Transport 3.8 had a slightly rough idle for several years, and I
finally took LMP's advice and changed out the ignition module because of the age of the car. He STRONGLY believes it is important to change out the ignition module AND the crank sensor after 10 years on a vehicle.

I bought an ignition module that had new coils attached on ebay for cheap. The odd thing is, the slight miss at idle was gone. Period.

My 3800 GTX has alot of mods, including a 150 hp nitrous, and I had some bogging on right turns, which finally got worse, and I replaced the dual platinum plugs with another set of Autolite 103AP.(4 heat range colder)
To rule out the fuel pump they say to tape a fuel gauge to your windshield, and drive the car.
Upon reading the first few lines of your problem, I was strongly thinking the ignition module. (obviously you already checked your plugs and wires first?) KEV-just something to think about
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Old 11-26-2009, 06:34 AM   #4
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Re: '94 3800: dies while driving...

I have tried almost everything to cure our TS rough idle/misfires:
- full tune-up (new spark plugs + wires)
- new rotor + distributor
- new fuel injectors
- new filters all around (air & fuel)
- new PCV valve
- cleaned EGR
- checked vacuum/replaced filter
- checked IAC
- recently changed the Ignition Module due to complete failure

None of the above did anything about my rough idle condition.

Last edited by Meridivs; 11-26-2009 at 09:54 AM.
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Old 11-26-2009, 09:07 AM   #5
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Re: '94 3800: dies while driving...

When the van does die, Does the speedo still report speed? Or does it drop dead?

You have replaced quite a bit... My next guess would be the PCM... The computer that was used in the 94-95 3800 Powered GM cars and vans was not really known for longevity.. They have a pretty good failure rate.. But they almost never fail completely.. They fail enough to have you chasing your own tail trying to correct for different issues.. I have 3 of those PCM's sitting on the floor right now and all 3 of them have failed... I am on my last and Final 94-95 PCM as we are getting ready to rewire my 95 Bonneville with the 97 OBD2 PCM to avoid the failures..

I can't say this is your problem for sure, but its sounding a lot like it...
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Old 11-29-2009, 10:50 PM   #6
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Re: '94 3800: dies while driving...

Please note almost nothing -if anything at all- from Meridiv's post apply to my case.
I'm still trying to debug this problem, but there's something baffling me:
I built a proper ALDL cable and ran the EfiLive evaluation again, and I confirmed the weird readings I was getting before: The RPMs as read by the PC are very low (~180 or so), despite the tach on the IP displaying a normal idle speed (~800). Also, the spark advance (SAP) is reading ~25 deg BTDC. The weird thing is that if this was the result of bad communication through the cable the program should complaint about checksums and whatnot... right???
Regardless of the interface, it looks like the fuel pump is running for too long after turning the key to "run". How long should it operate before starting the engine? If I want to replace it, are there good and bad brands to look for/stay away from? Any store I should consider? Ebay? Rockauto? Junkyard?
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