High idle woes

[VTExplorer]

You gotta help me guys. I'm going crazy here....I've read a ton of idle-related threads but I can seem to pinpoint what my problem is.

Once my '96 is warmed up, it'll idle pretty good in the driveway, around 1000 rpm (a little high). Once you start driving its a different story. If I shift into neutral, or just push in the clutch at say 30 mph, the engine speed will rise to between 1500 and 2000.

If I coast down a hill in neutral with it idling high, once I reach the bottom of the hill and stop for a few seconds, the idle will drop to around 1000. Does this mean a brake booster-related vacuum issue?

I checked for vacuum leaks, both visually and with a propane torch. No luck. I cleaned the IAC. When the IAC is unplugged it idles at between 500 and 750. The TPS is reading properly (0.97 volts). Is there anything else I can do besides take it to the shop?

Al

 

[verlane24]

Just fixed this....

Maybe this won't apply to your engine, but I had the exacty same problem with my 93 for a few weeks. As time progressed, I also started having problems with warm startups..... I had to crank it for 5 or 6 seconds and push on the gas to get it to start.

I never got a CEL, but pulling codes gave me 116, or ECT temperature too high. I had just replaced that about a month ago, but must have gotten a bad one. Replaced it, and the fuel pressure regulator(had been meaning to do that for a while.), reset the computer and now all is well.

Although I cannot be sure since I replaced both parts at the same time, but I think the high idle was from the faulty ECT sensor. The hard starts might have been either one, but I'm not really sure. The ECT is like $20, so it's a cheap thing to try.

good luck
brian

 

[Opera House]

I have a comment when you say

it idles 500-750 with the IAC unplugged. The idle should drop down to the point the engine almost or does die. That tells me you have a bad vacuum leak. Either the IAC is still leaking, vacuum hose problem, brake booster, PCV valve, or throttle body is stuck. If nothing else it is the lower intake gasket.

Clean the throttle body first. You can remove the IAC and tape over the port. Remove and capping vacuum tree ports one at a time to eliminate sources of the leak.

 

[VTExplorer]

Thanks for the suggestions, it's been too damn cold and wet to crawl around under the hood. This may be a problem I just learn to love for a while.