I posted a while ago about a high idle issue I was seeing on my '90 XJ. As background, if I simply turned the key on and cranked, I was pretty much guaranteed to see idle sitting at 2K+. If I turned the key to on for 15-20 seconds before cranking, idle would settle nicely at 800. The issue is that over the course of a drive, the idle would start to rise (as high as 3,000RPM in one case). If I shut it down for a minute and fired it up again, it was back to normal.
I finally had a chance to take a closer look at it today and while looking for vacuum leaks found that if I moved the vacuum line going to the map sensor it would bring the idle down. I initially thought I had found a vacuum leak but I was wrong. It seems that with the vacuum on the MAP sensor, I get high idle. If I pull the vacuum line off the MAP sensor, the idle drops (and stays) at 500 rpm. This makes no sense to me. With the vacuum line pulled off I am flooding the intake with air so I expected the idle to rise, not fall.
Anyone have any theories about what is going on here?
I finally had a chance to take a closer look at it today and while looking for vacuum leaks found that if I moved the vacuum line going to the map sensor it would bring the idle down. I initially thought I had found a vacuum leak but I was wrong. It seems that with the vacuum on the MAP sensor, I get high idle. If I pull the vacuum line off the MAP sensor, the idle drops (and stays) at 500 rpm. This makes no sense to me. With the vacuum line pulled off I am flooding the intake with air so I expected the idle to rise, not fall.
Anyone have any theories about what is going on here?