After completion of my big refurbishment project, the car fired right up. It ran fine at first, but after a few days of driving it had me thinking the idle was a little peculiar. Nothing major though. Then I began running the A/C. The idle problem became much worse, with frequent stalling. Let me jump right to the cure for those of you who have heard enough already about the symptoms:
1. I forgot to tighten the clamp on the air cleaner hose assembly at the TB. Any leak there throws off the mixture because it short circuits the MAF, similar effect to a vacuum leak. When the motor is hot and vibrating at idle, the leak is worse, so the effect is worse. It's very easy to overlook this clamp with the Mark 1 eyeball since it has tangs that keep it in place even when loose.
But starting the car after this was corrected revealed the problem was not fixed. On to part 2, which is related:
2. The air leak was a problem, but not the majority part of the problem. The real problem occurred shortly after first start. What had happened was the EEC had computed an idle program that did not compensate for the load from the A/C since it was turned off. So, I followed the idle program reset procedure, but with a very important wrinkle...I turned the A/C on to Max Cool before engine start. Viola! All idle problems are gone!! No parts replacement required!!!
For those who are not bored, some more about the symptoms. At cold start, the engine would start fine, but RPMs would bog to 500 before recovering to normal fast idle. Driving was normal except that when coasting to a stop or rounding a corner, the RPM's would drop and sometimes it would stall. With A/C off, it would not stall when the RPMs dropped. Running the code checks showed everything normal, no errors at all.
In working this problem. I read back through years of old posts that discussed very similar idle problems. I read many scenarios where parts were recommended or changed; MAF, IAC, DIS, speed sensor, TPS most frequently. Those parts changes often did not help. I read a few posts about vacuum leaks creating idle problems, but I had checked those hoses and connections thoroughly. Finally I happened to read a post about MAF issues, and the author mentioned problems caused by a loose connection at the TB. I sat bolt upright! I could see that clamp in my head plain as day and I knew I had not tightened it. Alas, I was disappointed when I started the car after tightening the clamp the next morning, and the RPMs bogged just as before. But I had also read an old post about the load imposed by the A/C (considerable), and I had read about the idle reset procedure. Since my problem was exacerbated with the A/C on, I thought just maybe the idle program needed to be redone now that my air leak was corrected. So I disconnected the battery and turned the headlights and key to on for a few minutes to clear the computer, per the procedure. Then I turned the lights and key to off and reconnected the battery. Next I turned the key to On position, but not start. I set the A/C to Max Cool. I started the car and immediately turned the lights on, again following the reset procedure. Sure enough, the idle reprogrammed itself correctly with the increased load. I could not believe such an elusive problem with no codes was so easy to fix. Now the idle responds correctly, just like it always has since the car was new. My experience makes me wonder how many SHOs with similar problems just had an incorrect idle program because the computer generated it when the engine was too lightly loaded with the A/C and lights off.
1. I forgot to tighten the clamp on the air cleaner hose assembly at the TB. Any leak there throws off the mixture because it short circuits the MAF, similar effect to a vacuum leak. When the motor is hot and vibrating at idle, the leak is worse, so the effect is worse. It's very easy to overlook this clamp with the Mark 1 eyeball since it has tangs that keep it in place even when loose.
But starting the car after this was corrected revealed the problem was not fixed. On to part 2, which is related:
2. The air leak was a problem, but not the majority part of the problem. The real problem occurred shortly after first start. What had happened was the EEC had computed an idle program that did not compensate for the load from the A/C since it was turned off. So, I followed the idle program reset procedure, but with a very important wrinkle...I turned the A/C on to Max Cool before engine start. Viola! All idle problems are gone!! No parts replacement required!!!
For those who are not bored, some more about the symptoms. At cold start, the engine would start fine, but RPMs would bog to 500 before recovering to normal fast idle. Driving was normal except that when coasting to a stop or rounding a corner, the RPM's would drop and sometimes it would stall. With A/C off, it would not stall when the RPMs dropped. Running the code checks showed everything normal, no errors at all.
In working this problem. I read back through years of old posts that discussed very similar idle problems. I read many scenarios where parts were recommended or changed; MAF, IAC, DIS, speed sensor, TPS most frequently. Those parts changes often did not help. I read a few posts about vacuum leaks creating idle problems, but I had checked those hoses and connections thoroughly. Finally I happened to read a post about MAF issues, and the author mentioned problems caused by a loose connection at the TB. I sat bolt upright! I could see that clamp in my head plain as day and I knew I had not tightened it. Alas, I was disappointed when I started the car after tightening the clamp the next morning, and the RPMs bogged just as before. But I had also read an old post about the load imposed by the A/C (considerable), and I had read about the idle reset procedure. Since my problem was exacerbated with the A/C on, I thought just maybe the idle program needed to be redone now that my air leak was corrected. So I disconnected the battery and turned the headlights and key to on for a few minutes to clear the computer, per the procedure. Then I turned the lights and key to off and reconnected the battery. Next I turned the key to On position, but not start. I set the A/C to Max Cool. I started the car and immediately turned the lights on, again following the reset procedure. Sure enough, the idle reprogrammed itself correctly with the increased load. I could not believe such an elusive problem with no codes was so easy to fix. Now the idle responds correctly, just like it always has since the car was new. My experience makes me wonder how many SHOs with similar problems just had an incorrect idle program because the computer generated it when the engine was too lightly loaded with the A/C and lights off.
