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Discussion Starter · #1 ·
Hello all, first post on here, I own a 1990 production date mx-6 and ever since I saved it from the junkyard I've been having a weird problem. When I start the car it jumps to like 2500rpm and stays there until the car warms up. It used to stay high but I adjusted the idle screw down to the factory recommended 750 rpm, I guess some monkey was futzing with the carb before me. The funny thing is when I start the car when it's already hot, or if it was like yesterday and the sun made it 102° inside the engine compartment it doesn't do it. Any help? I assume there's some cold start temperature sensor somewhere that's broken but I have no idea where to look for it.
 

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You mentioned Carb but since it's a 1990 and you are in the USA I will assume you meant throttle body and the car is fuel injected.
You haven't mentioned if it's turbo or naturally aspirated.

The number one suspect and most likely the cause of your high idle warm up issue is the ISC/BAC valve on the throttle body, it has two parts. 1 is an electric motor that regulates (maintains) idle rpm by controlling the amount of air to the engine, if your car is maintaining 750rpm once warm this part is functioning.
The other is a mechanical air bypass that operates on coolant temperature the way a thermostat does but in reverse, the cooler the bypass the more air it allows past it, as coolant warms up the bypass closes.
Unfortunately both parts of the valve must be changed together and a new gasket is required.

There is a chance the BAC valve is gummed up with oil and dirt and possibly opening too far when cold.
Removing the valve requires removing 4 Phillip head screws, best to use a socket screw driver head on a ratchet. The head must fit properly in the screws or they will strip. Before attempting to loosen them they must be cracked loose, put the screw driver socket in the screw head and hit the socket with a hammer a couple of times, they are small fine threaded screws going into aluminum but they have been seated for more than 30 years, they don't need to be hit very hard but if they aren't cracked loose the head will strip and you will have to remove the throttle body and use vice grips to remove them.

Do not remove the throttle position sensor above the ISC/BAC valve because it will require proper adjustment when reinstalled.
Clean the spring and plunger assembly inside the ISC/BAC using brake cleaner and an old or discount tooth brush, and all 4 ports around it, the lower two are idle control and the upper two are warm up idle.

You should order a gasket before removing the valve or get a sheet of Felpro Gasket material (the thin stuff) and an exacto knife, outline the old gasket on the sheet and cut it out. The factory gasket had a thin layer of bonding material on the surface, you can use spray on gasket tack (thin layer on both sides) or high temp gasket maker and a very thin layer (not a bead), just put a bit of gasket maker on both sides and use your thumb and index finger to smear it all over the surface of both sides.
The old gasket can be reused if it comes of in tact, it must be lightly scuffed on both sides with fine sand paper and gasket tack or maker must be used.

If you can't get you ISC/BAC valve working properly and can't find a good price replacement I can't sell you a used one, cut a gasket for it and send the 4 mounting screws with it, I will guaranty the part but I will not test it on a car before sending it. The best I can do is remove the one on the 626dx engine I pulled out of the car I bought for a shell, I only drove the car twice after buying it to get it to my shop, then I stripped it to a bare frame. The car had a thousand problems but warm up and idle wasn't one of them.
If it doesn't work properly I will send another valve but you must pay the shipping.
 

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Discussion Starter · #3 ·
You mentioned Carb but since it's a 1990 and you are in the USA I will assume you meant throttle body and the car is fuel injected.
You haven't mentioned if it's turbo or naturally aspirated.

The number one suspect and most likely the cause of your high idle warm up issue is the ISC/BAC valve on the throttle body, it has two parts. 1 is an electric motor that regulates (maintains) idle rpm by controlling the amount of air to the engine, if your car is maintaining 750rpm once warm this part is functioning.
The other is a mechanical air bypass that operates on coolant temperature the way a thermostat does but in reverse, the cooler the bypass the more air it allows past it, as coolant warms up the bypass closes.
Unfortunately both parts of the valve must be changed together and a new gasket is required.

