1985 Corvette setting the idle
Thread Starter
Drifting
Joined: Feb 2013
Posts: 1,462
Likes: 48
1985 Corvette setting the idle
Hi everybody, I'm trying to set the idle on my 1985 Corvette 4 plus 3. Question do I use the idle screw on the throttle body to set the idle and then adjust the TPS? I have an FSM but a friend has it.
Thank you.
Thank you.
Le Mans Master
Joined: Sep 2013
Posts: 6,192
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From: near the thumb in the mitten
There is a procedure in the the stickies. You need to verify intial timing is correct. You need to be sure throttle bores are clean of carbon build up. Then check minimum air rate or idle 450 +/- 50 reset with screw if necessary. Then TPS. If you have scanner you can verify IAC counts are 20-25.
ECM controls idle speed unless you turn screw far enough that it can't. At that point TPS may not adjust back to .5v
Get your book back.
ECM controls idle speed unless you turn screw far enough that it can't. At that point TPS may not adjust back to .5v
Get your book back.
Thread Starter
Drifting
Joined: Feb 2013
Posts: 1,462
Likes: 48
Thank you for the help. The Idle should be 450? To 500? Am I understanding that correctly? The IAC presently says about 135. There are no error codes. Yet the car has a bad idle. The throttle body is clean. Presently it is running rich. The MAF is adjustable and I have it to the point where it says about 12 g/sec. I will try your suggestion and I will try to get my book.
thanks again
thanks again
Le Mans Master
Joined: Feb 2005
Posts: 6,479
Likes: 565
From: Clearwater Florida
..... Adjust the idle stop screw on the throttle body to get about 15counts on the IAC … don't forget to reset the TPS … check IAC counts in gear 20-30 is good … your MAF is too high should be approx. 7gps … my YukonXL2500 with an 8.1 idles @ less than 12gps … do the math to find the correct # for your engine CID and RPM …..
Le Mans Master
Joined: Feb 2005
Posts: 6,479
Likes: 565
From: Clearwater Florida
….. The IAC is an air valve … so are the throttle plates … the IAC adjusts its air bleed to maintain the idle as necessary … what you want to accomplish is having the IAC in a range where it can react to any operating conditions and still do its job … at 135 its near the end of its ability to control idle speed …..
Le Mans Master
Joined: Feb 2005
Posts: 6,479
Likes: 565
From: Clearwater Florida
….. The actual idle speed is controlled by the ECM … not the throttle plate screw … if the IAC counts hit zero just back the screw out until you see a rough average around 15 counts …..
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Thread Starter
Drifting
Joined: Feb 2013
Posts: 1,462
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C409, The car was running rich. My mechanic last night tested disconnecting the Burn Module. I have not heard back on how it worked. The MAF was reading in the 13/14 range. The car has an adjustable MAF so we now have it down to 10/11. And the car is not running as rich. We are going to try and get the MAF lower to the recommended 7/8. The O2 sensor is reading 350/425. And the IAC is steady at 135. Could the IAC be the problem?
If I disconnect the TPS. Can I set an idle with the throttle body adjustment screw. Then plug in the TPS and adjust it to 0.54 volts.
Keep in mind everything the TPS, MAF, MAT, CTS, Fuel injectors, fuel pump regulator, wires, cap, plugs, coil, knock sensor and O2 sensors.
I want to thank you all the help.
If I disconnect the TPS. Can I set an idle with the throttle body adjustment screw. Then plug in the TPS and adjust it to 0.54 volts.
Keep in mind everything the TPS, MAF, MAT, CTS, Fuel injectors, fuel pump regulator, wires, cap, plugs, coil, knock sensor and O2 sensors.
I want to thank you all the help.
Le Mans Master
Joined: Feb 2005
Posts: 6,479
Likes: 565
From: Clearwater Florida
..... As you open the throttle blades the IAC counts will drop … @135 too much air is being inhaled through the IAC at idle … the ECM controls idle speed by way of ignition timing and IAC to a pre set number in the tune … A tuner can change the idle speed but you can't change it just with the screw … you tune the idle with the screw to obtain the most effective range of operation of the IAC … if you leave it at 135 there is very little it can do to increase idle speed in cold weather or to maintain idle under certain conditions …..
Thread Starter
Drifting
Joined: Feb 2013
Posts: 1,462
Likes: 48
The car is idling rough. I have TunrrPro software and it is saying that the MAF is 12/14 g/sec and the IAC is 135 counts. The MAF is adjustable and we are trying to lower the MAF to below 10 g/sec. I asked my mechanic if we should disconnect the MAF. This morning he checked every wire running from TPS, MAF, MAT, O2 sensor all of which are new.
So this afternoon he was willing to disconnect the IAC. He said he needed to remove it to set the plunger. It broke. But, we ran the car and it idled better. The MAF read 12/14 g/sec and the idle was 800 but it did run smoother.
We are going to get a new MAF and see if that's the problem.
I have good fuel pressure 35/40 . And I have good vacuum 20 inches.
We have a bad idle. Obviously new plugs, wires, cap, rotor, coil and module. Any and all ideas are welcome. Thank you
So this afternoon he was willing to disconnect the IAC. He said he needed to remove it to set the plunger. It broke. But, we ran the car and it idled better. The MAF read 12/14 g/sec and the idle was 800 but it did run smoother.
We are going to get a new MAF and see if that's the problem.
I have good fuel pressure 35/40 . And I have good vacuum 20 inches.
We have a bad idle. Obviously new plugs, wires, cap, rotor, coil and module. Any and all ideas are welcome. Thank you
Le Mans Master
Joined: Feb 2005
Posts: 6,479
Likes: 565
From: Clearwater Florida
The car is idling rough. I have TunrrPro software and it is saying that the MAF is 12/14 g/sec and the IAC is 135 counts. The MAF is adjustable and we are trying to lower the MAF to below 10 g/sec. I asked my mechanic if we should disconnect the MAF. This morning he checked every wire running from TPS, MAF, MAT, O2 sensor all of which are new.
So this afternoon he was willing to disconnect the IAC. He said he needed to remove it to set the plunger. It broke. But, we ran the car and it idled better. The MAF read 12/14 g/sec and the idle was 800 but it did run smoother.
We are going to get a new MAF and see if that's the problem.
I have good fuel pressure 35/40 . And I have good vacuum 20 inches.
We have a bad idle. Obviously new plugs, wires, cap, rotor, coil and module. Any and all ideas are welcome. Thank you
So this afternoon he was willing to disconnect the IAC. He said he needed to remove it to set the plunger. It broke. But, we ran the car and it idled better. The MAF read 12/14 g/sec and the idle was 800 but it did run smoother.
We are going to get a new MAF and see if that's the problem.
I have good fuel pressure 35/40 . And I have good vacuum 20 inches.
We have a bad idle. Obviously new plugs, wires, cap, rotor, coil and module. Any and all ideas are welcome. Thank you
….. Carefully re-read posts #2 & #4 …..