My 85
Thread Starter
Intermediate
Joined: Jan 2008
Posts: 25
Likes: 0
From: El Toro California
My 85
when i start it the idle runs up and down like its hunting for a set speed
is this the throttle body like some one told me (I honestly dont know how that would work) or is it the Maf?
it runs between 500 to 900 up and down
and will the stock tach on the digi dash run higher than 5000 on the number read out?
and will the speedo go to 200?
ive got a bet on it with a friend....
Thanks guys
is this the throttle body like some one told me (I honestly dont know how that would work) or is it the Maf?
it runs between 500 to 900 up and down
and will the stock tach on the digi dash run higher than 5000 on the number read out?
and will the speedo go to 200?
ive got a bet on it with a friend....
Thanks guys
Pro
Joined: Mar 1999
Posts: 678
Likes: 0
From: Oslo, Norway
Hi I have an 85 as well and i was a novice in this. I had the same as you that the idle hunted.
There are a number of things that you should investigate.
Check for vacuum leaks: Check the hoses and plastic tubes under the plenum. If thats ok spray some start gas or propane over the runners at the manifold and see if there is a rpm change. If so there is a leakage there.
I took off the throttlebody and disassembled it and rinsed it for carbon deposites and also the IAC valve. Rinse the seat inside and ensure that the IAC operates. Oil the throttlebody shaft in the housing .
Check the throttle positioning Sensor for correct voltage .54 when the throttlebody is installed.
Do the minimum idle air adjustment as in the book and recheck the TPS voltage.
Check also that you have the right ignition timing following the procedure in the book. It should be 6 degrees BTDP.
If the idle is hunting now it might be the EGR system.
I had in addition to vacuum leakage a faulty EGR valve placed under the plenum. To check the mebrane I applied vacuum and it fell fast.
It shall hold the vacuum for about 20 seconds. When applying vacuum it should be possible to see the valve itself goingup and down all after vacuum or not.
Good luck
There are a number of things that you should investigate.
Check for vacuum leaks: Check the hoses and plastic tubes under the plenum. If thats ok spray some start gas or propane over the runners at the manifold and see if there is a rpm change. If so there is a leakage there.
I took off the throttlebody and disassembled it and rinsed it for carbon deposites and also the IAC valve. Rinse the seat inside and ensure that the IAC operates. Oil the throttlebody shaft in the housing .
Check the throttle positioning Sensor for correct voltage .54 when the throttlebody is installed.
Do the minimum idle air adjustment as in the book and recheck the TPS voltage.
Check also that you have the right ignition timing following the procedure in the book. It should be 6 degrees BTDP.
If the idle is hunting now it might be the EGR system.
I had in addition to vacuum leakage a faulty EGR valve placed under the plenum. To check the mebrane I applied vacuum and it fell fast.
It shall hold the vacuum for about 20 seconds. When applying vacuum it should be possible to see the valve itself goingup and down all after vacuum or not.
Good luck
Team Owner
Joined: Feb 2006
Posts: 24,460
Likes: 7
From: sw Ohio
if theres no loose or broken vacuum lines (or missing!) then the likely culprit is the IAC valve. I had a problem with the PCV vacuum line going into the manifold on my '85. It was old, brittle and would mess up the idle from time to time. Eventually it would just start up and die 
Drifting
Joined: Apr 2007
Posts: 1,339
Likes: 2
From: port charlotte fl
You can check the IAC valve by doing this.Get the car up to operating temp.Ignition on.Jump the A+B connectors in the aldl which will close it completely.Your fan should start when you do this.Remove the jumper clip.Ignition off.Unplug the IAC four pin connector and start the car.You may have to adjust the idle screw to give you a steady idle around 500rpm.If you can achieve a steady idle doing this you have a problem with the IAC valve.
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Team Owner
Joined: Oct 2004
Posts: 81,241
Likes: 3,063
From: altered state
St. Jude Donor '05
IAC valve is on the lower pass side of the throttle body. Has a plug and looks like a large nut that spins off. They get all carboned up. Once removed blow the passages out with some TB cleaner too.
Instructor
Joined: Jan 2008
Posts: 163
Likes: 1
From: Idianapolis Indiana
After you check all your vacuum lines just do the idle air adjustment on your throttle body. Exactly the way the book says step for step. Make sure you use the right numbers for your car. they all have different minimum airflow settings.
Le Mans Master
Joined: Sep 2006
Posts: 6,050
Likes: 147
From: Phoenix AZ
2021 C4 of the Year - Modified Finalist
Iac
the best way to check for a functioning IAC is to let the car idle, and turn on the air conditioning. If the freon is full, the compressor will load the engine, and the IAC will open to speed up the idle to keep it from dying.
then do the reverse. turn off the A/C, and if it starts hunting, the IAC seat is carboned up.
If the idle smoothed out with the AC on, it's the IAC.
If it didn't smooth out, unplug the ESC and see if it smooths out. That made my engine hunt for an idle.
Chris
then do the reverse. turn off the A/C, and if it starts hunting, the IAC seat is carboned up.
If the idle smoothed out with the AC on, it's the IAC.
If it didn't smooth out, unplug the ESC and see if it smooths out. That made my engine hunt for an idle.
Chris
