I have a 94 mustang V6 3.8. When I start it up it idles at 1800 rpm and drops to 1500 until warm then it is a steady 2000 rpm. Driving at 55 mph it runs at 1200 rpm. Once you put it back in park it is 2000 rpm. Doesn't have a miss and doesn't fluctuate. Could this be caused by a vacuum leak or something else?

 

hi, very well could be a vaccum leak but my best eduacated gues would be a sticking IAC valve(idle air control valve)-they can sometimes be cleaned by spraying carburater cleaner in the throttle body while revving up the engine,mark.

 

Instead of cleaning the IAC we replaced it. Startedit up and my husband drove it about 8 miles, did fine. After he pulled back in he started it up a couple times and it idled at 700 rpm. Two hours later I started it up and she instantly idled up to 3000 rpm, worse than before. He took the IAC off again and replace the gasket, plugged it back up, connected the battery, started it upidle speed was around 700 rpm, let it idle for 4 or 5 minutes and drove it 10 miles, pulled back in put it in park, idle speed was 700 rpm. Shut it off and started it back up and idle speed was 3000 rpm. What is going on??? Help

 

Did he clean the contacts on the connector for the IAC and use dielectric grease when reconnecting?

 

He cleaned the contacts on the connector for the IAC and use dielectric grease.

Here's the thing!!!

3 times he has put the new IAC on and it as worked properly. Each time has been after the car as sit over night. After putting it onit isidled at700 rpm, drive it or let it idle for a while (until engine is warm, it seems) then shut it off and start it right back up, idle goes to 3000 rpm. Now, you can let it sit over night with the new IAC and it doesn't matter, still 3000 rpm. I have unhooked the battery and IAC for an hour and that didn't change anything. Have not left either or both unhooked over night to see what happens.

Now, you can put the old one back on with the engine cold or hot and it is constant at 1800 rpm cold/ 2000 rpm of warm.

I am concerned about the new IAC.... It didn't come with a gasket, he had to make one. I would have thought it would come with one. So, I am wondering if someone had this part before me or does it not come with one. At this point I don't know if I got a bad IAC or if there is something else bad?

Why would the new one work for a little while?????????

 

Have you looked inside the throttle body bore? It is possible there is carbon built up around the throttle plate not allowing the plate to fully close. Also check to be sure all linkage is free and not hanging on anything. Stupid idea, but are you sure there is not a floor mat or something that may be holding the accelerator down a little bit? I have seen it happen a lot. There is a throttle body cleaner that you can buy in an aerosol can that works pretty good. We use the one made by 3M.

 

I will have him check inside the throttle body bore for carbon build up. I know he has check the linkage and it is free. I know he is using a throttle body cleaner, not sure of the brand.
At this point I wish it was the floor mat holding down the accelerator, stupid or not, that is the first things I checked. I'm not that lucky!