mark kibort

Have a friend that has been chasing a bad running 928S4 issue for over one year. what are the symptoms of the bad air flow sensor (MAF) is it full rich, as all of his plugs are black. changed fuel regulator (rising rate), vacuum connected, so it seems to be either the computer or the MAF. all plugs are wet and black, car runs rough, compression on all cylinders is about 160psi, but it really hasnt been run on the streets in a few months. spark flowing for all 8, and from the look of the plugs, fuel is flowing too. (just maybe a bit too much!!)

he has looked to see if the MAF is going through its "burn off" cycle, and i think he said he cant see the glowing red /white after turn off. Ive offered a suggestion to unplug the MAF and see if it changes anything.

any ideas from the gurus?

MK

John Speake

The early MAfs suffer from a design fault. This can result in over rich mixture. Some have the fault so bad that the wire glows red all the time the ignition is on.

Best way is for your friend to do the disconnect trick. If the car runs reasonably well (at least idles) then the MAF is the problem.

But the problem could equally be in the O2 loop. So if the MAF trick doesn't reveal the fault, proceed as follows -

Do battery disconnect. Unplug O2 sensor below the fuse/relay panel. reconnect battery. Start engine.

let us kniow how he gets on.

Lorenfb

Not only could the O2 sensor be forcing a rich condtion, but a shorted O2 input
(pin 24 to the LH unit) to ground could cause this.

You also need to check:
1. temp sensor not being open (pin 13) & indicating < 500 ohms hot & 2000-3000 cold
2. the fuel pressure is O.K.
3. the MAF should provide less than 2.0 volts (~ 1.5) at idle,
the max should be < 5.0 & ~ 4.0 to 4.7 @ WOT
4. the WOT switch (pin 3 of LH) is not closed or shorted to ground
5. obviously, a bad LH unit can cause an over-rich condition

Rich9928p

One of the main failure modes of the LH fuel injection module is the air - fuel ratio being too rich. This problem can persist for a long time and keep getting worse. If the fuel system is OK (i.e. injectors aren't dumping fuel), the lambda sensor is OK, the MAF is OK, the the LH is suspect and should be tested.

Driving with this failure mode could damage the catalytic convirters -- or worse -- cause a fire similar to when one of the two ignition banks fail.

mark kibort

great information!! thanks guys.

Im thinking its the LH, due to the symptoms Rich pointed out. However, the owner didnt see the "burn off" cycle, so it could the MAF. O2 sensor also could be checked , as well as that full throttle switch, but its hard to believe that it could be stuck in a closed position. OH heck, CHECK IT ALL !!

MK

John Speake

You will only see burn off if the engine water temp is at normal operating temp. And the engine has been run above 2000rpm.

If you put a diagnostic tester on it, you would save lots of time and effort !

Bill Ball

Mark;

If the guy is local we should do an easy part swap test (for the LH and MAF, at least), although Loren and John have given you some other easy tests. Oh, and if it is an 89+ I have John's Spanner for testing purposes.

Jim bailey - 928 International

Mark plug his mass air and LH into YOUR car, see what happens.

Rich9928p

I think we've cured Greg's 928, I drove over to his place and installed one of my rebuilt LH modules and it started right up. And to think, he was reduced to riding horses because his 928 wasn't running.

mark kibort

Thanks Rich, your a CHAMP!!! He called and was a pretty happy guy. The car has been running pretty rough for a couple of years now!! It was perfect that you were so close by, and had the experience in the computer tech.

Love to hear there is one less shark in the world running poorly!

the horse situation was funny too!!

Mk