1995 Ford Mustang fast idle

Thanks for your suggestions. The throttle plate was cleaned and everything else was checked and OK. I was thinking that since the idle is perfect when engine is cold and is fast when engine is hot, that the EGR was not opening when hot like it is called to do by the computer. The computer naturally thinks that the EGR is open and bumps up the idle to compensate. So I looked over the EGR controller and found that the EGR was not getting vacuum when it was suppose to. I switched the vacuum line coming into the controller with the line coming out (that goes to the EGR) and now the EGR has vacuum and opens up. The problem now is the idle speed goes down to 650 and runs rough. I do not think that it will pass smog like this. I turned the idle speed screw up as much as I can. Turning the screw any more and the idle goes from 650 to 1000 with just a touch of the screw. What do I do to bring the idle up to 750 when hot like it does when engine is cold?

Thank you for your help. You are right in that the EGR does not come on at idle and I checked the EGR lines and found I had them right the first time. The EGR was partially coming on at idle and dropped my fast idle when the car is hot to normal but was not good when car was cold. The EGR system is all back to normal and functioning. I have only 3 things that could cause the normal idle when engine is cold and 1,000 to 1,200 rpm when engine is hot, and they are the IAC valve, the TPS and the MAF sensor. I changed the IAC and TPS when I installed a good used functioning throttle plate body. I took off the IAC and cleaned it as well as cleaned the throttle plate. All this was done at first with a little change in performance (the car does idle better when cold and a bit lower when hot), but I still can not smog the vehicle since idle is above 800 rpm. My question is there a chance the MAF sensor is bad. This is the only thing I have not touched. I looked at it to see if it was dirty and it seemed OK. Maybe one of the used IAC or TPS parts are bad. Which one of these items would typically cause this problem? There are no vacuum leaks (this was the first thing I went through before the throttle plate body exchange). Thanks.

Yes, I believe it might be the MAF. I did change the air filter (stock and no K&N). I do have something strange happening that I forgot to mention. It came to me since you asked about the air cleaner. I hear, for the first time, a hollowing noise coming from the air cleaner area in front of the MAF. This is why I changed the air filter, thinking it had a air leak around the gasket, but the noise is still there and it is random. I only hear it sometimes during idle. Can this be the problem and is the MAF likely to be effected by whatever is causing is noise? Again the idle is only 200 rpm high when engine is warmed up and since I have done nothing with the MAF (except check it for dirt when I changed the filter) I am thinking this to be the problem or at least effected by whatever is causing the hauling noise. I do thank you for all your prompt replies.

Thank you for your help. I replaced the MAF sensor and all new air cleaner canister & filter and ducting. It did come down to 950 rpm at idle when warm (perfect when cold). So we are making progress. I think that the used IAF valve that I put on to see if that was the problem could also be bad. I did clean it with carb cleaner before installing it. Besides trying a brand new IAF, is there anything else? Everything I have done has improved the idle speed. It was first at 1,300 when hot and now down to 950. Smog will not pass unless it is below 850. Thanks

Thanks. I am looking at replacing the IAC again with a new one this time (I installed a good used one that I cleaned. I thought is was good). I unplugged it to see if the idle would drop and nothing happens, not even a code. Idle stays the same and no engine code. Is this normal? Also could the air temp sensor be the problem since the idle only goes up when engine is warmed up?

One more thing that I just tried. I unplugged the TPS when car was at idle, and nothing changed (rpm same and no code). I then unplugged the MAF and car died but again no code. Does it take a bit of time for the codes to show up when I unplug these devices? So I have no change when both TPS or the IAC are unplugged.

Just wanted to leave you with the solution to this problem in case others could use this to help solve their problem. Since I unplugged the IAC and nothing changed, I decided that the part had to be defective. All the other repairs I did thus far were needed and kept improving my problems, but as soon as I put on a new IAC, everything was perfect. Perfect idle at any temp. Now when I unplug the IAC, the car dies (this is normal). So thanks for hanging in there on this hair pulling journey.

Hello,
Thanks for helping me work through the fast idle problem It is doing good, but I took it into smog and it failed. Emissions were 6 times the approved levels. Car runs great and on the way home it through the code 175 (# 2 bank is rich). I changed the injectors, fixed any potential vacuum leaks, put new plugs in and new plug wires, and replace the upper O2 sensors. It runs even better than before, I had no codes for over a week and took it in to smog and on the way in it through the same 175 code. Could the lower O2 sensors (below the muffler) be bad? I do not get any sensor codes. Car runs great. Please help. Thanks

Thanks, I had new oil in it less than 200 miles ago. I will change it again just prior to smog test. I keep getting the engine code 175. I have done everything to think of. The lower O2 sensors, I will leave alone. I just cleaned the MAF sensor with the MAF cleaner. It looked good before, but who knows. I am not sure about the cam on the engine. We got it used and I did not open it up that far. There is a stock air cleaner on it. The engine has more HP than the original engine that we replaced and runs real strong but now that we solved the high idle issue I am dealing with the 175 code. I will let you know in a week or so of driving if the MAF cleaning worked. Thanks.