I know you're going to say, why are you worried about it but here goes anyway.
When I start my car cold it idles at around 900-1000 rpm for a few seconds then idles down to its programmed rpm of 750 rpm. BUT, when I start it when it is warmed up, the idle goes to 1400 then slowly drops to 1100 where it will stay for about 15-20 seconds then it will drop to 750 like it should. I feel it's a temp sensor of some sort but want to see what anyone else has to say. The vacuum is low at 12 inches but it is rock steady so I have ruled out a leak, plus my cam would cause a lower than stock reading. This is the one and only thing that is giving me any trouble, everything else is super fine. One more thing this condition exists if I wait 5 minutes or 5 seconds to restart.

 

I believe that mine does this.

 

I believe the default "park" position of the IAC is 160 steps. At 160 steps, that's equivalent to a pretty significant throttle opening.

When cold, it "needs" that much opening and fuel to be able to run [well]. When warmer, that has the effect of causing a higher idle at start-up. Combined with another algorithm that controls the number of steps it can back out per second, the idle drops in a controlled, slow behavior.

In older cars, this was the dashpot on a carb.

When I built my 383, I reduced the park steps to 100. I didn't really care for the huge start-up rev, didn't feel it was useful, and didn't like a higher rev until the oil pump got a chance to work.

OTOH, the stock programming delays startup until the engine has rotated a few times -- which also activates the oil pump. I liked a faster start, so I shortened the delay.

It's a complicated set of events that I'm sure GM thought out well. But, like the power of a 350 (vs bigger motors) some people aren't happy with the same decision -- and that's OK.

Your IAC may be sticking when warm/hot (and it gets "caught" still sitting near that park position at ignition). Try cleaning it or replacing it.

 

I have a spare IAC and it acts the same with both. Thanks for throwing some light on this, I am going to assume the guy who burned the chip programmed the idle the way it is and just learn to live with it.

 

As usual, with more information and looking at your profile, I see you have a built motor. To be honest, I don't know if you've had it done recently, for awhile, just noticed this issue or what.

But I will say there is a necessary adjustment to the throttle position screw with an increase in the cubes.

I can't remember all the logic, but I know the engine needs more air due to the increase in cubes. If you haven't cranked the idle position screw in, the IAC has to do WAY more work -- especially at start up.

You need to crank the idle set screw until the number of steps are 15 when hot and w/o the A/C or fan running. In that config, the IAC isn't forced to do more work -- and make itself as noticeable as you're seeing.

Right now, it's probably up in the 70-80 step range. So, when the car starts, it "thinks" it's not getting enough air, activates the IAC and kicks up the idle.

Based on your next reply, my next answer might be different!





(But I am sure your throttle blades aren't open enough if they're still at the factory setting!)

 

Gregg I have reset the idle according to procedure so many times since I got this car I can't even count that high. I have tried initial blade opening all the way from 700 rpm down to 450 and it has no bearing on this particular issue. I do have a lot of mods but it all boils down to a 383 with a very mild cam. I recently found out my BBK throttle body was causing some other issues so I am back to a stock 48mm which is in great condition, and this idle thing was present with both of them. I have had this issue now for years, and it was always something I kept telling myself I had to work on someday. Well someday came a few days ago but I have not accomplished a thing regarding any improvement. It may be the way it is but I just hate restarting after getting gas or whatever and having it rev up to 1400 rpm and the slooowly come back down to it 's set idle. What confuses me is why it does not do it on a cold start, it acts completely normal then.

 

The rpm method for setting idle didn't work for me. In fact, it made it worse. I used ECM tuning equipment to monitor the TPS and IAC counts. As such, I'm not sure if/what validity the rpm would have.

FWIW, My idle has to be at LEAST 800 to idle well at all warm-up temps. So, It could be my TB blades are open further? I will also say mine behaved similar to your until I found out about the need to open the TB more.

But, there are other possibilities. Another I can think of is a faulty temperature sender in the front of the intake. The ECM "sees" that one, while YOU see the one in the engine block. There are two. If the manifold sensor [erroneously] shows a cold temp at start-up, that could cause your issue.

I'm sure there are other possibilities -- including vacuum leaks (but I'd think you'd have that problem more often).


FWIW, As a WAG, I would crank the TPS screw til the idle speed starts to get too high. (Do this when it's warm and no A/C or fan.) Then back off the screw maybe 1/2 turn. Maybe 1/4? Then see what it does. The idea would be to get it open enough that the IAC really isn't "helping" anymore. If you don't have equipment to monitor IAC counts, that's the only way I can think of. The IAC is the only component that controls idle set-down.

 

If I set my idle at anything above 650-700 the engine will not idle below 1100 when warm. I'm going to try & set the idle with my AutoXray with guidelines you mentioned but I'm sure he results will be the same as I mentioned above. The temp sender at the front of the engine is what I am wondering about. I will test that sender and try your method then get back with the results.

 

Yeah, if an x-ray shows ECM data -- like IAC steps, that would do it.

FWIW, mine acted like your until I opened the blades.

Keep in mind, opening the blades just transfers where/how the air is getting in [at idle] not how much. That's until you get IAC counts to zero. past that, idle would get faster.

If you set your idle to 700 when warm, or even 800, it makes no sense to saw it won't idle below 1100...unless you're meaning during the warm up process -- before it gets to temp.

You should know that the ECM controls the idle. And, it does so via the IAC. That's what the IAC is, an IDLE controller (via control of air input). That's why it's called the idle air controller. When you get the blades open so far it NEVER drops to your desired idle, that's when it's open too far.

