Have a 2003 Mercury Mountaineer 4.6L V8. Just last weekend, I replaced the IAC valve after the engine started to drop in rpm (down to 500) and then stall out completely. Well after I did that, now whenever I start it up, it revs to between 2000 and 2500RPM which I know is not normal then after about 5-10 seconds drops down to 1000rpm then sometimes 750-800rpm. Does it every time I start it and doesn't matter if A/C is on or not. My gas mileage has gone from 16.7mpg city/highway combined down to 15.4mpg, because of this issue.

Another issue is when I am coasting or coming up to a stop, the vehicle seems to want to stay at 1000-1500rpm or not really even coast. Tonight for example, I started to coast from 55mph to a stop light which from the time I turned off cruise control (tapping the brake) to the light is roughly 1/2 mile and some of it was up hill, well by the time I got to the turn lane, I was still over 40-45mph, normally I'd be around 30-35mph and barely need to use any brake to slow down for the turn. Then whenever I stop completely, the rpms are at 1000-1500rpm and I have to hold the brake, just so the vehicle doesn't roll into an intersection. It'll also stay there when I'm going through a parking lot. Normally its at its normal idle range when putting along in "Drive" in a parking lot.

I asked this over in a Mountaineer section on FTE and honestly they were of zero help, only got one response and this is frustrating because this is eating up gas and can be a real safety problem going through crowded parking lots or stopping at a light or stop sign. I've been thinking maybe a vacuum leak somewhere, but all the main hoses seem to be fine. I've already changed the plugs, since before they were running too lean (gapped them to .052-.056), cleaned the MAF sensor and like I said the IAC is brand new. Real kicker is, there was no check engine light or message in the message center display, so running a OBD-II scanner was completely useless in figuring out the issue since no codes were being thrown.

A few months ago when it first started getting warm outside and we had the A/C running, the engine would just randomly rev up to 2000rpm...this was in a drive-thru and when it happened, one it scared me because it was so sudden and almost took out the back of a car, luckily all I had to do was put more pressure on the brake peddle and killed the engine and started it back up, that seemed to keep it to normal, however it happened randomly several times...I asked on FTE and they said, IAC valve. Well now that its replaced and doing this now, I'm at a complete loss on what is going on. My patience is running pretty thin right now on this.

Also, I did clean out the throttle body with cleaner, and got as much carbon out as I could. That has done nothing as well.

 

Thought it might also be the throttle cable sticking, but that doesn't seem to be the case at all. Could it be the boot on the pcv or the valve itself? I don't have excessive oil consumption, just the normal 1 quart every 1000+ miles.

 

So I guess I was wrong and it is caused by the A/C compressor. Not sure if it's from it having a small refrigerant leak or that the compressor is going bad. All I know is, it runs like it should when the compressor is off, but when it's on it high idles and doesn't slow down.

 

Still having the high idle issue and my gas mileage has gone from 16.5mpg to 14.77mpg since I've worked on it. How likely is it the IAC from O'Riley isn't too compatible with a Ford? Or is it something else?

When I start it up in the morning, it goes to 1500rpm, sometimes drops to normal idle and sometimes stays at 1000rpm even while in gear, causing me to have to put more pressure on the brakes. When I start it back up, it idles at 2000-2500rpm for about 5-10 seconds and still runs at 1000rpm like any other time.