I just picked up an 01 z28, all stock except for a level 2 4l60e and a 3800 stall nitrous converter (installed by prior owner). On a cold start the engine won't idle unless I keep my foot on the gas until the engine warms up.

I installed a known good iac from my brothers 98 z28 and it started up and idled perfect on a cold start. Then I parked it for a few days to fix the water pump, at which point the idle issue returned even with my brothers iac. So I cleaned out my IAC and installed it, everything was great, I parked the car again for a couple of days and then the cold start idle issue returned again. I cleaned the IAC again today, manually pushed the plunger in about an 1/8th inch and there's a slight stumble at startup but it corrects itself and starts to idle fine.

I'm new to these cars so you'll have to pardon my ignorance. I've noticed there's some sort of air pump running at cold start up, it has to do with the thick rubber hose that runs along the driver side of the car to the front bumper. It seems when that shuts off the RPMs are able to stabilize easier. I can't figure out what's going on with the idle. I have no check engine light. I'm at a loss as to what I should do. I suspect idle speed is too low for the IAC to correct on cold start up or maybe iac is bad?

When I had a fox body 5.0 I used to be able to set my base idle by disconnecting iac, adjusting throttle blade to set RPMs to about 500, reconnecting IAC and confirming TPS voltage is between .9 and 1.0 volts. Is there any such procedure for resetting base idle on these cars? I keep searching these forums trying to find anything on setting idle speed or base idle but I'm not finding a whole lot.

 

A procedure that works on 99% of application is this:

0. Fully warm engine
1. unplug/disable IACV so that no air can enter the engine through it if possible. Sometimes there is a hose you can crimp shut, or you can change it in the computer to be fully shut. The idea is you want NO computer control for the following steps.
2. now set the minimum idle RPM using manual adjustment, either at the TB or IACV (depends on the kind of engine, some have both adjustments available) like you did with the mustang.
3. the minimum idle RPM is approx 100rpm below your desired idle speed, fully warm. So if you want the engine to idle at 850rpm with the IACV controlling it, then your fully warm setting should be 750rpm, that way when you re-plug in the IACV it only has to add 100rpm worth of air.

4. a mistake would be setting the minimum to something like 500rpm. That would force the IACV to open much more to let in far more air, and it will cause the computer difficulty moving it open/closed trying to stabilize the idle. The IDEA here is to make the IACV do as little as possible, and we do that by getting the warm idle minimum airflow as close to the desired idle speed as possible.

5. obv plug everything back in, your done.
6. some engines have multiple cold start idle adjustments. for example the sr20det, has a wax filled element which sits in the coolant, and as the coolant warms up it expands and lowers the idle speed, independently of the IACV control, which also stabilizes the idle by adding/removing airflow. The factory did this for the reason I just pointed out: to help make the IACV do as little as possible. If the IACV is having to make big movements/big changes it will be less reliable at holding the idle stable.

 

Do you have the ability to scan and log IAC counts, tps voltage, etc? You're working blind otherwise. I doubt the IAC is causing the trouble.

 

I'm assuming I can probe the back of the tps with a volt meters on these cars to check voltage but I don't have anything to check iac counts. Is there a specific scan tool I need that can do that or can any advanced obd2 scan tool perform those functions?


Thank you, I will give this a try later today.

 

Some OBD scanners have the ability to scan most parameters. I used my HP Tuner pro as a diagnostic tool. You can backplane tps with a multimeter, but I bet it's fine. Don't do what the other guy said, it's wrong, won't work, and potentially makes things worse. Get a scan tool and see what data you have first.

 

I back probed the TPS and you're right, it's fine. With throttle closed voltage is between .65-.7, throttle open and voltage holds steady at about 4.4v. I'm beginning to think the IAC is not the problem either, especially since I swapped on one that is known to be good and it still doesn't fix the issue. Checking IAC counts may just be a waste of time.

I'm getting very frustrated with this. All vacuum lines are connected according to the vacuum diagram. PCV hosing appears to be all new OEM. I swapped to a different MAF and IAC, neither made a difference. When I started it this morning, the cold start condition felt like a slight misfire that goes away when that air pump shuts off. I'm at a total loss where to go next. Maybe pull spark plugs and check compression, yank the intake and make sure the gasket is good. I don't know. I'm not going to bother with manually adjusting the throttle body screw, I found multiple threads earlier as well saying not to do that.

 

Intermittent problems like this are the worst. Get a scanner, it will help. You're going to have to check everything one by one otherwise.