High Idle, HELP!
09-07-2008, 12:02 PM
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High Idle, HELP! My car was idling at around 1,200 rpm's a few months ago, despite being tuned to run at 800. I thought that it was definitely the throttle position switch, but the previous owner, Patrick G, told me he had replaced the TPS and it didn't fix the problem before. He said that the issue was with a spike in voltage coming from something and causing the PCM to boost up the idle to compensate. We narrowed it down to a faulty shift light and so I removed it. This fixed the problem, but only for a while. A few weeks later the problem returned with the idle as high as 1,500 rpm now. This time the alternator was going out so I replaced it, but unfortunately I didn't do my research and just put in another stock replacement so it too went out in only a few more weeks. This time I replaced it with a big 420 amp with a big gauge wiring kit with a fuse in order to support my stereo and all my nitrous stuff. I also put an overdrive pulley on to compensate for my underdrive system. After installing the new alternator, the problem was helped, but this time not totally fixed. It now idles at about 1,100 rpm which is where I'm stuck at right now. Patrick is convinced that the ultimate problem lies in the small wire running from the alternator to the PCM. This wire had gotten caught under a spark plug during a change and is now kinked in a few spots. This problem is that I can't for the life of me seem to find a part number on this little wire and there is still a chance that that won't make this issue go away. It really kills my mpg to have the car sitting at 1,100 at a stop light and it is very annoying. When the car is first started up not having been run for a good while, the idle is usually at the 800 rpm mark. It is only after a few minutes of driving that it spike up and stays there.
09-07-2008, 07:31 PM
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Sounds like a good compound problem.
The Alternator turn on signal wire, on the red pcm connector, pin 15, is merely a wire. It is either good or bad. A simple continuity/insulation break down test will reveal it's condition.
If you are not maintaining enough voltage/current to maintain the memory of the idle/fuel trim data in the pcm then I can see Patrick's logic.
Properly maintained and tuned I would expect a setup like yours to easily maintain a 700rpms idle.
The second part of the problem is avoiding uncommanded airflow. Simply put if you have an unaccounted for leak that the pcm cannot "see" via the MAF or MAP then you will have a high idle.
If you are commanding 800rpms and are not getting it then just go back to the basic triangle. Fuel, Air, Spark. Assuming your fuel and spark have already been dialed in as in this case, the uncommanded airflow is the most likely suspect.
The Alternator turn on signal wire, on the red pcm connector, pin 15, is merely a wire. It is either good or bad. A simple continuity/insulation break down test will reveal it's condition.
If you are not maintaining enough voltage/current to maintain the memory of the idle/fuel trim data in the pcm then I can see Patrick's logic.
Properly maintained and tuned I would expect a setup like yours to easily maintain a 700rpms idle.
The second part of the problem is avoiding uncommanded airflow. Simply put if you have an unaccounted for leak that the pcm cannot "see" via the MAF or MAP then you will have a high idle.
If you are commanding 800rpms and are not getting it then just go back to the basic triangle. Fuel, Air, Spark. Assuming your fuel and spark have already been dialed in as in this case, the uncommanded airflow is the most likely suspect.
