KFM and the ECM Saga

So, what happened to the ECM in my car to make me go out and learn some of this stuff? Well the story begins one chilly evening in October...

I was leaving my health club one evening about 7:00PM. The temperature was about 50 degrees (Fahrenheit) outside. When I attempted to start my car, it would run for about 2 to 3 seconds, then die. After trying for a while, I had the car towed home. Once in the comfort of my garage, I tried diagnosing the problem as best I could. I thought the problem was fuel or ignition related so I changed spark plugs, fuel filters and other parts. On about three different occasions the car actually started, but it was weak and unstable. I took it out around my neighborhood, only to have to push it back home.

I finally decided that this was beyond my scope of knowledge and tools, so I had it towed to my trusty, exotic car mechanic. They tested several systems and finally wound up replacing the Throttle Position Sensor (TPS). This is a GM part and readily accessible. Later that same day I got a call: my car was ready! Could it be possible that the problem I tried to diagnose for weeks could be solved in one hour? I started feeling very humble about my limited automotive repair skills. I went to pick up the car and, again, it wouldn't start. My mechanic was puzzled. "We even test drove it up and down the street" he said. I felt redeemed. Sure, I still didn't have my car running, but at least my automotive ego wasn't feeling so hurt.

What ensued from that day was week after torturous week of testing components on my car. It's a good thing my mechanic didn't charge me for all the hours of labor (he considered it a learning experience), because I would've had to sell my house to pay him. I started doing more research, posting letters to the Lotus List, and making phone calls to experts. Even though all indications were starting to point towards a bad ECM, everyone assured me that they were very reliable and never went bad. I ignored their advice, took out the ECM, and sent it to SuperChips here in Florida. They make a chip upgrade for the car, so I figured they might be able to figure out what was wrong. They bench-tested it for a week, but couldn't find any problems. They returned it to me, I plugged it in, but my car still started and died.

The next test was to get a hold of a Tech-1 scanner and see if I could diagnose the problem with it. A local mechanic provided the tool, and my friend Curtis, from Carolina Motorcars, was kind enough to supply me the Lotus cartridge it required.

While playing with the Tech-1 one night, I noticed something wrong with the TPS. Before I explain what was wrong, let me explain what it is supposed to do.

The TPS is like a variable potentiometer. At closed throttle (0%) it should measure about .45 volts. At wide-open throttle (WOT - 100%) it should measure about 4.7 volts. The ECM has an analog to digital (A-D converter) circuit that translates this voltage to the proper percentage value so that the ECM knows how open the throttle is. It then uses this information to determine how much fuel it needs to deliver to the engine by controlling the timing of the injectors.

Although measuring the voltage at the TPS terminals with a voltmeter while opening and closing the throttle gave the correct values, the ECM (through the Tech-1) was reporting that the throttle was only varying between 0 and 11%. So, even though the throttle was 100% percent open, the ECM was only giving the engine fuel as if it was 11% open. This made the air-fuel mixture too lean, and the engine would die.

But the most amazing discovery was yet to be found. I accidentally left a work-light resting on top of the ECM for a few minutes. As I'm talking to my mechanic, I noticed the TPS percentage numbers on the Tech-1 started changing on their own! After a few more minutes the Tech-1 was reporting a full range from 0% to 100%, as it should. I told my mechanic that I bet the car would start and run now. It did. What I had discovered, totally by accident, was that the ECM A-D circuitry had become temperature sensitive. From that point on I could, consistently, make the car run by first applying heat to the ECM for a few minutes with a blow dryer!

I was confident enough that this was the cause of my problem, that, after 3 months of sitting in the shop, I used my little handy travel blow dryer, started it car, and drove my car home. In the weeks that followed, I attempted to have the ECM repaired. Later, I decided that it would be better to replace the unit. When I got the new ECM, I plugged it in, started the car, and it's been running ever since!


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