View Full Version : idle problems on start up

03-15-2009, 05:14 PM
hi, wondering if anyone can help with a problem.....
i have a 1990 supercoupe 35 anniversary edition. when i start my car after about 30s to a minuite of idling, the idle will drop to 500rpm or less and it will hardly run and will stall if i don't give it throttle. it does this when the engine is warm, or cold, but goes away after i start driving...mabey the first or second stop sign, you would have never thought there was a problem. i was told it might be O2 sensors...my thing is non-computer engines..not really famaliar with this stuff!! thanks for any help!

03-15-2009, 10:34 PM
A fellow Anny owner...cool.

While we're told that the computer controls the idle, there is a mechanical base idle adjustment that can need attention on higher mileage cars when worn components cause slight shifts in operational ability.

While I'm not sure this is your problem, if you feel comfortable under the hood, there is a simple process to perhaps help find out...

On the intake/throttle body, there is a small set screw (to open the plate more) on an arm, below, on the passenger side that determines the physical position of the throttle plate when closed (at idle). There are also two components in that same area - the TPS and the IAC. If you know what these are and wish to perform a simple test, proceed as follows.

Warm the engine first, then shut it off and pull the connector off of the IAC. Then start the engine...you may have to nurse the throttle a bit to get the engine to idle. It should idle, but expect it to be fairly low, say 500 rpm in PARK.

If not, you can attempt to adjust the throttle plate stop screw in minor steps until it does. Starting and stopping the engine each time of course. If this is all that is needed, you should maybe expect to add a turn at the most to bring the idle up to 500 rpm...again, you may need to nurse the throttle to get it to cooperate before it is willing to sit at that level. Keep track of any changes so you can undo if needed.

If you get this far, you can plug the IAC back in and the engine should start, run and idle without you touching the throttle. It should want to jump up a bit, then idle down to say 800 or so and then down lower to about 700 and then around 650 in drive.

This assumes no other issues such as a dirty air filter or MAF...vacuum leaks or faulty IAC or ECT, just as examples. Don't ignore that these can combine as well.

How old are the plugs & wires?
Have you tried to read engine codes?
Has any work been done on the engine lately?

03-16-2009, 12:27 AM
plugs and new, wires are old, have a big cone air filter and i attached the vaccum line to it, so no vaccum leaks..don't have check engine light on...so didn't think to check engine codes. not sure how either, i'm assuming it's a flash system but i could be wrong.
i swapped the tranny from an aod to a 5 speed...and i'm guessing it's started after that...not sure if that has anything to do with the idle issue. i got a 5 speed from the same year of car and apparently the comps are the same, so i'm thinking it has something to do with idle control...

now what you talking about, adjusting the throttle stop screw...that won't fix the problem when it warms up...cause then i'll be ilding at say 1200rpm..and that just opens the throttle more....if i'm understanding you correctly

how would you test the IAC valve and ECT?

03-16-2009, 01:23 AM
>cause then i'll be ilding at say 1200rpm..and that just opens the throttle more

The effect is not cumulative. Remember, the computer will step in - it will cycle the IAC to (help) control the idle and read the mixture when it hits the O2 sensors, etc. etc. But, there is a certain amount of 'at idle' throttle plate deflection required...if that isn't satisfied, the computer will struggle to make up the difference alone.

When you've gone thru the process I outlined, you'll see better how the electronics and hardware work together.

>how would you test the IAC valve

Disconnecting it will tell you outright if it is having any effect at all. Usually, they fail in such a way that you can't miss - they'll make a honking sound and the car won't be happy to idle. You can pull it and look inside for damage, carbon etc. Be careful to not destroy the gasket if you pull it to inspect.

>and ECT?

You can test it on the car by checking the ohms across the two terminals or volts by back probing while it is connected. See this link (http://www.fordfuelinjection.com/?p=28) for a value table for coolant temperature versus resistance, etc.

Again, please at least check the base idle before going too far inspecting sensors, etc. It's better to gather info and learn the systems first - it can be pretty easy to chase your tail with these cars if you aren't careful. Good luck with your Anny, in any case.

03-16-2009, 08:16 PM
thanks alot man, that makes alot more sense, i'll give her a try!