Join Date: Sep 1999
Location: Des Moines, IA
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Re: Sol-vac Idle Problem
I have the same problem occaisionally on my 90 YJ. I haven't resolved it yet, but here's what I've got so far. (I'm putting a major disclaimer on this post because it's been a while since I worked on this and can't locate all the information I had, so it's mostly from memory).
It's my understanding that one of the vacuum switches (4 or 10, don't remember which) indicates a deceleration mode to the computer, which causes the computer to step up the idle to reduce emmissions. I think that as soon as the computer goes to closed loop, due to low idle speed and a marginal vacuun switch, the computer detects deceleration and kicks the idle up. It's held there for about 5 seconds and released. As soon as it's released, the idle goes down which triggers the switch again. I was able to test this theory one time by disconnecting the vacuum switch while the problem was occuring and the problem went away.
I think that the vacuun switch does not funtion correctly when it is cold, when the underhood temperature gets warm enough the switch starts functioning correctly and the condition goes away. This would also explain why when it is really cold, it will stay on fast idle for the whole 20 min ride to work.
Since this isn't my daily driver, I haven't been in a big hurry to verify my theory. Let me know if you find out anything different with yours.