'92 Lesabre starting and PCM issues

I've been having a starting problem with my '92 Buick Lesabre recently... The car has 99,200 miles on it. It will start but then hesitates and dies after a few seconds. I'm able to keep it running if I keep the throttle pushed in a little, but if I let off for more than a few seconds, the engine just dies. When I keep the throttle in I can hold it at 2,000 RPM but the engine occasionally hiccups, drops a few hundred RPM real quick, then goes back up to 2,000 RPM. Sometimes it stays at 2,000 RPM even when I let off the throttle. The car ran completely fine up until this problem, and this is the first time I've had any trouble at all in four years. I've checked the fuel pressure and air intake and both seem to be fine. Battery seems fine, so does the starter. I also changed the fuel filter and spark plugs with no change in behavior. Spark plug wire resistances are well within spec. This problem has not gone away since it started about three weeks ago.

The check engine light is on (for the first time ever) and blinking very erratically... I purchased an AutoXray 5000 scan tool and hooked it up recently. It said that the PCM was reporting the Throttle Position Sensor was reading high. After getting that diagnostic code, I used the AutoXray's monitor function that reports all of the sensor values over time. It said that the coolant temperature sensor and manifold temperature sensor were anywhere from -80 degrees F to 270 degrees F (they never matched either), were random with no pattern, and that the throttle position sensor was going from 0 to 5 volts, again random with no pattern. It also said my battery voltage was anywhere between 0 and 25 volts, and that my brake was depressed sometimes, or not depressed other times, and that the AC was alternating between on and off. It also said both of my Quad Driver units were on "high", but they should have been reading "low" according to the tech manual. Most of the other sensor readings were OK.

None of this makes a whole lot of sense, as I checked the coolant temperature sensor itself on the engine with a multimeter and it was accurate to within 2 degrees... I checked the battery voltage at the terminals and it is fine as well. I've disconnected and reconnected the battery wires, but didn't see a change in behavior. I never had the AC on or the brake depressed either.

I can hear relays clicking under the hood, sometimes very rapidly (much more clicking than normal). The radiator fans now come on when the key is turned on with the engine off, which never used to happen. I've seen other posts which have indicated that this is a sign of a bad PCM (aka ECM, ECC). The faulty sensor readings also make me wonder if this is the case, or maybe if there is a bad ground (or both).

The other issue I noticed when I got it to run for a few minutes was that one of the refrigerant lines became white and frozen over, even though I wasn't running the AC. There was a cylindrical tank with two lines running into/out of it (close to the windshield on the passenger side) that was also frozen over. The top line that ran into the tank was frozen. I'm not an AC expert so I don't know if this is normal or not. It's the first time I've seen either of those freeze over like that.

I've read another post which said that the blower motor can often draw too much current and kill the PCM. Do you believe that could be what has happened to my vehicle? What other things do you think might be causing the problem? I know that I need to check the PCM and other ground connections, but I'm not sure where they all are (I see the one on the engine but that's the only one I see). Any other ideas or suggestions? Should I get a new PCM? If so, does the '92 PCM need to have the EPROM transferred/flashed or can I just install any aftermarket PCM (Cardone, Borg Warner, Standard Motor, Superchips) out of the box? Or is the OEM PCM the only way to go?

Thanks a bunch for any help you can provide. I want to get this problem fixed! :)

Jon Abbott

This does sound PCM related however, I would continue to check the simple things before replacing the PCM. The PCM can react in strange ways to erroneous feedback from sensors. First, check for vacuum leaks, then be sure the spring in the TPS is not broken and the lever returns all the way home with the throttle closed position. Also check the TPS voltage through the range from closed throttle (.5 V) to WOT (4.0V+). The white wire is the 5.0 volt reference signal, the blue wire is variable voltage and the black is ground.

Continue to check for loose grounds or erratic voltage (high or low). If all sensors and voltage and grounds check out within specification, try a new PCM. The prom from your original unit will need to be transferred to the new or rebuilt unit, unless you purchase the PCM with prom installed from Super Chips. Prom swap is simple. Remove the PCM and you will notice a door on the back side with one screw holding it in place. Remove the door and the prom is in front of you. Press the clips back on each side and pull up on the prom to remove. Install by simply lining up pins and push down until clips grab the prom.

Good luck, and welcome to AF!

I've been having a starting problem with my '92 Buick Lesabre recently... The car has 99,200 miles on it. It will start but then hesitates and dies after a few seconds. I'm able to keep it running if I keep the throttle pushed in a little, but if I let off for more than a few seconds, the engine just dies. When I keep the throttle in I can hold it at 2,000 RPM but the engine occasionally hiccups, drops a few hundred RPM real quick, then goes back up to 2,000 RPM. Sometimes it stays at 2,000 RPM even when I let off the throttle. The car ran completely fine up until this problem, and this is the first time I've had any trouble at all in four years. I've checked the fuel pressure and air intake and both seem to be fine. Battery seems fine, so does the starter. I also changed the fuel filter and spark plugs with no change in behavior. Spark plug wire resistances are well within spec. This problem has not gone away since it started about three weeks ago.

