OK so after fooling with the idle set screw, not being able to get Idle set to 550. Normally Idle stays in the 1000 range when in drive and 1300 when in park.

When I get time I have tried the idle reset and failed! I was not able to keep motor running with out giving it gas once the IAC was disconnected. So yesterday I tried something different, I turned the idle set screw forward, since I had it back.

What I am thinking is, since idle set screw was so far back the motor could not breathe so by giving it gas it would open up the throttle body and let air in. Well after moving screw forward like 2 turns I noticed the motor did not stall. Unfortunately I did not have time to finish the job yesterday because I had to run. But I have noticed the car still idles high but better than before, now it seems when I come to a stop the Idle slowly comes down to 1000. Not like before when it was just @ 1300 in drive and 1500 when cold.

What I am wondering... before since the idle set screw was so far back, throttle blade closed. Was the ECM setting the idle to like a default trouble safety seting when it is not getting enough air in the throttle body it opens up the IAC to let more air in because it sees trouble?

I hope to go home today and set idle since I will have more time, just wondering should I continue to move set screw forward to see what happens? Mostly does this make sense?

thanks!!

 

..... Check/set the timing , then set the minimum idle per the instructions in the factory service manual , then readjust the TPS ... if there are no other tune or mechanical issues , your car should idle as intended .........

 

That screw does NOT set the idle speed. The ECM does that by moving the IAC. What that screw does is set the AIR FLOW through the throttle body at idle. You need to do a minimum idle adjustment:

http://forums.corvetteforum.com/1570563689-post1.html

There is some controversy over whether or not to disconnect the EST connector. Leave it connected.

 

Timing has been set to 8* with no pinging at WOT. TPS was set to .54 and 4.42v at WOT. Can not get the 4.5 at WOT.

Yes, I have done the minimum idle adjustment several times and nothing, If I back the idle set screw out too much car stalls. when I have it so the RPM's are roughly below 1000 motor runs rough and seems idle is searching. After I connect the IAC and drive car for 15min idle is exactly like it was before. no change. Should I try replacing TPS??

thanks again!!

 

That 4.5 number is an Internet myth. I looked at the code in my ECM and the way it works is that the ECM converts the TPS voltage to a percentage. It considers anything over 70% to be WOT. That calculates out to 3.5 volts (based on a 5 volt reference voltage). You're fine.

 

Thanks for the info, so then The tps is good, but Idle still off? Any way to check ECM?

 

CFeria,

It really sounds like you have an IAC issue, the IAC an cause a high or low idle speed. To be quit honest, its very hard to set the minimum idle at 450-500 RPM. Without a scanner its difficult.

I set my minimum idle about .040 past initial contact on the throttle control arm (not in any manual), its extremely important to have the TPS set at the .54 vdc so the ECM can control the idle function (My TPS is set at .61 vdc for performance)

 

The only problem I had is that the engine kept dying right after I started it. I screwed in the adjusting screw a couple of turns and then had no problem. I did set mine with the EST connector unplugged. From what I've heard, that can make it hard to keep the engine running. Some FSMs say to unplug it, others don't.

 

As always Thanks for every reply,

Now, 'Vett1990' I really do not think it to be the IAC, I say this because I did replace the IAC, as well as clean the IAC passage in the TB. After replacing IAC, and after disconnecting the battery for .30 min. The motor ran exactly the same, pissed me off because that means there was nothing wrong with the old IAC. The only other thing I can think of it the ECM???

 

Be a good idea to check your EGR (exhaust gas recirculation valve)
If it jams and does not close that can cause idle issues and stalling.

I have no trouble setting the base idle speed to 400 rpm in drive, and yes i do disconnect the timing bypass connector. The ecm tries to adjust the timing and that increases the rpm's, Is the brake effort the same ? if the hose or the brake booster is leaking that will also affect idle.

Another thing, the harmonic ballancer outer ring with the timing marks on it can slip, setting your timing to 6 or 8 degrees before top dead center may not be correct if it has slipped. I would move the distributor anticlockwise slightly till you hear it ping on acceleration, then back it of slightly and the timing will be in the ball park.
Have you checked for any codes? also did you clear the codes by disconnecting the battery for at least 10 seconds.

Is the fuel pressure ok?

The idle should be like this, start the car and it will go to a high idle 1,200 rpm for a few moments and slow down and stabilise to 600 rpm. The ecm does have a temperature versus idle offset and hot it will vary a little from hot to cold.

Good luck

 

'gerardvg' Thank you, I have not checked EGR, but I will. Please tell me what I should look for?

To date, I have set timing to 8* I do remember when I was checking the timing it was set to 6* this was with the motor warmed up after a 30+ min ride, timing bypass disconnected, parking brake up and car in drive. When checked it was good, it was showing 6* so I thought I would bump it to 8*. Then after this I shut down, put jumper in ecm connector under dash on driver side, 2 top right connectors, waited 30 secs then disconnected IAC, shut down, removed jumper, removed timing by pass again, then restarted and tried to get idle as low as possible but once I dropped below 1000 the idle would begin searching from about 700 to 1000. After reconnecting IAC and driving for 10+ min. notice the idle is high like 1500 but then drops after a while back to what is was originally which is about 1100 in drive and 12-1300 in park.

I do notice when I start it up each morning when it is cold the idle goes up to about 1500 but stumbles a bit until it is warmed up in about 5 min. then Stays at about 1200 in drive until fully warmed up where it drops to 1100+ or - but it is steady after motor is warmed up.

Have checked for codes but have nothing, even before doing a reset by disconnecting the bat I did check for codes but had none.

 

With the IAC disconnected the ECM cannot control idle. Sounds like an air leak somewhere??

BTW, have you used TB cleaner on the TB & throttle plates?
If not already cleaned you might remove the IAC and carefully (leave no lint) clean the bore to remove any carbon buildup.

If all else is OK, the engine should idle down to base idle of 450rpm, though may be on the verge of stalling.

A vac gauge might help locate the problem as it sure sounds like an air leak somewhere.

 

Correct the IAC can not control Idle, but if it was a vac leak I don't think it would die once the idle set screw is backed out a certain amount, once the throttle plates go back far enough the motor doesn't get enough air. If there was a vac leak I don't think it would die and I would probably hear the air from the vac leak since its the only place it is getting air from since the throttle plates are shut closed? Is there a way I can disconnect main vac lines and run motor to rule out the vac leak? I have tried several times going over the vac lines and the seal around TB by spraying with tb cleaner as well as with propane, but got nothing rpm never went up or down, stayed the same. Only other thing I can think of is what was mentioned above about egr? But don't know how to test for that?

I have used tb cleaner a tooth brush and a rag to clean TB and port or location where iac screws into. As weel as even tried seafom thru the brake booster vac line.

Idle only hunts/stumbles at start up when cold. Once warmed up Idle stays between 1000 thru 1300.

 

Maybe the tach in your car is off? They aren't the most accurate at the best of times.

Give a light tug on the wires going into the IAC connector. One of mine had broke and was making contact intermittently and was causing my car to idle high at hot and not idle at all cold. Since you have disconnected the connector it may also be a good idea to Mae sure a pin hasn't punched back enough to make a loose connection.