'84 - Unstable Idle/Dies under Load (Video Inside)

kostya.lyt · 12-11-2013, 03:48 PM

I've got a few quick questions regarding an '84 Targa I'm looking at buying. I've tried searching, but haven't found anything with similar issues. A few points:

- Car hasn't run in a few months. Ran good 4-5 months ago.
- Hasn't been driven regularly in 4-5 years.
- RPMs change when taking off oil filler cap (I understand that's good).
- Fires right up.
- Wants do die when trying to take off in gear, no matter how much gas you give.
- Pops slightly out of the exhaust.
- Battery was dead and had to be jump started.
- Tach wasn't working.

'84 Targa - Poor Idle - YouTube

Thanks!

kostya.lyt · 12-19-2013, 09:20 AM

I ended up buying the car, so now I can describe the problem a little bit better.

Fires right up, no problem. Usually settles into the poor idle for a few minutes and then, like a flip of a switch, idles smooths out and the car runs perfect. Could this be something temperature related?

Also, the tach still isn't working with a fresh battery. Could this be related?

Thanks!

kodioneill · kodioneill's Garage

Cylinder head temperature sensor bad.

Hunt3R · Hunt3R's Garage

"Fires right up, no problem. Usually settles into the poor idle for a few minutes and then, like a flip of a switch, idles smooths out and the car runs perfect."

You mean from a cold start? If so, the DME ignores the O2 sensor for the first few minutes when warming up so that "flip of a switch" could be when it starts reading the signal and then tries to adjust the idle and mixture. So perhaps your base idle and/or base A/F mixture are too low, hence the rough idle and dying as you try to pull away. Hard to say though since you do not even know what RPM it's idling at, you really need to get that tach fixed. And if it is the mixture or idle that's off, why? There's a bunch of possibilities there, just because you remove the oïl cap and the idle drops but doesn't die does not necessarily mean that you do not have a vacuum leak, it just means you don't have a really bad one.

And yeah, a bad head temp sensor is a possibility too and is easy enough to check.

kostya.lyt · 12-20-2013, 08:12 AM

Did some more testing yesterday, looks like the motor "comes to life" at about 85-90 degrees Celcius.

Is the base idle and base A/F adjustable, or is that programmed into ECU/DME? Is there any other way to check the head temp sensor besides resistance?

On another note, the tach and the "shift" arrow are both dead. Every once in the while the needle jumps up and then right back down. I'm assuming the signal comes directly from the ECU, right?

rick-l · 12-20-2013, 11:13 AM

Quote:

Originally Posted by kostya.lyt

On another note, the tach and the "shift" arrow are both dead. Every once in the while the needle jumps up and then right back down. I'm assuming the signal comes directly from the ECU, right?

The ignition signal (3 pulses per revolution) and the fuel injector pulse width go into the tach (microcontorller) to control the dial and the light.

+1 on checking the temp sensor. Is a basic tune up in the works?

kostya.lyt · 12-23-2013, 06:48 PM

A basic tune-up is definitely in the works, but I want to fix the more major issues before moving on.

I spent some time on it a few days ago and it ended up being the common broken oxygen sensor connector. Now cold idle is fine but acceleration is a bit sluggish and surges a little bit, I understand this is usually a "lazy" oxygen sensor, is that right?