The check engine light is on (for the first time ever) and blinking very erratically... I purchased an AutoXray 5000 scan tool and hooked it up recently. It said that the PCM was reporting the Throttle Position Sensor was reading high. After getting that diagnostic code, I used the AutoXray's monitor function that reports all of the sensor values over time. It said that the coolant temperature sensor and manifold temperature sensor were anywhere from -80 degrees F to 270 degrees F (they never matched either), were random with no pattern, and that the throttle position sensor was going from 0 to 5 volts, again random with no pattern. It also said my battery voltage was anywhere between 0 and 25 volts, and that my brake was depressed sometimes, or not depressed other times, and that the AC was alternating between on and off. It also said both of my Quad Driver units were on "high", but they should have been reading "low" according to the tech manual. Most of the other sensor readings were OK.

None of this makes a whole lot of sense, as I checked the coolant temperature sensor itself on the engine with a multimeter and it was accurate to within 2 degrees... I checked the battery voltage at the terminals and it is fine as well. I've disconnected and reconnected the battery wires, but didn't see a change in behavior. I never had the AC on or the brake depressed either.

I can hear relays clicking under the hood, sometimes very rapidly (much more clicking than normal). The radiator fans now come on when the key is turned on with the engine off, which never used to happen. I've seen other posts which have indicated that this is a sign of a bad PCM (aka ECM, ECC). The faulty sensor readings also make me wonder if this is the case, or maybe if there is a bad ground (or both).

The other issue I noticed when I got it to run for a few minutes was that one of the refrigerant lines became white and frozen over, even though I wasn't running the AC. There was a cylindrical tank with two lines running into/out of it (close to the windshield on the passenger side) that was also frozen over. The top line that ran into the tank was frozen. I'm not an AC expert so I don't know if this is normal or not. It's the first time I've seen either of those freeze over like that.

I've read another post which said that the blower motor can often draw too much current and kill the PCM. Do you believe that could be what has happened to my vehicle? What other things do you think might be causing the problem? I know that I need to check the PCM and other ground connections, but I'm not sure where they all are (I see the one on the engine but that's the only one I see). Any other ideas or suggestions? Should I get a new PCM? If so, does the '92 PCM need to have the EPROM transferred/flashed or can I just install any aftermarket PCM (Cardone, Borg Warner, Standard Motor, Superchips) out of the box? Or is the OEM PCM the only way to go?

Thanks a bunch for any help you can provide. I want to get this problem fixed! :)

Jon Abbott
Good info by Hotz28 as usual, one thing you may try with the scanner hooked up is try tapping the pcm firmly on the case and watch the sensor inputs, if any respond to this the pcm is done.

If after doing all your diagnostics, you find that you need to replace the PCM, I have one that I am not using. I bought it off of ebay thinking I would need it, but I don't. I have two 92 Lesabres that both have perfect PCMs. I once had some quirky electrical problems that turned out to be the Light Control Module in one case, and the Ignition Module in the other. Anyhow, I have never used the PCM but it was said to be in working order when I bought it. I will sell it to you for what I paid for it, which is about $40 shipped. You can reach me faster by email at [email protected].

As to your cars problems, it does sound like a PCM that is wigging out. However, other problems could be doing that to the PCM. As for the frozen AC lines, that is usually caused by moisture in your AC system (which is a no no), or it could be caused by a compressor that is cycling on too often. The accumulator (the silver device near the passenger windshield--its job is to store excess refrigerant and oil, as well as remove moisture from the system) you mention is cold to the touch when the AC system is working. Is yours cold or truly frozen? Being that you did not have the AC system turned on it should not have been very cold. Perhaps your compressor was receiving a faulty signal telling it to engage the clutch, thus turning on the compressor. This faulty signal could come from a PCM that was acting up. However, replacing your PCM may or may not fix your problem.

This does sound PCM related however, I would continue to check the simple things before replacing the PCM. The PCM can react in strange ways to erroneous feedback from sensors. First, check for vacuum leaks, then be sure the spring in the TPS is not broken and the lever returns all the way home with the throttle closed position. Also check the TPS voltage through the range from closed throttle (.5 V) to WOT (4.0V+). The white wire is the 5.0 volt reference signal, the blue wire is variable voltage and the black is ground.

Continue to check for loose grounds or erratic voltage (high or low). If all sensors and voltage and grounds check out within specification, try a new PCM. The prom from your original unit will need to be transferred to the new or rebuilt unit, unless you purchase the PCM with prom installed from Super Chips. Prom swap is simple. Remove the PCM and you will notice a door on the back side with one screw holding it in place. Remove the door and the prom is in front of you. Press the clips back on each side and pull up on the prom to remove. Install by simply lining up pins and push down until clips grab the prom.

Good luck, and welcome to AF!

Thanks for all the good info (the yellow alpaca-like creatures in your picture inspired me as well!). I did just as you said, checking the PCM and sensor grounds and voltages/resistances through the black 24-pin PCM connector and at the sensors, and everything checked out fine. The TPS spring was fine. I found a salvage '92 Lesabre computer off eBay, swapped out the PROM, and it worked beautifully! I was grinning from ear to ear while I drove it down the street the first time. The car drove like nothing ever happened to it! Gotta love electrical problems. :)

Thanks to everyone who replied, you have all been a great help!!