high idle + surge under load? IAC, vacuum problem maybe?

[macwarrior]

Hi there,

I have a 94 4cyl. When running fine, it idles rock-steady at 800. Recently, though, it's started doing two things:

1. It idles really high - 2000 when it's warming up for about 10-20 seconds, then drops to 1500 "normally" if the car is just sitting in neutral with no pedals depressed
2. I'm sitting in neutral, it's idling at 1500, and I put my foot on the brake. Immediately the engine drops to about 700-800 RPM then shoots back up to 1200, then drops again. It does this about once a second. It sounds like I'm revving the engine - just goes vrrroOOOOm...vrrroOOOOm...vrrroOOOOm...vrrroOOOOm... as long as I have my foot on the brake. Take my foot off, it revs back to 1500 and sits there. This doesn't happen when the engine is above idle and the brakes are applied, even in neutral...only once the revs fall off to about 1500.

I've checked some of the stuff on this board, for instance http://www.mx-3.com/phpBB3/viewtopic.php?f=3&t=64418" onclick="window.open(this.href);return false;. That thread says pretty much what I'm thinking - it's a vacuum problem or the IACV. I originally thought it was the IACV, but with the power brakes causing fluctuations like this, it seems more likely that it's a vacuum leak or something? Also, I think (someone correct me on this?) that the IACV is responsible for the fast idle during warm-up, and since that's working "properly" (albeit high), the IACV must be okay.

I took it to the mechanic (unrelated problem - the car drives fine with the engine like this, just annoying and burns gas); his thought was also that there's a vacuum problem. He suggested that some valve (forget which one) might have carbon buildup or similar, and that I should fill the car with hi-test and run it out on the freeway for a while to try and burn it off.

Well, 300km later and a tank and a half of premium and it's still idling high and revving with the brakes applied. So -- what parts of the vacuum system could be causing these two things? High idle is almost certainly a leak somewhere, but I haven't been able to figure out why I get the cyclic revving when there's a load. Are there other tests I could do to try and figure it out?

Thanks a lot.

[andrewlcraft]

I'd love to see an answer to this, too. My '94 does the exact same thing. The throttle body has been removed, it and the IACV thoroughly cleaned. TPS was replaced at the same time for a different problem. Battery cable ends new and clean. All the basics new and in good shape, plugs, cap, etc. I was thinking either vacuum, there is a valve built into the hose to the booster, or possibly a voltage thing, since it seems worse the more electrical load there is. Once you turn on the lights, defroster, blower motor, etc. it gets worse with the more load that is added. Any suggestions would be great. This is the first time I've seen the EXACT same complaint, though lots of search results related to idle problems on the 4cyl. cars.

[eyezpinned]

i had a similar problem... turned out to be the iac. basically if you turn on the air conditioning and your idle drops and stays low your iac is either stuck closed (try an off car tb cleaning) or busted.

[andrewlcraft]

i had a similar problem... turned out to be the iac. basically if you turn on the air conditioning and your idle drops and stays low your iac is either stuck closed (try an off car tb cleaning) or busted.

Not to drag a thread back from the dead, but I've got an update on this problem. As stated above, when the ac kicks on, my idle would just drop. For kicks, I simply disconnected the IACV, and the idle surge under braking went away completely. Cold starts are just fine, ac and other loads act just the same as they ever did. Not that this is exactly a fix, but it gets rid of a very annoying problem until I can track down a functioning IACV for a decent price.