Bad IAC?

I have a problem with idling. Symptoms:
When idling with the transmission in "D", the engine speed will go up and down (only ever reaching about 800 rpm) and may eventually die.
Oddly, if the A/C is on, it idles well -- it stay at about 800 rpm.
I would think that this is a problem with the IAC valve, but the IAC valve is only about 1 year old. I took it off and cleaned it with carburettor cleaner and blew it out with comrpressed air, but this made no difference.
Usually, it will idle OK in neutral, but sometimes, after idling for a while, it will start its cycle of the revs dropping to below 500 rpm and back up to 800rpm, occasionally dying.
The problem is worse when the engine is cold, but it will also happen if the engine is hot (usually, after idling for a short while).
Is this consistent with a bad idle air control valve or something else?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
you didnt say what car/engine you have , but do you have any codes present?
http://www.minibite.com/america/malone.htm
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Tue, 30 Oct 2007, snipped-for-privacy@webtv.net wrote:

'94 Thunderbird, 3.8L V6. The CEL is NOT on (except on starting the engine, as one would expect), so I assume no codes. I don't have a scanner to check, though.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

When my IAC went out in my 97 CV, there were no codes, just the symptoms you describe. I didn't try running it with the A/C on, as it was in the dead of winter. I deduced it was the IAC at fault, found a site that told me how to clean it, but through my fault or dumb luck, it made the problem *much* worse, so I knew I was on the right track. I bought a new IAC and all was well again.
You might want to drop by Advance Auto/AutoZone/etc. and have them scan for pending codes. They'll do it for free.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

I've never had any long-lasting results from cleaning an IAC, either. The ones I tried cleaning lasted from 1 week to about 2 months before I ended up buying a new one.
SC Tom
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
SC Tom wrote:

I've cleaned mine (97, 4.6) four times. Last time about 7 months ago. Still working great at 170 K.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
so I assume no codes. I don't

If you look in the thread titled "93 Taurus Still Idles Too High " there is a link to a scanned page that will show you what to do to read the codes using your check engine light, or a test light if you would rather do it that way.
Autozone and the like are a good resource for getting the codes read, if you have one around, or you can do it yourself, if so inclined.
FWIW, I would kill for a low idle speed problem on my Taurus :-)
HTH,
John
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Tue, 30 Oct 2007, John wrote:

The reason that I replaced the IAC valve about a year ago was that it was idling at around 2000 rpm
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Mine does that, but only after I drive it on the highway for a few miles. Turn off the engine, start it up, and its just fine until I go fast again. <shrug>
Got a new IAC and TPS. Doesn't seem to matter.
Gonna have to run some wires into the interior from those, and a few other devices, and monitor some levels to see if I can figure what is out of whack.
What grinds me the most, is simple lack of basic documentation about how things work, what voltages to expect under what conditions, and what would be considered too much/little.
With my old VW van and even an old Chevette, all the data I need to troubleshoot is out there. The same info for the Taurus seems to be some kind of national secret. Maybe the NY Times can fill me in? :-)
John
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
motor manuals have good info. public libraries,used to have them they are called motor manuals and are blue.
http://www.minibite.com/america/malone.htm
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Basics. Is the engine warming up to about 190?. need a scan tool(if new enough to see data). Check to see if the computer is seeing if the car is in/out of gear. Check to see if the computer knows if the AC in on or not. check if the computer knows how cold the engine is and how cold the air is going into the engine. MAT, CTS....start there.........if its an older EEC IV then if you do it correctly, (KOEO, KOER, you will get a code if something is wrong. You must do it by the book.(very important).
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.