My computer-controlled engine keeps stalling

Question:

I own a 1993 Pontiac Grand Prix with the 3.1 liter engine. My problem is the engine will fail sometimes, usually when cruising down the freeway and after having been driven 15 or 20 miles. I’ve noticed that it will happen when I’ve taken my foot off the gas pedal but not necessarily braking. The last time it happened I was able to restart after coming to a halt on the side of the freeway by giving the vehicle a lot of gas, getting it to start momentarily, and jamming it into gear. It would lurch a few hundred feet then stall again. I repeated this twice more and coasted downhill into a gas station right off the freeway. I waited 15 minutes, restarted the vehicle and drove 10 miles home without incident as if nothing had happened.

Researching this I have found the Idle Control Valve (ICV), the Manifold Absolute Pressure Sensor (MAP), or the Torque Control Clutch (TCC) are my biggest suspects but my symptoms don’t seem to quite fit any of these items. This problem has never happened on a short journey of 5 miles or less and never in cool weather. The restart after 15 minutes or so works every time.

Finally I decided to check if the stalling caused a Diagnostic Trouble Code (DTC) to be set and stored on the computer. The vehicle has an OBD1 adaptor and I tried the welladvertised test of connecting the A and B connectors and watching for a flashing code on the dashboard. I connected the sockets and turned the ignition key to “on”. I got no flashing lights but the engine cooling fans turned on!? So this is another head scratching mystery and I only mention it here because I wonder if it indicates that I have a more serious Engine Control Module (ECM) problem.

This low-mileage car (65,000) has been cared for over the years. I’m the third owner and I knew the previous two owners. I have not replaced any parts yet and would like to get some more evidence of the likely culprit before replacing parts unnecessarily.

Answer:

I must admit that it is beyond my expertise to unsnarl computer problems by email. Perhaps a fellow car buff can help with this. How about it readers? I’m sure John would be most grateful if you could determine the probable cause of his problem from the information he has provided.

But before getting too deep into the computer sensors and software, I can only suggest you start with the basics if you have not done so already. What you seem to be experiencing is vapor lock, or a fuel problem. Vapor lock is when the fuel in the fuel delivery system gets hot and vaporizes, causing the system to suck in air. I would check the entire fuel delivery system to make sure it is nowhere near the exhaust system or any source of heat.

I would also check for leaks and even a pinhole leak in the fuel line or flex hose. I once owned an MGB that sprung a tiny leak where its steel fuel line rubbed against a shock absorber. It drove me crazy until I figured it out. Another rather obvious thing to check is the fuel filter. It could be clogged. And I would blow out the fuel line from one end to the other, just to make sure there was no debris in it. And, finally, I would try a can of injector cleaner available from auto parts stores.