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Help! 90 C4 Won't Maintain an Idle at Cold Temps! 1st Time Owner!

Joined
Apr 21, 2018
Messages
8
Location
IL
Corvette
White 1990 C4 Targa
Hey Guys (and Gals),

I bought a pristine '90 C4 a couple of weeks ago with 80K on it. I've been driving it daily without issue. Friday night, I parked the car as normal. Saturday morning, it wouldn't start. I checked the fuel pump, and it was working. I had compression as well. I did some digging, and decided to replace the ICM and the rotor. After that, the car started- but wouldn't idle! I feathered the accelerator for a bit, and when the car heated up to around 190 degrees, I could take my foot off the pedal and the car would maintain an even idle on its own. If I shut it off, I could start it with no problem, and it would idle on its own.

This morning I went back out to drive the car to work (it was cold out), and again, it wouldn't maintain an idle on its own. I warmed the car up, and drove it to a repair shop. They haven't been able to find out what's causing this? Do you guys have any thoughts? This is my first Vette, and I'm looking at it like a learning experience- or, as my one car buddy said it, "Daily driving a C4 is considered an extreme sport by most people."

ANY help would be appreciated. My thought is maybe it's the throttle position sensor...?

Thanks in advance.

UPDATE: I'm starting to wonder if it's not the Coolant Temp Sensor? I noticed my second fan does not turn on, even with the A/C on.
 
Last edited:

Hib Halverson

Technical Writer for Internet & Print Media
Joined
Jan 10, 2001
Messages
13,452
Location
CenCoast CA
Corvette
71 04 12 19
Is the check engine light on?

Do you have the factory service manual?
 
Joined
Apr 21, 2018
Messages
8
Location
IL
Corvette
White 1990 C4 Targa
Is the check engine light on?

Do you have the factory service manual?


I ordered it and it will be arriving on the 28th, but there is no check engine light.

Even stranger...today it fired up and ran just fine!
 
Joined
Apr 21, 2018
Messages
8
Location
IL
Corvette
White 1990 C4 Targa
Is the check engine light on?

Do you have the factory service manual?

Hib, I broke out the FSM today and ran diag on the car per the instructions inside. To wit:

SES Light on when ignition on, light flashes code 12 when grounded.

The FSM suggest this is a faulty ECM, but I know that there are a lot of things that can masquerade as an ECM failure. I don't to start throwing parts at her.

My thought is maybe that the vehicle's ground is struggling, from the battery?
 

Hib Halverson

Technical Writer for Internet & Print Media
Joined
Jan 10, 2001
Messages
13,452
Location
CenCoast CA
Corvette
71 04 12 19
You do not have a faulty ECM.

You may have read the FSM incorrectly.

If you ground pin B of the ALDL, go key-on/engine-off and the SES flashes code 12, then no codes are set.

Whatever is causing the problem is not an engine controls fault.

Wouldn't hurt to check the engine controls ground. It's a big bolt just above the oil filter. Make sure the connections are clean and tight.
 

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