1990 Pontiac GP engine computer listening to wrong advisors?

She sports a 3.1L. multi-port fuel injected engine. She was a strong runner until, suddenly, one day, she stalled. After a lot of troubleshooting, only a modicum of progress. Now, start and it runs very roughly for a couple of seconds; then, stalls. Repeat. Pull the fuse for the EGR valve / purge valve, and she will idle (though it sounds a little rough).Get on the throttle, from idle, she runs very roughly, and dies. /// The laundry list: Changed: TPS, IAC, CTS, MAP, air filter, fuel filter, spark plugs and wires, timing chain (triply checked the indexes), ECM engine computer (re-used PROM). No trouble code in the ECM. Exhaust pipe disconnected to check for plugged catalytic converter. Wasn’t. From the ECM connectors, resistance checks were good on every wire. Back-probe, for voltage, of ECM connectors revealed correct voltage values. The ECM shows no trouble codes. A PRIMARY QUESTION: WHY is the computer energising the EGR valve at idle? What USUALLY (sensor inputs) induces the ECU to energize the EGR valve, besides the ect sensor input? //// From the past shop work orders, this car has a long history of stalling. It’s had several fuel pumps, fuel filters, cps, spark plug and wire sets changed over the years. The factory scan tool would, of course, be NICE to have. Comments?

Got a laptop with a serial port? Go to http://obddiagnostics.com and read about the features. The cost of the equipment, if you already have the laptop, is small if it leads to a solution. Note that one feature is that the free program can read pending codes.

Thanks for the idea, WhaWho. That site is for an OBDII scan tool program. The factory scan tool I’m referring is the one especially designed for the 1990 era Pontiac (and other Chevy’s). It’s, of course, OBDI. One of my main questions is, “Why is the EGR valve being energised at idle?”. Then, what NORMALLY causes the EGR valve to be energised (and at what engine operating conditions)?

In the earlier days, a wiring harness problem was a nightmare because a computer couldn’t diagnose it. Now, a computer still can’t diagnose it. To fix the problem; you might have to go shopping at the junkyard. Since any one of those parts is worth the same as a 1990 car; try replacing the car and keeping all the extra parts for spares. It might be easier than probing the wires to find the one bad one.I know that any problem can be fixed, but by who? And how much? These questions are simple but huge.

You know what, I am just sort of shooting from the hip on this, but here-goes. I had a 1991 Chevy Corsica with the 3.1. It was exhibiting these exact same symptoms and running super, super rich. What ended up failing was the PROM module in the ECM. Sounds crazy, but replacing the entire ECM, PROM and all with another from an identical car solved the whole issue. Worth a shot.

(For what it’s worth, the PROM failed within about 2 weeks of the car’s voltage regulator failing and letting the voltage spike up to 16+ volts.)

The idea of the PROM being the problem occured to me. I don’t understand why a new (or, tested-good) PROM doesn’t come with the replacement computer. Used computers containing the PROM are available for as little as $25 from the junk yard— condition unknown and unwarranted. Where else could one get a PROM… hopefully, a GOOD one?

Why are you assuming the EGR is the problem? I did not tear the diagram apart, but a quick look shows that dependent upon which EGR valve your car uses you are also killing power to the fuel injectors when you pull that fuse.

Have you verified that you have fuel pressure, and maintain it, when cranking the engine?
What about a fuel pump relay or faulty pump wire connector?
Run a hot jumper wire to the red pump prime lead and see what happens. This will bypass the relay and serve to eliminate (possibly) that item.

The EGR valve, itself, is NOT the problem (it’s the DIGITAL EGR valve). The problem is the ORDERING (energising) of the EGR valve to open at IDLE. This is NOT supposed to happen. /// At first, I was, also, thrown off by the labeling of the fuses on the diagrams of the fuse box and the wiring diagrams. The fuse which is labeled IGN FUSE comes from the IGNition switch; but, it goes to the EGR valve and the purge control valve. I followed the wires, from here to there, with a multimeter. The engine RUNS with that fuse pulled. It doesn’t run with that fuse set./// The fuel pressure, with a fp gage sitting on it, is fine … static and running.

The Haynes TECHBOOK 2108 states, “The computer uses information from the coolant temperature sensor (cts), throttle position sensor (tps), and the manifold pressure sensor (map) to regulate the EGR valve solenoid. During cold operation and at idle, the solenoid circuit is grounded by the computer to block vacuum to the EGR valve [stopping EGR flow].” /// If those signals are of correct values, then, wouldn’t it be the PROM which is set wrong?

If I were you, I’d go pick up a junkyard ECM and see what changes. The PROM calibrations are car-specific, so do try to match up model and year with your car.