1990 Grand Prix loses power then quits

Tiny
MIRADA_75
  • MEMBER
  • 1990 PONTIAC GRAND PRIX
The vehicle is a 1990 Grand Prix, 3.1 L MFI, auto with overdrive, 264,000 km odo.

One day while driving home from work, the car ‘bucked’ really hard and was nearly impossible to keep running. It would idle around 2500 rpm and then want to stall.
I was able to start it by holding the throttle half-way to the floor. The SES revealed two codes, a 23 and a code 53.

After doing a bit of investigation, I replaced the mixture control solenoid (based on the 23 code) and this did not fix the problem. I had the car towed to the shop that I usually go to and they diagnosed the problem as a faulty ECM (bad voltage inside the unit) and they replaced it with a used one.

I drove the car for 3 days and it ran well but idled a bit rough as normal. The 4th day I was driving for about 3 minutes and all of the sudden it started to lose power. I let off the throttle and it stayed running but as soon as I got back on the throttle it quit. From then on it would only start and run for about 1 – 2 seconds.

I had it towed to the shop and once there it would start and idle very poorly but any attempt at pushing on the throttle and it would quit. They immediately did a fuel pressure test on it which revealed proper pressure with no fluctuation. Since the ECM that was put in a week earlier was a used one, they thought that before doing any complex diagnostics they would just swap in another one to rule that in or out. Putting in another ECM made no change to the condition.

The shop did a diagnostic test on it and they said the only fault was a MAP fault which they attributed to bad vacuum because the engine was running so bad. The car had to sit for a day or so and when they got back to it they started it and it ran just fine. They brought the car in while it was running fine and started to wiggle and probe the wires to the ECM. They also tested for voltage decay to the ECM on the wiring harness but that showed everything O.K.

They drove the car around (mostly short trips) and could not get it to duplicate the quitting or rough running condition. They inspected it some more and recommended changing the Crank Shaft Position Sensor which I authorized them to do. They changed it (the sensor was very brittle and it had several cracks around the body of it) and of course when they pulled the car out and drove it, it was O.K. They drove it some more the same day and then it started to run rough (surging and idling poorly) again. They got another used ECM and put that in and then it ran good again (mere coincidence I think now). They drove it several more trips that day and did not have any more problems.

I picked up the car from them after work and I decided to drive it that evening before any important trips (like getting to work). I had been driving it for ½ an hour under various load conditions and then on the highway at 80 kph it lost power and would just idle roughly and then quit, while it was still coasting I put it in neutral and restarted it. I revved it and put it in gear and then it took off. After this brief surge, it quit again and then it would only start and run for 3 seconds maximum. I opened the hood and tried to start it again. While it ran for 3 seconds maximum, I observed lots of blue sparks jumping from the wires to the block and wire to wire. I put a brand new set of wires on it and it made no difference. I let the car sit overnight and when I tried to start it in the morning it was still no difference.

It has since gone back to the shop again and they are investigating the problem again.
My instinct is that despite the good fuel pressure test result in the first place, this is still a fuel problem.

Please help! Myself and the garage that is trying to repair it are struggling to figure this out.

Sorry for the long-winded post, but it is important that you know all that has happened
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Friday, October 26th, 2007 AT 2:00 PM

2 Replies

Tiny
MIRADA_75
  • MEMBER
I went to the shop this evening and the owner and I spent an hour working on it. We put the E-test sniffer in the exhaust and hooked up the scan tool to the ECM.

The E-test sniffer shows that oxygen levels are 4 or 5 times what the the carbon dioxide levels are and the hydrocarbon emissions are very low. The car is running very lean according to this.

The scan tool shows that the ECM is telling the injectors to open for 12.8 ms which is very long considering the car was idling, so the car should be running more rich.

So now the question is, if the emissions are lean and ECM is telling the injectors to put lots of fuel into the cylinders - where's the fuel?

Also during this testing we had a fuel pressure gauge which was steady at about 35 psi and when we opened the tap at the end of the hose there was good flow of fuel.

I also had someone suggest that the ignition module plays a part in controlling the injectors and if the module is faulty it could cause this. Is this true?
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Saturday, October 27th, 2007 AT 12:13 AM
Tiny
MIRADA_75
  • MEMBER
Just to close this post out - the problem was found. The injector on cylinder #6 was found to be contributing little or nothing. When the harness was pulled off this injector the engine didn't run any different and as soon as it was plugged back in, the engine quit which indicated a short to ground.

The injector was replaced and now the miss that it had is gone as well as the problem of the random stalling.
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Sunday, November 4th, 2007 AT 9:21 PM

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