Cosmo Kramer

So worked on the car a bit more. Sprayed starter fluid fluid all around intake and TB, no vacuum leaks. Splitters and brake booster all good, vacuum to EZF at idle showing 19 hg. Plus when I introduce a vacuum leak (pull small line off brake booster) the idle only raises 100 - 150 rpm. To cause a 1200 rpm high idle I would have to have a leak equal to 3 - 4 small lines, which I definitely don't have.

So some more symptoms/diagnosis to share:

1. Driving with ram air tubes off and hot engine idle is another 100 rpm higher.
2. Idle "hunts" a bit (like the computer tries to lower it and it pops back to 1200), maybe a variance of 100 rpm.
3. Idle switch is working verified at EZF.
4. Blue temp sensor tested and OK.
5. Overall the car feels a bit lazy, runs nice and smooth, but down a bit in power considering what it is supposed to be. Seemed to pull better when I first drove it in January and it was cold out.

I am starting to think about the MAF or O2 sensor. The car idles lower when it is cold (around 1000, which I think is correct) it bumps up after it starts to warm up and never goes down. Could it be when the O2 goes into the loop after warmup something isn't reading right? Is there a temp sensor that would have the LH stuck on cold idle?

Thoughts from the gurus?

dprantl

If it runs better when cold, then it is running lean when not cold. When the car is cold the temp II sensor makes the car run richer. Try disconnecting the O2 sensor and see how it runs. It could also be an aging MAF. Also, how did you test the tempII sensor? Sometimes they start to fail in weird ways only at certain temp ranges. I would just replace it because it's easy and not expensive.

Dan
'91 928GT S/C 475hp/460lb.ft

Cosmo Kramer

I tested the temp sensor cold and while it was warming up and it stayed within spec. Is there any other sensors that measure temperature?

When the O2 sensor is disconnected does that put it in "open loop"?

dprantl

That is the only temp sensor that directly affects mixture all the time. There is an intake air temp sensor on the bottom of the air box. My tempII sensor failed in only certain situations and other times it was just fine. It was very intermittent.

Yes, disconnecting the O2 sensor will make the car go into open-loop mode.

Dan
'91 928GT S/C 475hp/460lb.ft

Cosmo Kramer

Thanks I will give it a whirl. How about disconnecting the MAF, will that tell me anything?

dprantl

Theoretically, the car is supposed to run in a limp-home mode when the MAF is disconnected. In practice, I always see LH cars just die when it is disconnected...

Dan
'91 928GT S/C 475hp/460lb.ft

PorKen

Did you measure each terminal to ground? (There are two sensors in a LH Temp II.)

Imo000

The car should run very bad or nto at all if the MAF is disconnected or at least it did with me.

Cosmo Kramer

Yes checked both terminals to ground. I am going to do it again to make sure.

The idle was all the way up to 1400! Then I found this wire disconnected (first pic). That dropped the idle to 1100 - 1200 on the tach and restored some power, not too sure what sensor it goes to.

OK second pic confirming this is the O2 wire that I disconnected. No change in idle for this one and performance was identical, like I didn't even disconnect it.

JHowell37

How old is your O2 sensor? Sounds like that is where the trouble is. If you unplug it, and nothing changes, then it probably isn't functioning anyway.

Imo000

Ted,

You can replace the 3 wire O2 with a single wire unit and they only cost $30 and see if it makes a difference. If it's old, it could use a new one anyways. That's what I did on the '85 when the supercharger went on.

Hilton

MAF - lots of "lazy" running cars pep up nicely with a MAF that works.

Anyone who you can borrow a MAF from to test for a sho0rt drive?

Based on my 89's history, the MAF can just about last 8 years (non-cat car - having an O2 sensor should extend this a little).

If you're planning on keeping the car for more than a couple of years, its well worth it and will eliminate a lot of potential troubleshooting.

Cosmo Kramer

Yes, I have one I can try. Gonna give it a shot and see what happens.

Mike Frye

Ted,

Sounds like you have a bad O2 sensor. If there was no change when you unplugged it, then I think that's your smoking gun. You might also find that it's in the connector. That Chinese finger trap design of a plug is just begging to be yanked on. I found that I had a bad connection inside the plug and when I cut it open and fitted a new plug, it was making connection and the idle smoothed out noticeably (this was the cure to my surging idle when cold).

Keep us posted!

Cosmo Kramer

Another clue, I just pulled my LH connector and measured the resistance between pin 14 and 25 (MAF adjustment) was reading over 1000 ohms, spec is 382 I believe. Maybe that is why the O2 didn't change anything when I disconnected it. From what I have read the higher the resistance the richer the mixture. My car definitely is not running rich, it is has symptoms of running lean. Perhaps a mechanic or the PO was adjusting it to compensate for a failing MAF?