GT Idles great when cold, then problem when warm (Page 1/2)
Jay Laifman DEC 14, 09:44 PM
I searched old threads and did not find an answer.

As my title says, I have a 1987 GT, V6. It idles perfectly when cold. It drives nicely at all times. But once it gets warm, it won't hold idle. If I give the throttle a little pressure and get the rpm up, it idles fine. It just won't do it automatically.


I seem to recall a long time ago, I learned that some state of the ECU changes once it gets warmed up. And that effects idle. I just don't remember what that was. I do have a cable to read the ECU for probably 20 years ago. I just don't know if the old IBM laptop I had with it still even works! I'm assuming none of the current OBD 1 or 2 readers are helpful.
Gall757 DEC 14, 10:36 PM
When your engine is cold it is running on pre-set values (called open loop)....but when it is warm, the computer is reading the engine sensors and sending signals (called closed loop). So some sensor is giving bad information to your computer. The prime candidates are the O2 sensor, the MAP sensor, and the Coolant Temperature Sensor. The O2 sensor is not expensive so you can just replace that, and then probably the CTS would be can test it, but they are not expensive either. If you need help locating these things, just ask.
Jay Laifman DEC 14, 11:39 PM
Patrick DEC 15, 02:36 AM

Originally posted by Jay Laifman:

I just don't know if the old IBM laptop I had with it still even works!

The battery probably won't hold a charge... but just keep it connected to AC and give it a whirl. The ancient Dell laptop I use with WinALDL will survive the Apocalypse.

Jay Laifman DEC 20, 09:43 PM
I got the laptop to work and got numbers.

I will pull out the workshop manual to see if I can determine what it means. In case anyone here has some thoughts, that would be great.

I don't know what's relevant. But here are some highlights:

On the Sensor Data tab, all sensors were showing readings. For the ones I know, they were correct (like coolant temp, rpm, voltage, advance (well, within what appears correct).

I did not drive the car because the battery won't hold charge and my extension cord isn't long enough! But this might be important.

It shows two error codes:
24 VSS Vehicle Speed Sensor
32 EGR Exhaust Gas Recirculation

Thanks for any thoughts.
Jay Laifman DEC 21, 11:41 AM
According to the Manual, the car may show the VSS flag if the car is not moving - and I was not moving. Unfortunately, the laptop is so old, the battery has no life anymore. The laptop died before I could get a reading. I'm going to try to use both of my 100' extension cords, get the car down the street, plugged in and drive to the house and see if I get a reading. Yikes!

Years ago, I was running the car without the VSS buffer which caused rough idle after warm up. But not no idle. Still, a suspect that I will have to check out.

On the EGR flag, from what I've read, it doesn't cause any major issues. Someone on this site indicated that he sold a bypass for the EGR system. But he doesn't list it on his website anymore. I've emailed him to see if they still have it anyway.

[This message has been edited by Jay Laifman (edited 12-21-2018).]

Gall757 DEC 21, 12:11 PM
The code 24 could be a faulty contact in the VSS connector. The code 32 should not cause any performance issues and is usually the last thing that gets fixed. Blocking it off is often more trouble than getting it to run right.
Jay Laifman DEC 21, 12:37 PM
Thanks. I did just spend some time following all the vacuum lines. Sure enough, the 1/2" little tube attached to the EGR servo was completely gone. We'll see if that fixes it.

I did visually inspect the connections to the VSS and the VSS buffer. Both looked good. But I can connect and disconnect them to see if that reveals anything. The bummer is that these connections are getting brittle with age - and I broke the latch on the MAP sensor connector. I gotta remember to be more careful! If that's even possible.

I will try my short drive with long extension cord idea to see if I'm at least registering speed too.
2.5 DEC 21, 02:50 PM
Does it warm start ok? Does it smoke at all like it is running a tad rich?
When you touch the throttle you are letting air in which can offset getting too much fuel.
Jay Laifman DEC 21, 10:24 PM
I don't see any smoking. But it's hard from the car.

As I said, I'm going to try a long long extension cord tomorrow to try to see if the VSS registers with a short motion (using the WinALDL cable and reader on an old IBM laptop). That's because the laptop battery is no longer any good.

I tried to use my newer lap tops. But they don't have that old style of serial connector. So I have to plug the WinALDL into a USB port instead of the serial port, and the laptop is just not seeing it. I found this where the guy says to select USB Serial Converter. But that's just not showing up on my laptops where he says it will be. Does anyone know how to do this?