1994 Grand Prix
 

O.K. guys I need some help. My 94 GP 3.1 liter with 128,000 miles is acting up. At idle the engine is smooth and everything is fine. While driving along on flat ground without a big engine load, everything is fine. When I go up a hill the engine hesitates or feels like it is cutting out. The same thing happens when I am doing 60 mph and change lanes to pass a car on the expressway. As soon as I induce a load it seems to cut out. It has never died on me. I replaced my fuel filter. I replaced my spark plug wires. My plugs have 35,000 miles on them. At night in the dark with the engine running, I misted water on the 3 coils and 6 spark plug wires. I accelerated the engine and saw no arcing. I also used a Matco diagnostic tool and found no engine codes but I did see this.
Quad driver 1 - OK
Quad driver 2 - OK
Quad driver 3 - OK
Quad driver 4 - Bad

What is a quad driver? I had one guy say not to worry about that. Its not my problem.
Any ideas would be appreciated. Thank you.
Paul

Could the mass airflow sensor. I had a similar problem with my 1990 GP, which had the 3.8L.

I guess anything is possible with these cars. I am going to hook it up to a new OTC reader and see if anything is different. Driving me nut but I won't give up. Thanks
Paul

Quad drivers are four switching transistors in one package inside the computer. They're used to control things like the fan relay and converter clutch solenoid. I agree -- ignore that indication. If a transistor isn't used (as in connected to an output) it will display as bad.

It would help to know if it's a misfire or not. A misfire feels like a repetitive pulsation, almost a vibration. You can tell that one cyinder isn't firing. Fuel-related problems feel more random, or the engine totally loses power.

Carbon tracking on the plugs is common. You'll see black lines all the way down the insulator from the tip to the shell. This causes a misfire under load. It also ruins plug wires almost instantly -- the carbon track transfers to the plug boot and causes a conductive path down the boot. If this is the case, it's critical to replace plugs and wires together.

The MAF sensor is a possibility, although it's more common for them to cause idle problems. Edit: Not exactly true. They tend to skew lean at idle/light load and rich at WOT.

I'd be curious about the fuel pressure. With a gauge hooked up, drive it the way that makes it act up. Also, watch the O2 sensor reading under heavy load. The O2 sensor reading should stay rich at wide-open-throttle. If it doesn't, there's a good chance the fuel pump isn't keeping up with demand. It's important to test this stuff while the condition is occurring. Testing at idle doesn't tell you anything.

Thanks LPete. I printed out your response and will check accordingly. FYI the car feels like a miss as you described. Good info on the quad drivers. Thanks and I'll let you know when I know. If you can think of anything else that might help, I would appriciate it. Thanks again.
Paul

Ok I got it figured out. It had to get worse for me to figure it out though. I did an ohm test on all three coils. The posts that the plug wires snap onto are the secondary coils. You have to remove them individually and flip them over to check the resistance on the primary coil of each one. Anyway on the coil that fires cylinder 2 and 5 there was no resistance at all. Just OL. Open loop. Sometimes it worked and sometimes it did not. The other two coils showed 5.54 and 5.74 ohms.
I installed another coil and everything is smooth now.
Also a little more info in case someone in the future is looking for an answer.
I installed a fuel gauge to the fuel rail before I found the coil problem. I hooked up the gauge to my 1994 Grand Prix 3100 SFI and of coarse the fuel pressure was zero. I turned on the key and the pressure jumped to 44 psi. As soon as I started the car the fuel pressure dropped to 38 psi at idle. Rev it up and it goes back to 44 psi.
The drop is correct from key on to start. With the key on only, the pressure is high because there is no vacuum to operate the fuel pressure regulator. As soon as the engine starts the regulator receives a vacuum signal and opens and psi drops. My Haynes manual says a drop of up to 10 psi is normal.
I installed the gauge in a manner that I was able to close the hood and watch it. I had it wire tied to the windshield wipers. The pressure was very steady through the RPM range. It spent most of its time at 44 psi as I drove home today on the interstate at 65 MPH. Coming to a stop it would smoothly dip to 38. Also I left the gauge on for about 2 hours at work while the engine and key was off and the pressure stayed up.
Well I learned a little today and hope my experience will help someone in the future.
Paul