How to lower idle rpm on a 1984 C4
#1
Advanced
Thread Starter
How to lower idle rpm on a 1984 C4
Specifically how do you get to the idle adjusting screw ????
I took the top of the air cleaner housing off..
Does the bottom of the air cleaner housing need to be removed to adjust the idle ---- or what ???
I took the top of the air cleaner housing off..
Does the bottom of the air cleaner housing need to be removed to adjust the idle ---- or what ???
#2
Team Owner
Pro Mechanic
Idle speed should be controlled by the Idle Air Control valve & ECM. If the idle speed is beyond the ability of the ECM/IAC to control it, then you most likely have a sizable vacuum leak. Fix the vacuum leak.
.
Last edited by Tom400CFI; 03-16-2018 at 04:43 PM.
#3
Advanced
Thread Starter
Thanks I appreciate all the advice I can get. I saw images where adjustment can be made straight down after removing a plug. My shop manual has the adjustment from the back almost horizontal. I will take the bottom of the air cleaner off and get a good tach. I like the Carter AFB, I have about a half dozen, The Holleys remind me of flushing a toilet.
This C4 is the first gas injected, I have several diesel injected equipment.
This C4 is the first gas injected, I have several diesel injected equipment.
#4
Team Owner
Pro Mechanic
#5
Safety Car
The idle speed is controlled by the computer. The adjustments on your throttle bodies are used to balance the two throttle bodies. I have 240,000 miles on my '84, and those adjustments have never been touched! That's why they welded them at the factory!
Tom is correct. Find your vacuum leak. Think of it this way: If things are right, the computer will control both fuel and air to give you the correct idle speed. If you have a vacuum leak, the computer can't control the air flow, so the only thing it can do to maintain the proper air/fuel ratio, so the engine will run, is to provide more fuel, so the idle speed will therefore be too high! Way different than a dumb carburetor!
#6
Advanced
Thread Starter
Thanks Hot Rod Roy and Tom
The manual I got was downloaded from Tradebit for $19 I would not recommend it as the images are poor and apparently some of the data is incorrect. Maybe the computer chip was reprogrammed.
I will check for vacuum leaks, WD 40 works good for that, although cheaper I don't like to use propane, especially in boats.
The reason I want to lower the rpm is it shifts hard into reverse and forward plus I don't like it idling that high.
The manual I got was downloaded from Tradebit for $19 I would not recommend it as the images are poor and apparently some of the data is incorrect. Maybe the computer chip was reprogrammed.
I will check for vacuum leaks, WD 40 works good for that, although cheaper I don't like to use propane, especially in boats.
The reason I want to lower the rpm is it shifts hard into reverse and forward plus I don't like it idling that high.
If that's what your shop manual suggested, you need a new manual! What book do you have? If it's a Chilton or Haynes, please relegate those to the bottom of your bird cage.
The idle speed is controlled by the computer. The adjustments on your throttle bodies are used to balance the two throttle bodies. I have 240,000 miles on my '84, and those adjustments have never been touched! That's why they welded them at the factory!
Tom is correct. Find your vacuum leak. Think of it this way: If things are right, the computer will control both fuel and air to give you the correct idle speed. If you have a vacuum leak, the computer can't control the air flow, so the only thing it can do to maintain the proper air/fuel ratio, so the engine will run, is to provide more fuel, so the idle speed will therefore be too high! Way different than a dumb carburetor!
The idle speed is controlled by the computer. The adjustments on your throttle bodies are used to balance the two throttle bodies. I have 240,000 miles on my '84, and those adjustments have never been touched! That's why they welded them at the factory!
Tom is correct. Find your vacuum leak. Think of it this way: If things are right, the computer will control both fuel and air to give you the correct idle speed. If you have a vacuum leak, the computer can't control the air flow, so the only thing it can do to maintain the proper air/fuel ratio, so the engine will run, is to provide more fuel, so the idle speed will therefore be too high! Way different than a dumb carburetor!
Last edited by nlneilson; 03-16-2018 at 08:36 PM.
#7
Team Owner
Pro Mechanic
How high is "high"? Should idle ~600 RPM or so.
#8
Advanced
Thread Starter
800 + sometime. when i blip the throttle it will stay at 1200 to 1400 for a short time before dropping below 1000
the dash reading is inconsistent, I need a good tach.
I called a local AAmco about the shifting, he has worked on many Vettes, he will do an initial check at no charge.
#9
Melting Slicks
Here is a link to some great pictures showing the procedure for balancing the throttle bodies. The article is for a 82 Crossfire but the same basic procedure. It will show where the plugs and adjustment screws are if you ever need that:
http://www.thecubestudio.com/Crossfi...yBalancing.htm
On my 84 I had vacuum leaks all over my engine including the top plate of the manifold. I got my idle back by fixing these leaks.
I did re-balance the throttle bodies after reaming out the throttle shaft bores on my throttle bodies and I installed bushings for the shaft to run in.
http://www.thecubestudio.com/Crossfi...yBalancing.htm
On my 84 I had vacuum leaks all over my engine including the top plate of the manifold. I got my idle back by fixing these leaks.
I did re-balance the throttle bodies after reaming out the throttle shaft bores on my throttle bodies and I installed bushings for the shaft to run in.
Last edited by zachaeous; 03-17-2018 at 01:50 PM.
#11
Safety Car