If you've got it programmed to idle at between 450-600 rpms, that might be part of your problem. I've got a very mild cammed 383 and I couldn't get mine to idle [well] that low.

Though it's able to idle at 600 when running, hot, and "stable". I tried to get it to 700 for awhile, but it's a bit "stumbly" until hot/stable again -- even after a hot restart. I could see a tuner using the IAC to compensate -- and get one to idle at 650-700. Maybe that was his technique? I ended up choosing 800rpms instead.

I'm not sure I could figure out how to set the idle screw w/o being able to see the IAC steps. If you can't see them and your idle screw/programmed idle speed has changed, I can see where you'd be having issues.

 

I'm going to try setting idle your way in the morning and I am going to check the temp sensor too. I will be reporting results later tomrrow.

 

Gregg, should I disconnect the TPS before opening the blades?

 

I'll be watching this thread as I am fighting the same problem!

 

Good Morning Gregg,
It is official, this thing has whipped me bad. There is no way it would let me set the blade opening like you mentioned so the idle issue has to be programmed into the chip itself like you said. I could not get the steps below 70 and the idle was over 1000 and was climbing higher every time I opened the blades some more. Right now they are at 40-45 and I set the idle the usual way with the engine at 400 rpm in drive. I am done fighting something that will not respond to anything I try. Thank you for all your help & suggestions.
desertvette, I can tell you this much, I have a whole bunch of spare parts I have amassed over the years, ecm, tps, iac, etc. I have not spent a dime on any of the things I have changed and all the parts are good & test out good. I have changed all the above mentioned parts with absolutely no change in the fast idle restart when hot. Since it does the vary same thing, every time, with different parts, I feel it has to be in the programming of the chip.

 

That idle vac at 12 inches seems way too low, though I am unsure of what cam you are running. You had mentioned "mild". Others may chime in on what your vac should be. Stock is 19 I believe.

 

ECM retracts the IAC everytime you shut it off to allow plenty of air for a quick restart (which also pumps more fuel into it), so to some extent, your symptoms are normal and easy to view with a Scan. Taking 15 to 20 seconds to return to Targeted Idle does seem prolonged - most are there in a matter of seconds. Anyway, I'd start with a Scan to capture Targeted vs Actual vs IAC counts. I seriously doubt it's a Temp Sensor Issue, though you could look at that value too along with the Park/Neutral and A/C signal or anything that could trick it into thinking there's a load that doesn't exist. AND, if it's still using a Cold Start Injector, troubleshoot whatever shuts it off - bi-metal thermostat? - as if it's taking too long to shutdown, then the revs are going to stay up until the fuel stops flowing.

 

(I wonder if your timing is set right?)

As you open the TB blades, the idle SHOULD stay the same. It's just that the air will be pulling from the TB more. And, the IAC should close down -- reducing the number of steps/counts you see.

By doing that, the IAC doesn't have to drop a far to "home position" and you see a less-pronounced float and it sets down to idle. This makes we wonder about your IAC and/or your chip.

Got any idea what idle speed is on your chip?

 

This is what I was getting at in post #3.

The amount of time to return to the low target of 15 steps is very much dependant on where the IAC is at start-up and how long the IAC stays there. It's supposed to be at 160 steps,,,but it's also supposed to shoot for 15 almost instantly. But, the programming does mandate a maximum travel rate for the IAC motor.

Maybe the chip tuner shortened your startup time (via fewer pulses) but didn't lower your IAC steps for "park". Seems like I saw similar behavior until I dropped mine to 100 (for the default).

Again, 160 steps is OK for cold starts since it "needs" more air/fuel to run at the same rpm. When it gets warm/hot, that makes it rev more.

After a buiild, sometimes people focus on things that were always there...but they just noticed it. I was like that. OTOH, maybe yours is a bit more excessive. But the things that would control it would be programming and IAC [proper] function. It's also possible the tuner entered a slower IAC slope -- just to get you down to your "stock-like" idle without it dying. Again, most builds need more idle. Since you don't have that, he may be getting you to it via the IAC. It sounds like a very good tactic IMO.

 

Temp sensor checked good. My Auto-Xray is just a basic unit and it does not tell me a lot, I could copy the readings but I doubt it would tell you anything. I'll check the cold start but it is not supposed to even be operating with the engine above, I believe 125 degrees so that should not be it. I can start it with the injector unplugged & see what it does.

Timing is bumped to 10 degrees. When I tried opening the throttle up the engine stayed at steady rpm for a turn or two then started climbing immediately and the steps never went down to speak of. Idle speed in the chip is 750 rpm.

As far as I can see it has to be the way the chip is programmed. Whatever it is, I have done all the messing around I'm going to do, with all the spare parts I have and the way nothing changes when I swap those parts tells me it is the tune. I am going to unplug the cold start & see what that does just for curiosity's sake.
Again thanks to you & SunCr for your assistance.

 

Midnight 85 - As you know from my other thread I have the same problem. One thing you should know is that I did not have this problem before I raised the min idle to 450 from 400. I don't know if this is a clue to anything or not.

Guys, my other thread on this is:
http://forums.corvetteforum.com/c4-t...need-help.html

My problem was intermittant stalling at idle due to a TSB on the ECU relearning the idle speed when going less that 5mph and TPS less than 2%. TSB was to raise the min idle to 500 rpm.

 

Thanks for the info but I have set my idle in so many different rpm ranges over the past few days It isn't funny. Actually I have got the best results with it at 400 rpm in drive. My chip is programmed for a 750 idle though so low speed stalling is not an issue.