High idle on 1984
Thread Starter
Intermediate
Joined: Oct 2020
Posts: 33
Likes: 2
High idle on 1984
Hello,
I am new to the forum and just bought a 1984 with a 4+3 trans.
I knew the car was idling fast when I test drove it but figured it was an easy adjustment--I was wrong.
It idles up to 1600 rpm maybe slowing to 1100. Also, when I blip the throttle, it takes a few second to idle back down. I followed the instructions to balance the TBs but that didn't help. I could not get the manometer to read 6 inches of H2O I believe since I am in Denver at 5280 feet. I could only get the reading to about zero inches of H2O. .After the adjustment, removing the IAC plugs, the engine wouldn't idle down. I have the throttle plates literally closed so I figure the problem is the IACs. The last owner just put in new ones along with the TPS. I set the TPS per instruction but found if I moved it around, I could get the idle a little lower, so I don't know what the voltage is reading right now.
I am wondering if the IACs are not getting the correct signal to close. Is this possible? What sensor could be bad that is not sending the signal to the ECM for the IACs to close?
I hope this makes sense and would appreciate any help possible. it is difficult to make a smooth shift when the engine is idling so high.
Thanks!
I am new to the forum and just bought a 1984 with a 4+3 trans.
I knew the car was idling fast when I test drove it but figured it was an easy adjustment--I was wrong.
It idles up to 1600 rpm maybe slowing to 1100. Also, when I blip the throttle, it takes a few second to idle back down. I followed the instructions to balance the TBs but that didn't help. I could not get the manometer to read 6 inches of H2O I believe since I am in Denver at 5280 feet. I could only get the reading to about zero inches of H2O. .After the adjustment, removing the IAC plugs, the engine wouldn't idle down. I have the throttle plates literally closed so I figure the problem is the IACs. The last owner just put in new ones along with the TPS. I set the TPS per instruction but found if I moved it around, I could get the idle a little lower, so I don't know what the voltage is reading right now.
I am wondering if the IACs are not getting the correct signal to close. Is this possible? What sensor could be bad that is not sending the signal to the ECM for the IACs to close?
I hope this makes sense and would appreciate any help possible. it is difficult to make a smooth shift when the engine is idling so high.
Thanks!
Pro
Joined: Apr 2003
Posts: 552
Likes: 13
From: Oz
To prove air is going through the IAC's you can block the irregular shaped hole above them with your thumbs and see if the idle comes down.
After being tampered with, the IACs need to be seated to reset the closed position. I can't remember the numbers, but I believe you have to drive it above 40mph for this to occur. The ECM then drives them all the way closed and resets the zero position.
You could also have a vacuum leak somewhere. Check all the hoses. Disconnect and plug the vac hose to the brake booster too as it could be pulling in air though a failed booster diaphragm.
Also check around the top of the intake lid.
Good luck
After being tampered with, the IACs need to be seated to reset the closed position. I can't remember the numbers, but I believe you have to drive it above 40mph for this to occur. The ECM then drives them all the way closed and resets the zero position.
You could also have a vacuum leak somewhere. Check all the hoses. Disconnect and plug the vac hose to the brake booster too as it could be pulling in air though a failed booster diaphragm.
Also check around the top of the intake lid.
Good luck
Pro
Joined: Apr 2003
Posts: 552
Likes: 13
From: Oz
Safety Car
Joined: Nov 2005
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From: Arizona - If you don’t know CFI, STOP proliferating the myths around it...
Like Ben has said, the IAC is a controlled vacuum leak. You say you tried balancing the TBs, did you reset the TPS sensor back to .525v? If not, the ECM has now real idea where idle is in relation to throttle plate position. It also does like Ben mentioned, must be driven over 35mph for a couple miles and the IACs should be relearned/reset. If not, you have other issues and I would look for a vacuum leak at the TB base gaskets, top plate gasket or elsewhere. GL and post results. 
Last edited by Buccaneer; Oct 16, 2020 at 03:42 PM.
Thread Starter
Intermediate
Joined: Oct 2020
Posts: 33
Likes: 2
Thanks for the reply. I did set the TPS to .525 volts plus or minus .003 volts. I plugged the vacuum line going to the air cleaner since I found a vac leak. I have the initial advance set to 10 btdc--I think I'll put it back to 8 degrees. I suppose that won't make much difference though. I'm going to torque everything down tomorrow and check further for vacuum leaks but it seems pretty tight so far.