There is a chance the BAC valve is gummed up with oil and dirt and possibly opening too far when cold.
Removing the valve requires removing 4 Phillip head screws, best to use a socket screw driver head on a ratchet. The head must fit properly in the screws or they will strip. Before attempting to loosen them they must be cracked loose, put the screw driver socket in the screw head and hit the socket with a hammer a couple of times, they are small fine threaded screws going into aluminum but they have been seated for more than 30 years, they don't need to be hit very hard but if they aren't cracked loose the head will strip and you will have to remove the throttle body and use vice grips to remove them.

Do not remove the throttle position sensor above the ISC/BAC valve because it will require proper adjustment when reinstalled.
Clean the spring and plunger assembly inside the ISC/BAC using brake cleaner and an old or discount tooth brush, and all 4 ports around it, the lower two are idle control and the upper two are warm up idle.

You should order a gasket before removing the valve or get a sheet of Felpro Gasket material (the thin stuff) and an exacto knife, outline the old gasket on the sheet and cut it out. The factory gasket had a thin layer of bonding material on the surface, you can use spray on gasket tack (thin layer on both sides) or high temp gasket maker and a very thin layer (not a bead), just put a bit of gasket maker on both sides and use your thumb and index finger to smear it all over the surface of both sides.
The old gasket can be reused if it comes of in tact, it must be lightly scuffed on both sides with fine sand paper and gasket tack or maker must be used.

If you can't get you ISC/BAC valve working properly and can't find a good price replacement I can't sell you a used one, cut a gasket for it and send the 4 mounting screws with it, I will guaranty the part but I will not test it on a car before sending it. The best I can do is remove the one on the 626dx engine I pulled out of the car I bought for a shell, I only drove the car twice after buying it to get it to my shop, then I stripped it to a bare frame. The car had a thousand problems but warm up and idle wasn't one of them.
If it doesn't work properly
Car'
[
I actually also thought that was the problem. I completely disasembled the valves you were talking about and sprayed em with carb cleaner, I know it's not a carb, but I also didn't know what to call it since it's a seperate part from the fuel injectors. Completely cleaned it out and made sure all the valves we closing and opening properly. My car's not a turbo. That did not fix the problem, it didn't even affect it in the sightest, although my car does run better now because it was incredibly dirty it didn't fix that issue.
 

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Try checking your coolant when cold. If there is not enough in there to reach the temp sensor, that can affect idle.
 

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[
I actually also thought that was the problem. I completely disasembled the valves you were talking about and sprayed em with carb cleaner, I know it's not a carb, but I also didn't know what to call it since it's a seperate part from the fuel injectors. Completely cleaned it out and made sure all the valves we closing and opening properly. My car's not a turbo. That did not fix the problem, it didn't even affect it in the sightest, although my car does run better now because it was incredibly dirty it didn't fix that issue.
That still doesn't mean it's not the BAC valve.
-Unplug the wire harness connector from the air flow meter (VAF) and start the car when it's cold and see if the RPM's are at 2500.
-If they are turn off the car and remove the accordion intake pipe from the throttle body, leave the VAF disconnected and start the car.
-Block the upper BAC hole in the throttle body with your finger, if idle drops to 750rpm than the BAC valve if faulty, if RPM drops by 500-800 rpm the BAC valve could be functional.
-Block the lower hole, rpm should only drop a little bit, If it drops a lot the ISC valve could be shot.
-Block both holes, rpm should be bellow the 750RPM you set idle at. (Did you ground the test connector when adjusting the idle screw and setting idle?)
If RPM drops bellow set point with both holes blocked than the ISC/BAC valve is your problem.
If not than we need to start looking at the vacuum solenoids on the firewall and a few other things.
What ever is causing the high idle is getting switched when the engine is at operating temperature, vacuum leaks usually don't seal at operating temp and in my experience with N/A F2's vacuum leaks after the throttle body usually cause the car to stall at idle.

Have you checked the accordion intake pipe and resonator chamber for crack and leaks?

RPM could vary when the intake and VAF are removed because the amount of air at the idle bypass and ISC/BAC at the throttle body mouth.
Normally when the BAC valve is faulty and causing high warm up RPM if the ISC valve is functional rpm will go to 2500 and down to 1800 and back up to 2500 over and over again, the ECU detects idle is too high and closes the ISC valve, because idle is still too high the computer opens the ISC valve and tries closing it again over and over until the BAC valve heats up and idle is within the control range of the ISC valve.
 
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