NickT

I have been trying to get to the bottom of a rough running engine and am now thinking about the O2 sensor.

I have:
- Replaced cap/rotors/plugs.
- Check plug wires.
- Swapped out the MAF with a loaner.
- No air leaks in the intake, it holds pressure if I block off the MAF inlet and blow in one of the vacume lines
- Intake R&R with injectors refurbished by Marrens.
- New fuel filter.

It does:
when starting from cold it idles high at 1200, ARM1 shows first orange light.
After the car warms up a bit and the ARM1 starts to hunt, the idle slowly drops and starts to bounce between 500 and 750 RPM. Sometimes a slow bounce sometimes quickly. The engine doesn't stall for the most part. It seems that when the ARM1 shows the first blue light the RPM's at at the lowest, maybe a tad under 600 and as it leans out, the RPM's pick up again.

If I disconnect the O2 sensor I assuming that the LH will run in open loop and if the O2 sensor is suspect, then the should should run a bit smoother.. That make sense?? I think I will order one anyway as I do not know the age of it and I have had the car 6 years and 80K miles on it myself.
O2 sensors, I'm assuming do weaken with age, just like everything else.

Any other thoughts??

She's an '88 5Spd.

TIA..

Peter F

Got the tip from John Speak to first disconnect the O2 sensor plug and then disconnect the battery for a short moment.
Then connect the battery again and go for a drive, this should put the computer in a default mode.
If car runs better it is propably the O2 sensor.
I swapped mine out and it improved things alot, not so strange considering one of my three leads were broken....

Good luck/Peter

aaron3

I had a idle problem that would come and go. I tried just about everything that you listed too. Finally, I just disconnected the O2 sensor and the problem has gone away. I have a new sensor but haven't put it in yet. I had similar issues as you. Low, cycling idle, sometimes would die. Problem wasn't consistent.

The only thing about your problem that is strange is that the O2 loop isn't active until the car warms up so the real high rpm when cold is strange. You can simply disconnect the O2 sensor and see if it makes a difference. The car will run in open loop.

I have a feeling that alot of the idle problems with the 928 are actually O2 sensor problems although everyone (myself included) seems to go after the tough things first (intake rework, ISV, MAF, etc.).

John Speake

Hi NickT,
Mark has it right - O2 sensor is ignored for about the first 4-5 minutes after a cold start. Hi rpm at start might be sticking ISV - did you check this out/replace when you worked on the manifold ?

Replacing the O2 sensor is good preventitive maintanance.

Regards

Lorenfb

All early narrow band O2 sensor fuel injection systems have some surging (or hunting)
because of the way the system switches from lean to rich as it centers around the
AFR = 14.7. The later wide band O2 systems respond in a more linear way without
the surging/hunting of the earlier systems (50 - 75 RPMs).

John Speake

The loop may hunt, but the idle speed on a 928 should be pretty steady...

NickT

I did clean out the ISV and checked it out before putting it back in service.. This was the end of last summer so things could have changed since then.. Really don't want to pull the manifold again

John Speake

Three less painful things to try :-)

1) Check temp sensor 2. If it fails, the LH ECU will set itself to "engine hot" default. This may be a problem

2) Do a battery disconnect reset.

3) Try the O2 sensor reset for the hunting when warm.

4) It may be the O2 sensor response has slowed up.

Regards

John

NickT

Thanks John, appreciate the ideas.. I have already done 1 & 2. Going to disconnect the O2 sensor when I get home and see what happens...

NickT

I disconnected the O2 sensor and the car ran way worse... Hardly maintained an idle at all... Drove the car for about a mile and plugged it back in. Getting really bummed with this issue..

martin D

Nick
A friend's '92 GTS had the same problem you have described, it was cured by replacing the rear (on engine) fuel damper p/n 930 110 602 01 (same 87-95), that's the one on the (your) drivers side (pass. side is press. reg.).
No.3 in 4th pic down on Tonys great site

if not, maybe the other damper on the front of the engine ?

NickT

Thanks... thought about those.. I have a spare cap for my fuel rail, going to tap it and put a pressure guage on it and see what is going on there.

Thanks everyone for these ideas...

Lorenfb

"The loop may hunt, but the idle speed on a 928 should be pretty steady"

Not necessarily, since it all depends on the response time of the idle control system loop.
Remember, the fuel injection is a control system with two feedback elements which affect
the output (RPM). The loop response time and feedback amplitude determines whether oscillation
occurs and to what extent (surging).

WallyP

If the idle speed on an S4-up 928 isn't rock-steady, there is something wrong.

NickT

I didn't go to work yesterday so my shark sat in her tank for a good 24 hours in the dark... This morning I start her up, high idle, 1200 for 30 seconds or so... then.. then then.... 750 ROCK SOLID. Drove to work 23 miles over back roads, stop lights, stop signs.. 750 ROCK SOLID. The car even felt better, more get up and go.. Puts some welly into it and did a few 1st gear foot to the floor runs and after.. 750 ROCK SOLID.. I have not changed a thing. When I put it in the garage last, the idle was surging up ad down as "usual".
Go figure.. I'm sure it will come back, maybe after work... But strange that this morning it was near perfect.