Erratic Idle

97JF · May 2, 2013

I've had a problem slowly develop on my 98 GT, occasionally when stopped at the traffic lights the RPM briefly goes up to about 1500 then back down again and the car surges forward a little, not drastically though, and also sometimes when I start the car when hot the RPM's will go up to sometimes 2000 then straight back down to normal again, any ideas ? Thanks

MustangChris · May 2, 2013

replace your intake air control unit, clean your mass air flow while you're there, and let us know if that helps out

Could be a vacuum leak, could be the IAC, etc. etc.

it sounds like this is an automatic??

97JF · May 2, 2013

Thanks for the reply, I will try and check the things you mention, yes it's an auto

the5.ohh · May 2, 2013

Join the club lol. Replacing the iac will probably help like MC said, clean your MAF an let us know how it goes. If that don't work I'd check for vac leaks

Ferocious · May 2, 2013

I have a working IAC that I will give you for cheap if you need one.

Also check your TPS sensor and throttle body. Make sure it's not sticking. And check the stop screw just in case.

MustangChris · May 2, 2013

by "check" the stop screw. he doesnt mean "fiddle-faddle" with it. if you fiddle with the stop screw all sorts of issues can come from it (such as a bad idle)

97JF · May 3, 2013

Thanks for the replies, what causes these problems ?

silverv6 · May 3, 2013

^because mustang.. When mine did it i made a restrictor plate (make the holes of the iac smaller) out of aluminum and it seemed to help

97JF · May 3, 2013

Can keep disconnecting the battery have anything to do with it ? Reason I ask is that the electrician fitted an Isolator switch to the car as there is a slight drain on the battery so everytime i'm not using the car I use the isolator switch to disconnect the battery and this problem I think has only started occurring since i've been doing this

tarafied1 · May 4, 2013

I have a similar issue with my 99 5 speed. Recently bought it. It doesn't act up unless I drive a long distance. It will rev 2000 or more for no apparent reason. If I shut it off and restart it then it will behave for a little while and then do it again. If I unplug the IAC valve when it's revving high, it will die. I did get it to restart with the IAC unplugged but it idles really low, about 500 rpm or less. Sometimes it just bumps up to 1200 or 1500 for a little while and then settle back down. Most of the time it idles at 800 or 900 rpm. I'm thinking I need a new IAC, probably original and car has 140,000 miles on it.