haromaster87

I started a thread here a little while back and I talked about my '86 GT revving itself when it hit a bump or something. Well since then that very problem caused me to get in an accident(the car revved while I was making a corner) and has persisted to be a major pain in my a$$. Right now the car has been doin alot of sittin there while I acquire parts to fix it. The other day I backed it out and jump started it. Fired right up and when I pulled it in the problem hadn't changed one bit. Whenever the car hits a bump OR when the hood is dropped, it revs. But not a quick up and down rev, it revs to 2000rpms, and holds there then it settles back down to idle. It then occured to me, this is very similar to what the car does when you first start it(usually on a cold start). I'm correct in saying that most cars rev up a bit then settle down on start up, no? So in the previous thread everyone was focused around the fact that unmetered air was making it into the engine and revving it- Vacuum leak. While the car was running I did the old carb cleaner vacuum leak test and came up empty. But if it where a vacuum leak it wouldn't rev the exact same way everytime. And it wouldn't rev and stay. So i'm startin to think it's something electric. Probably a loose connection or something. Right now the battery's dead so when I can get it started again i'll get a vid of it doing this. For now though, what could come loose and would make the car think it just started again? I just replaced the ignition switch not too long ago but i'll check it again. There's gotta be somethin else though. It might be around the engine bay too, because the force of the hood dropping can cause it too happen.

Please, any ideas you have, toss em out there. I will finally have the car running perfect once I track this. Other noteable thing's I replaced are the IAC and the TPS. While my idle improved dramatically when I replaced the tps, it didn't help the problem.

Thanks,
Tony

mjr46

there may be a bad connection at the IAC motor or a short somewhere regarding such......while the car is running go around and wiggle any part of the harness and or connectors you can see and see if the problem reoccurs while performing what we call the "ford wiggle " test...it sounds as if the problem is a connection / short as it is sensitive to vibration/movement/jarring motions

haromaster87

^^Ah, gotcha. Well, when you start the car, and it does the rev thing I described(revving to 2000rpms and settling down) is it the IAC that causes that? It'd probably be that part that has the loose connection. The "ford wiggle" eh? Guess Fords are notorious for lousy wiring? lol

mjr46

on cold start idle will be higher than normal due to various inputs to the ecm from various sensors .....ect, act, ect ect....will be key inputs so that ecm knows to bump idle.....also something else that could cause issue is the idle up feature for when the a/c is turned on.......since the a/c created a load on the system the ecm knows to raise idle when the a/c is turned on......like I said first go around and wiggle harness and relays and connections....you need to make sure it is not something like a faulty connection or short to ground in the harness sending a improper input to the ecm that'll tell it to bump idle up.......the ole' theroy of garbage in garbage out applies in such cases = improper input to ecm can result in improper output command to a component

HACKGT350

mjr your so smart. i wanna be just like you when i grow up. lol.

he is right it does sound like a loose connection and the IAC is one of the main things that will increase idle on start up. if you have a bad connection or a wire that is rubbed open or broken somewhere on that circuit it could trigger the valve to open and let more air in then you want.

the wiggles test isnt just for ford its for all car makers out there. every car out there can have some kind of wiring problem and wiggle tests can come in very handy trying to narrow down what the cause could be. you could also get a little hammer or even the handle of your screw driver and tap the IAC itself and see if anything happens.

good luck, keep us posted with your findings

mjr46

Thanks for the vote of support

HACKGT350

no problem

haromaster87

Alrighty, i'll check that tmrw or somethin when I can jump the car again. I know it's fairly normal for cars idle to bump on start up, every car i'v ever seen does it. But once my car is just idling at 900 or so rpms, a bump or jar causes it to rev. But not just a random rev, it's just like it does when you start the car. Cause when you start it the car doesn't rev up then just fall to idle, it revs, and holds, then it kinda slowly decreases rpms till it gets to idle. And thats what it'll do when somethin bumps it. So I just gotta find the connection thats faulty and that sends that signal to the ecu to bump idle.