Strange Problem

[Battery]

My 96 Mark VIII sometimes has a very idle at stop lights. If I shift into neutral it goes smooth, but rough again as soon as I put it back into drive. Any ideas what this could be?

 

[unity]

Things that commonly affect idle are vacuum leaks and the IAC (Idle Air COntrol Valve). Seeing that is smooth in neutral my first guess is vacuum leak. Higher RPM = less vacuum. The leak may just be enough at 600 rpm but less noticeable at 800 rpm. Is there any difference from when its cold to hot?

Other things: wires, coil packs, etc..

I would to a vacuum test first if you can, maybe even a fuel pressure test. I always check those first out of habit!

 

[MatthewDavid]

Try cleaning the IAC valve... I just did it and it changed A LOT!!... what I did was I took the valve off... filled it with carb cleaner... just got that pyroil stuff... let it set for a bit... poured it out... then just sprayed it for a bit till it was fairly clear... I also used a pipe cleaner and got in there a bit... couldn't get it perfect but I got it a lot cleaner...

Let it dry for a bit before putting it in... oh yeah don't tear the gasket either!!... after all that I put it back on and started it up and no more surging, it idles a lot better, and it's more responsive!

 

[unity]

Slight problem. The IAC is the hardest thing to get to on a Gen I. Its a rather lengthy project that can take more than an hour if its your first time. So swapping it on and off may be more effort than its worth until the other items are ruled out. On 97 & 98, five minute job!

 

[The Rev]

On a gen 1 where is it? How much do they go for, can I get em anywhere, etc

 

[unity]

It on the passenger side of the throttle body, almost up against the firewall. Not sure on price, but I bet you can get one at any parts store. I dont think this is one you need to go OEM with, aftermarket I believe is ok.

 

[Roadboss]

The IAC can be bought at NAPA and they have one that also is from the OE to Ford, it runs about $75.00, but it is a good quality one. Some of the cheapies end up getting leaks in them. Just my experience in the parts business. I had to pay my local guy to get in there to replace mine, as he has a little elf on staff to get in those tight places, but he only charged me an hour labor which was Christian.

 

[Cobramarkviii]

my car use to do that when I would stop with it in drive but it would go up and down and when I put it into neutral it would stop, and sometimes it will never do it at all, so it is prob the IAC ? at first I thought it ws the tranny, but I dunno