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Old 06-26-2006, 10:02 AM   #1
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Default 3100 No-Start

<I know this isn't 3800 related, but I could use some fresh brains on this>

Well, this one threw me for a loop... I had a friend call up and have me head over to his house - his sister'* 95 Skylark won't start, and he wanted me to give it a go and see what I could figure out. (3100 60deg v6)

Just to throw it out there, it sounded from the get-go like there was a weak spark issue.

First step, tried to turn it over. Cranks slowly (had to jump it later) and doens't really try to turn over on it'* own. After about 15 seconds of cranking (maybe a bit less), it backfires through the intake.

Second step, remove plugs. They're NASTY. Someone put some Bosch platinums in there. They're so black, if you didn't know spark plugs, you wouldn't have even known there was a conductor in there... they were that bad. Throw new plugs in (the front, didn't have time to tear into the rear bank).

Still does the same routine - doesn't try to turn itself over, backfires through the intake if you keep trying for more than about 5-10 seconds.

Third step, threw my spark checker into the system between plug wires and coils. Nice and bright... stronger spark than my perfectly running 3800... but it'* not consistent, and it doesn't seem to be sparking as fast as it should for the RPM'* it'* cranking at.

Fourth step, head online. Found a TSB which addressed micro-arcing between the coil and the spark plug wires - the boot didn't fit snug, which caused arcing between the terminal on the coil and the inside of the plug boot. It could cause pitting in the terminals on the coil, and eventual failure.

Fifth step, head back out to the car. Take a look @ the terminals. Sure enough, you can see small pitting on the end of the terminals, and there'* black marks on the inside of the plug wires.

Now, before I spend money just throwing parts at this thing (or rather, having my friend throw money at it for his sister), I decided to compare it to my 3800'* coils... so,

Sixth step, compare to 3800 coils. They look identical. I've got the small pitting and the same marks on the inside of my plug boots (this, BTW, is why you should use dielectric grease, which I had in mine originally, but didn't replace the last few times I've pulled the wires off to check things).

So, I'm not convinced that I need to replace the coils... so step 7...

Seventh step, check resistance across the terminals of the coils... they are all fairly uniform, falling around 584k ohms each. My 3800 is at about 612 k ohms.

Eigth step, running out of time, need to get home to the family. Friend mentions that master mechanic friend of his sister mentioned it might be a messed EGR valve. So I take off the EGR valve, clean it up, throw it back on. One of the solenoids had been sticking a bit, and the passages were so dirty it'* amazing that anything passed through there at all.

Ninth step, try to run it again. No go. Same as before. Doesn't really try that hard (although every now and again, it sounds like it might want to go), and if you keep going for more than 5-10 seconds, it backfires through the intake.

Tenth step, head home, have a cold can of coke. I was out of time for the day (it was a short day), and decided to do some more reading around before poking around any other systems.

Based on the weak spark, here'* what I'm thinking could possibly be wrong:

1. bad coils - I don't think they're bad though, they provide a good strong spark when they actually spark.

2. bad ICM - it could be that the ICM isn't providing the coils with the proper electrical current as often as required to fire the plugs

3. bad PCM - again, the PCM may not be providing the ICM with the proper information for firing.

4. bad wires - didn't take the time to check resistance across the wires, but I don't think they're bad... which reminds me, I forgot a step in there - I sprayed a mist of water onto the wires whilst cranking, and didn't get any arcing between the wires, so they *should* be alright.

and while it'* unlikely...

5. bad starter. I doubt this one, because he said that he could get it to start when it was dropped off at his house (flat towed), but it wouldn't keep running...

So to me, it seems like the issue is firmly rooted in the ignition. I pulled one of the fresh plugs at the end of our fun, and you could smell the fuel and the exhaust gas on it, so we're getting fuel in the cylinder, and the EGR is working.

I do have a check engine light, but I don't have a scan tool, and I can't get it somewhere for a free code-read. Do these have the screwed up PCM'* like Bonneville'* do in 94/95, or is it actually a strait OBD1 PCM? If it is, can it be flashed with the paper clip method?

Any thoughts on the subject would be very much appreciated...
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Old 06-26-2006, 10:57 AM   #2
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Well, with me owning 2 3100'* in the past, this hasn't happened to me. It does sound like a crank sensor to me. Sometimes a broken crank sensor will let the car start, sometimes it won't. i would run a code reader on it before replacing parts. May be anything.
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Old 06-26-2006, 11:32 AM   #3
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can these PCM'* be done up with the paper clip, or do I need to get an actual code reader on location...
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Old 06-26-2006, 01:27 PM   #4
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Quote:
Originally Posted by mkaake
can these PCM'* be done up with the paper clip, or do I need to get an actual code reader on location...
Mkaake buddy you have your hands full....

The 3100 Vin M is an SFI V6... It should have both the crank sensor and cam sensor simmilar in a way to the 3800... The coils are actually the same( with diffrent numbers )...

I agree that you could have a bad Crank sensor, bad ICM or possibly a bad PCM... Any of these are in my book at the moment... As the 3100'* get older its possible for the crank sensor to go bad and sometimes crack... Can be a PITA....

The 95 3100 should be a full on OBD1 PCM, but it does not have the Diagnostic flash feature as GM no longer had that pin in the ALDL after 1993...

Let us know what else we can tell you...

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Old 06-26-2006, 02:18 PM   #5
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Quote:
Originally Posted by jr's3800
Quote:
Originally Posted by mkaake
can these PCM'* be done up with the paper clip, or do I need to get an actual code reader on location...
Mkaake buddy you have your hands full....

The 3100 Vin M is an SFI V6... It should have both the crank sensor and cam sensor simmilar in a way to the 3800... The coils are actually the same( with diffrent numbers )...

I agree that you could have a bad Crank sensor, bad ICM or possibly a bad PCM... Any of these are in my book at the moment... As the 3100'* get older its possible for the crank sensor to go bad and sometimes crack... Can be a PITA....

The 95 3100 should be a full on OBD1 PCM, but it does not have the Diagnostic flash feature as GM no longer had that pin in the ALDL after 1993...

Let us know what else we can tell you...
Tell me you'll buy it for 1200 bucks?
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Old 06-26-2006, 02:20 PM   #6
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Um no thats ok...lol

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Old 06-26-2006, 11:42 PM   #7
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Just had a similar problem on my daughter'* 91 GP with the 3.1 (240k miles) and it was the crank sensor. It was pretty well cracked. Replacement was only $20, and I had her boyfriend do most of the work. He has zero car experience, and still only took him about 30 minutes to do it. It sat on the lower part of the engine back by the firewall. I don't know if it'* in the same position in '95 or not. Hardest part was that you have to work by feel, as you can't really see it well.
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Old 06-27-2006, 08:23 AM   #8
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very good. thanks for the heads up... I'll go ahead and give that a go.
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Old 06-27-2006, 12:48 PM   #9
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Quote:
Originally Posted by Archon
Just had a similar problem on my daughter'* 91 GP with the 3.1 (240k miles) and it was the crank sensor. It was pretty well cracked. Replacement was only $20, and I had her boyfriend do most of the work. He has zero car experience, and still only took him about 30 minutes to do it. It sat on the lower part of the engine back by the firewall. I don't know if it'* in the same position in '95 or not. Hardest part was that you have to work by feel, as you can't really see it well.
Yup thats where the crank sensor should be located... the 3100 was merely a slightly improved 3.1.... Better breathing ability, instead of MPFI it was now SFI, And the engine got the roller cam and roller lifters...

Mkaake, let us know how it goes

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Old 07-05-2006, 02:33 PM   #10
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Find the problem???
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