Major Malfuntion,, P300 & P1200 - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 03-20-2014, 02:00 AM   #1
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Angry Major Malfuntion,, P300 & P1200

Major problem with the old 96 Bonneville SE, 3.8, 143k miles,, have codes P300 & P1200,,, and the car barely runs,, and I'm about to blow it up for real,,, and I have next to no hair left.

For a couple of weeks(a month ago), daughter complained about the car surging,, and a lose of power. I finally got a chance to look at it,, and it was basically missing. She was also complaining about an oil leak/smell,,, so a couple of weeks ago I changed the lower intake gasket & a new upper plenum(was bad). Fired it up,, ran good, but #3 had a slight miss,, according to the exhaust sound & snap-on scanner P300 code. (only code at that time).

Also, she was still having a problem with low Voltage,, according to the gauge on the dash,,, I replaced alt with a good used alt,,, still had low voltage,, installed a 2nd used alt,, same thing. Tested the 1st used alt at O'Rileys,, showed 14.5 volts & tested good.

So to try and fix the miss, I did the following:

1. Checked compression on #3 cyl,,, 200psi
2. Replaced #3 plug (though only had 2k miles on it, platinum plug)
3. Since #5 wasn't missing, swapped #3/#5 wires (still missing)
4. Replaced #6/#3 coil,,, brand new one & spare used (still missing)
5. Removed ICM, cleaned mounting plate, installed new ICM (still missing)
6. Checked ICM connecting harness connector,, looked great,, no corrosion & all wires looked to be in place.
7. Swapped #3/#5 injectors (still missing)
8. Sent her on her way(ran out of time)

She drove the car for a week,,, she said the car died on her a couple of times, but would start right back up,, until the other day when she started it up and it would barely run. So I hooked up the scanner & got the codes P300(multiple misfires) and P1200 (injector circuit fault),, and now for some reason, #4 was the cyl that was missing the most,,, but all cyl. were missing a bit,, #3 was the 2nd worst.

Talked with fellow mechanics & they thought might be having a computer problem,,, so unhooked the harnesses from the pcm and ohmed the wires from there to the #3 & #4 injector harness connectors,, they measured OK,, and had 11.83v on both of the other injector harness ends(that was measuring from the harness to the battery ground). So with that we ordered a new computer, had it flashed & installed it,,, car still barely will run($110 wasted). If I try to rev up the engine,,, it'll rev up to about 3k & then acts like it'* out of gas.

Had previously hooked up the snap on fuel pressure gauge,, it showed a steady 44 psi(48-55 psi is suppose to be the spec pressure, but the gauge has always seemed to read a bit on the low side on other cars in the past). with the car running(barely), I could push the bleeder button on the gauge, gas would flow out of the tube steadily, gauge would drop to "0", but there was no difference in how it ran,, so I assumed gas wasn't the problem,, and once I released the bleeder valve,, gauge instantly returned to 44 psi.

After installing the new pcm,, scanner said same 2 codes,, but it was back to mainly #3 cyl. missing,,, I could actually remove the #3 wire from the coil with it running & no change,,, not firing at all. Also noticed on the scanner TPS working, Oxys working(lean/rich/lean/etc), map working,,, only thing thing off kilter was the ST & LT fuel trim,, approx, 125/128,,, way off,,, along with Once Again,, the voltage gauge still way low 9-10v,, but when I checked the voltage at the battery,, 12.8-13.5v.

So,,, something is way wrong with the voltage,, somehow, somewhere,,, so I started rechecking the cables,, both battery ends looked good & are tight. By the battery there are 4 small ground wires attached to a small stud on the fender,, took that apart,, they were all clean,, re-tightened them up,, little ICM ground wire is also tight. The Pos cable to the alt is also tight,,, couldn't see the main ground wire on the block by the starter just yet,, kind of figured it had to be good since it turns over so easily. I assume the main Pos cable hooks to the starter,,,again,, couldn't get to that one yet thought to see for sure.

Somehow,,, with the car barely running,, I drove it to the shop tonight,,, 5mph max,, and I was lucky to get it going that fast.

I wrenched for many years,, but suck at electrical stuff,,, I've been told & have read that the P1200 code can be caused by a sucky running engine,, so I need to figure out the #3 miss,, and the low voltage gauge inside the car,,, just don't know how to go about that or what to look for,,, I probably should check out all of the fuses tommorow,, but that would just be too easy of a fix,,, just never seen an engine run this poorly before,,all of a sudden like,,,

I need some serious help or a good TNT supplier!

Thanks, Doug
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Old 03-20-2014, 06:32 AM   #2
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How are you determining that the miss is cylinder 3? Does the scanner your using have a missfire graft? My other question is have you checked for a plugged cat? I simply remove the O2 sensor and see if it runs better.
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Old 03-20-2014, 08:29 AM   #3
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I'm having mixed ideas and cannot understand why you only have 11.8 volts on your injectors. That should be at least battery voltage because your injectors are HOT all the time when key is ON. PCM controls the GND to the injectors.

