inokiyo

Thanks guys!
I am going to buy an O2 sensor today, and hoepfully have it installed this weekend. I will keep you updated with the results.
Thanks again for your tremendous help!!
Ken (inokiyo)

inokiyo

Hi guys!
Well, the O2 sensor did not arrive, so installation will be ...whenever it arrives.
John, I did the O2 sensor check by disconnecting the battery and the O2 plug a few times today.
1st time -
After 20 minutes of hwy driving, the motor was not running rich anymore, but idle was rough, sort of going up and down from 800 to 400 rpm. Disconnected BTY and O2 plug, the idle smoothed out! Re-connected everything and went for another spin. Drove nice, as usual.
2nd time -
Came back off the hwy and idle was doing the same thing again, but this time was running rich, black exhaust came out every time it hit the high rpm. RPM range about 600 to 200 rpm.
did the disconnect, idle smoothed out, but running rich. I heard you need the drive the vehicle for the computer to re-boot, so went for another spin, about 15 minutes on highway.
3rd time -
Came back, rough, idle, almost dying, so dis-connected everything again. Idle smooths out, but runs rich. Re-connected everything to drive vehicle into garage, with every slight accelleration, the motor dies.
I am still hoping it is the O2 sensor, do you think these symptoms prove otherwise?
Also, how long of a drive is required to re-boot the computer?
Once again, any help, advice, criticism would help.
Thanks!!
Ken (inokyio)

John Speake

Hi Ken,
It doesn't sound as though the O2 sensor is the problem. The "adaptation" is only in action when you are running closed loop with an O2 sensor.

Either the MAF or the LH ECU could be the problem.

Do you have a digital multimeter ?

inokiyo

Hi John,
Thanks for your message. I don't have a digital multimeter, but I will see if I can get one from the local radio shack. I will see if I can test the MAF and LH once I do that.
I thought I might have needed a longer drive for the computer to reset itself, but sounds like that is probably not the issue. I will install the O2 sensor anyways (once it arrives), do you think changing the temp2 sensor a good idea as well?
Thanks again!!
Ken (inokiyo)

John Speake

If temp sensor 2 is faulty, it will only affect the running during the warm up.

If the sensor is open or short circuit, the ECUs recognise this as a fault condition, and revert to the default settings for the engine at normal temperature.

Try removing the engine airfilter element. Switch on the ignition (don't start the engine) and look into the MAF inlet (best if this is done in the dark) See if you can observe the MAF wire glowing red after a minute or so......if so the MAF is the problem.

inokiyo

Hi John,
I will do all that you have said. I am going to get a digital multimeter and do the checks this weekend. Looks like there is another 928 with a similar problem.
https://rennlist.com/forums/928-forum/262499-help-a-newbie-89-s4-intermittent-rough-idle.html
I will check the ignition monitoring relay on my 928 as well, just in case!
Thanks John, I will get back with the results!!
Ken (inokiyo)

inokiyo

Well, the O2 sensor hasn't arrived, so I tried to diagnose the problems by doing the following;
(Initial start up with engine cold was with rpm steady but running rich, exhaust is blackening my wall.)
1. Checked the MAF wire to see if it was golwing red after a minute, not glowing
2. Disconnected battery and MAF wire harness, re-coneccted battery and started the motor, was running rich and idle very unstable, would stall at times.
3. Disconnected battery, reconnected MAF, reconnected battery and started motor, still running rich, but idle stable.
I tried to check voltage at pin 2 and 4 on the MAF, but the motor would always stall before I reached the MAF connector.
I will try this again tomorrow, with a friend who can reve the motor while i do this, also check MAF volts at pin 5 as well.
Could this be more of a LH problem?
any advice would help1
Thasnk in advance!!
Ken (inokiyo)

John Speake

Hi Ken
Do you still have the O2 sensor disconnected ? I would recommend leaving it like that.

Yoiu only need to slip a wire into the connector while plugged into the MAf for pin 5. The other end of your meter can go to the body of the MAF (ground)

You're looking for a typical voltage of 2.7v at idle. If it's much higher at 700rpm then the MAF may be faulty.

It is possible for a faulty LH to give too rich a mixture - there is a fault where it thinkls the WOT switch is closed, and so enriches the mixture, even at idle.

inokiyo

HI John,
Thanks always for the info!
Should I have the O2 sensor disconnected even while I do the diagnostics?

John Speake

It doesn't matter either way.

Mattman928

I was having the same problem as you on my 928 GT. I originally thought it was the O2 sensor. I had it in the shop for an alignment, so I ask the tech (someone I trust!!) to swap out the O2 sensor while he had it up on the lift.

After listening to the engine, he said he didn't think it was the O2 sensor. He hooked the car to a smoke machine, and sure enough, found that the vacuum line had become disconnected from under the intake manifold. He's running some more tests, but that seems to have fixed the problem.

Given all the intake work you had done, you may have the same problem!

- Matt

inokiyo

Well, I installed the O2 sensor today, and no luck, still running rich. I also checked the voltage at pin5 of the MAF. the only reading I got on the digital multi meter was a slight voltage of only .43 to .5 volts. am I doing something wrong? I slipped 2 strands of wire into pin 5 and + on wire, - to ground. The ignition switch was on, motor not running.
I notice now that the motor is constantly running rich, the idle is stable at cold start though. After the motor warms up, rpm at idle starts to go up and down, in a rhythmic pattern, not intermittent.
Ken

John Speake

Hi Ken
You should get aboiut 1.8v with igntion on, 2.7v at idle......recheck connections .....:-)

Mattman928

Check your WOT sensor. If it's constantly runnning rich, then the sensr may be sending the signal that the throttle is wide open.

inokiyo

I checked the voltage at pin 5 of the MAF, ignition turned on and engine off, the voltage was 1.7~1.8V, engine idle at first reading was at around 3.1~3.2V, then 2nd reading with engine running for about 3 minutes idle was at 2.8~2.9V.
IS this enough variable to diagnose the MAF is faulty, or the dreaded LH brain? Something else?
The engine ran very rough with the MAF unplugged ( this was done a few weeks ago).
And what/where is the WOT sensor?
Sorry guys, still learning the 928 lingo!
Ken