Okay guys I have this weird high idle problem and need ideas. (Stock 305 TPI)When the car is cold it idles at 1250-1500 rpm. After I drive it a bit and it warms up it will drop down to where it is supposed to be. However it will only drop down when I'm sitting at a stop sign or red light. It's idling high then just drops down. I can drive to the store and only be in there for a few minutes and when I start it hot, it will do the same thing. I can at that point the car in gear and wait 10 seconds or so and it will drop down. Only seems to drop when it's warm and in gear stopped. My commute to work is only 1.5 miles and I let the car idle for 4-5 minutes before I drive to work. When I pull into the parking lot and stop and put it in reverse to back into a parking spot it usually drops down to proper RPM so it doesn't take a real long time.

I have smoked the engine, no vacuum leaks. I have done the proper adjustment to the IAC, I have adjusted and checked the MAF. The CTS is new and working properly. I have tried a different ECU with the same exact symptoms. I have run a datalog when cold and Tuned Performance tells me the ECU target RPM is 700 but it's running at 1500. I'm at my wits end here. The car runs so damn good but this is driving me nuts. I'd be happy to send the datalog to anyone who'd like to have a look.

Any suggestions would be appreciated.. Thanks!

Edit: IF any of this matters I have removed all smog crap and the CAT. AND, back when I was getting the car running I found out the throttle body, to the best of me knowledge is from a C4 Corvette. It was missing a vacuum port so I drilled it out and added a port to hook iy up properly. Only things that be UN stock.

 

If you can read the datastream, monitor the desired/commanded RPM versus engine RPM.

You may discover that the ECM is telling the engine to idle at that RPM for the first 20 seconds (or longer) on startup.

https://www.thirdgen.org/forums/tpi/...ml#post6218379

 

Check your TPS position, and back off of the fast idle screw in small increments.

It more than likely was increased by the previous owner.

- Rob

 

I've checked the TPS and it's dead on. Plus I think you are supposed to set the minimum idle at the IAC when it's hot right? When it's hot it idles fine, as long as I put it in gear first then let it drop.

 

You mean read the data stream on the TunerPro while the engine is running?

 

Alright, because you didn't mention checking the TPS% in the first post. Your startup in the bin is programmed to increase RPM momentarily, then back down depending on coolant temperature. If you find it is in fact dropping down to a normal RPM, but dropping down randomly, and at various times, did you at least try cleaning the IAC as opposed to just setting it?

- Rob

 

My Bad, I was 3/4's way through a 12 hour graveyard shift when I posted this. I typed MAF, meant TPS. Also forgot to mention the IAC is new. It is not random at all. It drops down after a little run time and only when it's in gear, stopped. After it's warmed up a bit. While datalogging it cold it read 80' as the coolant temp which would have been right on as it's pretty warm here and it was hot in the garage.

 

What is your IAC steps and TPS percentage reading when you're datalogging during that higher than normal idle you're referring to?

- Rob

 

You are losing me... Is that something I can see live using TunerPro? Which screen do I use to see that info? Total rook when it comes to that datalogging stuff. Can I email you a log?

 

Your TPS percentage and IAC steps need to reflect a balance between the two. With a throttle body from a C4, it is possible that the data is skewed heading to the ECM, and it's possible someone tampered with the fast idle screw and raised it, and the ECM is trying to compensate. Make sure the fast idle screw and throttle blades are fully closed, then adjust and set your TPS and IAC accordingly. I just want to see what both are reading before (high idle) and after (normal idle) anyway after the vehicle has already been fully warmed up with the problem still persisting. Attach your datalog here in the thread so everyone can take a peek

- Rob

 

The mask is $6e, I can't figure out why the desired commanded idle speed is 700rpm and the iac counts are high. The tps voltage seems normal.

 

Just curious, check to see if there is any obstruction between the C4 Throttle Body and your plenum, make sure the IAC passageway is lined up correctly. I haven't looked at a C4 TPI tb in years, and don't remember if there were any issues w/compatibility when using them with our plenums...