What is your voltage at the battery when at idle vs 2000RPM?
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Old 03-20-2014, 10:20 AM   #4
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I know #3 is missing, due to Both the scanner "Live" reading,, and also by just pulling that spark plug wire off of it'* coil. Haven't had the car on a hoist yet to check convertor,,, but that still wouldn't affect the no spark on #3 or the voltage difference on the gauge.

Mike,,, 11.8v WAS the battery voltage at the time(not running, pcm unhooked). Haven't checked the voltage difference yet at the battery,, but inside of the car when I gave some gas,, voltage went up to 12v approx. Seemed to have good #3/#4 injector grounds & good voltage.
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Old 03-20-2014, 11:18 AM   #5
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Then its possible your ICM isn't getting enough voltage.


IMO, I would trouble shoot your low voltage condition first. Start with the battery cables at both ends. All of them. Even take the boots apart.
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Old 03-20-2014, 12:48 PM   #6
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Quote:
Originally Posted by dlsimon212 View Post
I know #3 is missing, due to Both the scanner "Live" reading,, and also by just pulling that spark plug wire off of it'* coil. Haven't had the car on a hoist yet to check convertor,,, but that still wouldn't affect the no spark on #3 or the voltage difference on the gauge.

Mike,,, 11.8v WAS the battery voltage at the time(not running, pcm unhooked). Haven't checked the voltage difference yet at the battery,, but inside of the car when I gave some gas,, voltage went up to 12v approx. Seemed to have good #3/#4 injector grounds & good voltage.
You don't need a lift to check the cat you just need to remove the O2 sensor and see if exhaust flows like crazy out of the O2 hole and the thing will run better. As far as you saying the injectors are getting a good ground is a little confusing because the way it is wired is each bank gets a 12v feed through the ignition switch and the PCM grounds the injector when it wants to fire it. So you really want to check for good voltage on the pink wire and if you suspect injector 3 isn't firing you need to check continuity from the injector back to the PCM.
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Old 03-20-2014, 01:59 PM   #7
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Have you checked for spark, on all cylinders with a spark tester?

Have you checked for injector pulse with a noid light, on all injectors?
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Old 03-20-2014, 02:41 PM   #8
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Your saying your battery voltage is 12.5 to 13.5 volts correct? Whats the voltage at your alternator? I like to see a constant 13.5 volts at all times when at idle. How is the connector at the alternator look, terminals loose, wiring insulation bare or cracked? Another thought is check the alternator ground, seen a few brackets lose connection with the engine assembly if a lot of corrosion is there. Try adding another path to ground for the alternator.
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Old 03-20-2014, 10:01 PM   #9
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Worked on it for about 3 1/2 hours today,,, and after checking a ton of wires for the injectors & pcm,,, found a loose connector on inj. #3,,, all of the injectors ohmed approx. 12.2,, so we cleared the codes and after removing the exhaust pipe from the manifold,, I had success at getting rid of the P1200 code,, yippie. Tried to fire it up,, not happening,, smelled like it was flooded,, and it was,, so I threw in another full set of plugs,,, fired right up, on 4 of the 6 cylinders,,, still showing #3 & #4 cylinders,,, misfiring,, especially #4.

I was charging the battery while checking most of the injector wiring,, and noticed that the new 3/6 coil was getting very hot,, while the other 2 coils were normal,,, so I replaced with another one we had lying around,,, it seemed to not warm up nearly as bad. Now that we got rid of the P1200 code,, started to run down Alldata'* list for the P300 code,,, did notice that the voltage reading still sucked on the inside gauge10 maybe,,,just off of the red.

I agree and think I need to check out the ground wire at the block,,, the alt that I last put on actually looked to be in very good condition,, very clean,, no corrosion anywhere. I didn't get a chance to check the voltage today while it was running,, but yesterday when I was checking it,,, it pretty much read the same, when I checked voltage at the battery,, and from the battery to the main wire on the alt. Yesterday I did check all of the wire connections at the battery, alt & grounds on the fender(except the 2-wire connector on the alt),,, but I do need to check out the lower connections.

I stopped looking today at the point where I need a spark tester,,, since I don't have one,, I usually just stick a new plug in the wire and then ground out the plug,,, noticed on alldata,, they want me to ground out the other wire on the coil (ground #6 while checking spark on #3,,, I can do that,, but I need someone to crank it while I watch for the spark.

And yes I did use a noid light before to test the injectors,,, but the noid light fit the connecter snugger than what the injector did,, until I fixed that problem.

Thanks everyone for your suggestions! Keep them coming!

Doug
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Old 03-21-2014, 02:40 PM   #10
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If you need to buy a spark tester, this one works well, I picked it up a while back.
Lisle Inline Spark Tester Lisle Inline Spark Tester
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