dkillourie

In the last few weeks, my '98 Mustang GT (automatic) has been revving really high when I start it. It happened at the gas station after I filled it, and it has happened when I've driven the car to the store or work, shut it off, and restarted it after a short time. It jumps up to 2,000-3,000 rpm.

If I leave the car overnight, it starts fine the next morning. It seems like it's only when it's been off for a few minutes to a couple of hours. The shorter the time between starting, the higher it revs. I don't know how high it will rev, because when it does this, I put my foot on the break and shift into reverse. The revs come down, but I'm afraid I'm going to cause damage to the transmission.

I've searched this site and I haven't seen this same problem. I've seen others have issues with low revs, or not starting. Could it be a problem with the IAC, or the MAF, or???

I installed an Accufab upper intake plenum and FRPP 70MM throttle body a couple of weeks ago, but it had been doing this before the install. It seems to be more frequent now.

tricky02GT

Noticed the same after I fitted my tb + plenum. Revs race at start of engine. Maybe because higher intake or maybe zeroing of throttle position sensor? I dunno. Wouldnt worry too much about it. The revs DO die down right? Few seconds maybe?

dkillourie

I never waited for them to die down, because they climb so rapidly. I shift into reverse, which brings the revs down, but I can't imagine it's good for the car. I get nervous when I see it jump like that. I didn't want to wait to see how high it would go.

It only happens if I had previously driven the car within a short period of time. But it did happen before I installed the TB and plenum, just not as severe or as often.

I guess I can wait a little longer next time to see if they go down.

Neko-

Possibly something with the choke? It might actually be getting too much gas through that.

I'm just tossing the idea out there, not that I'm all that knowledgable about the engines, so anyone feeling a need to correct me, please do so

theedge67

No choke on modern cars anymore, the fuel injection got rid of that. It's definitely the computer giving the engine too much gas at startup. The engine is all electronically controlled and the computer must be getting mixed up somehow. Why it is doing that is another question.

I would start with the intake aspects. Dirty MAF?? Check the air filter too.

Also, as long as the engine is not terribly cold, as in under 40F, don't worry about it revving up. If your oil is good and you are using the 5-20 like you should, it will be fine. You are just asking for problems shifting into reverse at those rpms!!

jarradasay

check your set screw in the throttle body. It may be out of adjustment due to your installation of new TB and Plenum. Too much air is getting past the TB at idle. Also happens when IAC's go out. Check you installation of TB and Plenum. Make sure everything is adjusted correctly. The ECU only sends as much Gas as is necessary for the amount of intake air it is reading. You also may want to check to see if your TB's butterfly is sticking.

Just my 2 cents.

dkillourie

Thanks, I'll check that out.

dkillourie

Update:

I stopped driving the car after I posted this (though, I did start it everyday to move it into the garage, while I took the other one to work). About two weeks later, I drove it on a Saturday, just to drive it. It seemed fine. I drove for a while, made a few stops, and it didn't act up at all. I decided to try driving it to work again, and have not had a problem again.

In the mean time, right after I posted this, I bought a new IAC and EGR valve. I figured better to be safe than sorry. It was only about $150 for both, and with 70,000 miles on the car, it couldn't hurt to replace them. They came in last week and my brother and I installed them (they went on real easy in about a half hour). Everything works fine. The old parts were a little dirty, but seemed OK.

I don't know what the problem was, but it's gone now. The air filter wasn't dirty and everything else looked OK.

DynamicmustangGT

could actually be your tranny ready to let go. My tranny went in my 98 taurus at about 150k and it was doing that right before it died.

dkillourie

Is that supposed to make me feel better?