Idle hunting: different symptoms for different conditions
 

After long winter, my kitty is out of the winter storage and I am back at playing with it! Over last few weeks I did a few simple projects: fixed headlights, changed bonnet gas struts, changed oil. Now I reached point where I need help from the community.

It appears that the car developed a condition of rough idle. When I just start the car (engine not warm), idle hunting is more severe: RPM needle jumps between 500 to 800 rpm continuously. After I drive the car for a while, it still hunts for idle, but not as severely: oscillating between 650 to 750, which is manageable. Also, when the car idles with transmission in Park, idle increases to 900 and also hunts around 900 a little bit (maybe between 850 to 950).

I don't think it is completely new condition developed after this past winter. I think I had rough idle last year, too. But somehow I feel the idle hunting at start up now is more severe than last year. Or maybe I forgot.

What do the symptoms described above point to as likely culprit?

(I did sort out vacuum lines last year. Also adjusted TPS voltage and cleaned throttle body a while ago.)

OK, you up there, and have emission junk e never got, whoopee.

Cold idle should be in the area of 1000rpm+, with a working AAV. Too slow and things can get odd.

The other hunting critter is the TPS is out of range at the idle end. Must be between 0.32 and 0.36, or the ECU has no idea where the throttles are.

The infamous "black goo" inside the throttle bodies, they need cleaning regularly, and that "goo" will cause idle issues, and even stalling.

The fuel trimmer "pot" inside the ECU, accessable via a hole is said ECU casing, needs a tickle of a click or 2 in one direction or the other to smooth the beast.

Spark plugs are wrongly gapped, too wide will cause idle issues, mainly hot, but I have had a few do it at cold, and they were set at 0.030", instead of the 0.025" needed.

Stale fuel.

Thank you, Grant.

Some of the things you mentioned were done/checked. I cleaned throttle butterflies and gapped them properly. I changed spark plugs and gapped them. I tinkered with TPS voltage signal and (try to set it) to appropriate range.

Things I have not done yet:
- test AAV (I thought that it impacts idle when hot, not when it is cold, no?)
- adjust fuel trimmer pot at ECU

On the ECU fuel adjustment, where exactly is that adjustment hole on the case of the ECU located? Can I access it without removing the ECU? And what does it mean "to adjust fuel trimmer pot"? Should I turn it clockwise or counter-clockwise? How much?

While searching old posts, I think I saw a write up about adjustment of fuel trimmer pot in ECU mentioned. Most likely by Grant? Unfortunately I was not able to find the actual write up/document. Anybody has it saved, by any chance? Thank you!

Just for you.

Have fun

Thank you, Grant! Much appreciated!

Following Grant's instructions (thank you, Grant!) I was able to feel the hole on the bottom of ECU. It was fairly easy. (Mine actually still had a rubber plug in it.)

I also wanted to actually see what the shaft of the potentiometer looks like so that I can make the tool to adjust it. So, I removed the nuts and lowered the ECU and then flipped it. For those who are interested, here is the picture of the hole and you can see the shaft of the pot inside it. Now I need to make the tool!

https://cimg5.ibsrv.net/gimg/www.jag...b166928524.jpg