- Rob

 

 

Here's a link to the thead I posted about three years ago. It also has a few pics of the TB.
https://www.thirdgen.org/forums/tpi/...-body-pic.html

Also, Tuned maybe you can post one of my hot run datalogs if you still have it. My daughter lost the charger to my computer that has them and it's dead for now.. Thanks

 

Okay, now take a good look at the pics you posted in that thread. The C4 charcoal canister purge line ties into the Corvette's PCV valve, but remember that the charcoal canister line is "timed", which is why it has a solenoid. The solenoid is controlled by the ECM. To use this throttle body for your F-Body, you drilled and tapped for a charcoal canister port that corresponds with the F-Body throttle body on the passenger side, but the question now is, how are you running it? You stated that you got rid of all of the smog equipment in your first post when you added an edit. Are you still running a charcoal canister, and if you are, is it still being ECM controlled by the ECM, or do you have it running as a constant source of vapor into the throttle body with no solenoid? More importantly, when you drilled for the new port and added the brass fitting into the C4 throttle body, did you look to see how it ties into the vacuum port on the inner side of the throttle body, and where it leads to?

- Rob

 

Is this solenoid part of the smog system? If so then, no I don't have it and the line is directly connected from the TB to the canister (which I am still running).Could this be acting as a vacuum leak? And if so, why would the ECU later fix it and drop the idle to where it's supposed to be? On my car, where is that port on the TB supposed to go on the inner side of the TB? We might be onto something here??

 

With no solenoid, the charcoal canister is providing for a constant source of vapor (fuel) and this is why your IAC is pegged open at times, the ECM is trying to compensate with air to keep the air/fuel as close to 128-BLM as possible. Remember, this happens at key points because it is ECM controlled, meaning when all parameters are met during idle, Closed Loop. Shutting the engine and turning it back on triggers Open Loop for forty seconds, initial start when cold and warm triggers Open Loop for 200 and 100 seconds, etc, as I explained above. Once Closed Loop is enabled.(O2 feedback), the ECM begins correcting for stoich. As for the ECM being able to fix it momentarily, it is just the matter of pulling fuel from the injectors. As a test, pull the canister purge line fromthe throttle body, plug the brass fitting, and see if your idle issues seize from happening...

- Rob

 

I will try pulling that hose and plugging the port at the TB. I was just looking at those diagrams. It looks like the solenoid you are referring to is on the canister itself? Is that correct? If that's the case then it should still be there because I haven't touch the canister itself. Thnks for your help, you are an early riser. It's 2am here but I work graveyard so that's my excuse..5am there?

 

Double check the solenoid connection, see if it was removed, or if it was left connected and still in tact. I haven't run a charcoal canister in a very long time, but If I remember correctly, the ECM essentially closes the switch (vapor pathway), not opens it. I think they did this to avoid it from staying closed if any ECM failure occurred so no excessive vapor build up, so no ECM terminal connector would keep it open. but I am not totally sure on that because again, I normally just disable it in the bin and run a check valve off the tank. Sadly I am up at 4:30am every morning, after so many years of getting up this early it becomes second nature...

- Rob

 

I haven't looked at that canister in a long time. Are there supposed to be electrical wires going to it? I'll check it out when I get home. I'll let you know what I find.

I usually get up at 4:30am too but I got switched to graves for the month of August. I'm too old for this crap but gotta pay the bills. My body doesn't know which way is up or down right now...

 

Always remember that the IAC is almost fully opened at just at and over 100 steps, and is fully closed at 0 steps. The higher the steps in the datalog, the more the ECM is trying to add air to compensate for excessive fuel being sensed in the exhaust stream. This only happens during Closed Loop. In Open Loop however, with no O2 correction being present, it doesn't matter if there is excessive fuel, the engine will run smooth, in fact it will run great and pull harder. This is because Open Loop is controlled by a TPS and RPM reference. It's only when Closed Loop is enabled when you will see high and low idle problems, and/or surging. That is because the ECM is trying to balance a range of air/fuel to meet a commanded ratio. I definitely can relate to you about working late, I had to stop doing that because it was too much mentally, even more so than physically at times...

- Rob

 

I'll try puling that line and plugging the port and we'll go from there. Fingers crossed.....

 

Pulled the hose and plugged off the port. Double checked all the wires and connections at the canister and everything looks right. Still no change. Started it cold and it idled at 1250 or so... UUUURG!

 

I guess my next step is pulling the TB and looking at the IAC port? I would be nice to know what it is supposed to like so I can compare what I have with what I should have..

 

 

Looks like the ports are both triangular. The gasket between TB and plenum at the IAC port seems to ride a little high and possibly could be blocking the airflow slightly. Maybe I ought to trim it down so it matches the port on the plenum? Here a couple pictures of my TB. You can also see where the brass fitting I installed vents to. Is this correct? I did disconnect it and plug the port with no change.

 

Anybody else out there have any ideas?? It's driving me nuts. Could a malfunctioning MAF create symptoms like this??