Ted928

I would like to make some measurements on the LH plug while it is connected to the brain. Does the plug come apart so that I can probe the back side?
My car is running way too rich. When cold it starts fine and then starts spewing rich smoke after a few seconds. After a few minutes it will choke and die. I have checked everything. Rebuilt LH brain did not help. Perhaps i have an intermittant open somewhere????
Thanks!!!

John Speake

Hi Ted,

You say the car is LH equipped, so I assume it is the early 32v motor ? (the last 16v US models have L jetronic, flappy air meter, don't they ?

I don't know if the LH plug comes apart easily.
Running excessively rich could be Temp senor 2 open circuit or back connction back to the LH brain. Or could be the MAF outpu voltage is way too high. To check the MAF, just disconnect the 6 way plug to it. If the engine runs much better, but not perfectly, then the MAF has gone.

If your car is L Jet equipped, then an open circuit flappy potentiometer would make it run very rich.

Hope this helps.

Ted928

John et. al.,
Thanks for your thoughts.
Yes it is 16V LH. I have checked the TCII resistance a number of times and it is always within spec. I want access to the LH plug so that i can jump across TCII, grounding it out since that should lean it out (possible their might be an intermittent open in this wire). When I disconnect the MAF meter the car will run better, but still spewing smoke/soot. I disconnected the MAF so that it would run well enough so that I could drive it out of mom's garage and on to a trailer so that I could bring it home. Their was rich smoke/soot everywhere. I checked the resistance on the MAF and it was 4.5 ohms (a little high since spec is 3.6-4.1). Does anyone else have experience as to what this value typically reads??? However, the MAF output voltage was 1.8V and increased when I blew air through it. If I could access the LH plug then I could read MAF output voltage while the engine was running.
I have a local friend with an 87 S4. I'll try swapping the MAF with him. Unfortunately, the LH brains are not the same so I can't swap that.
I am learning that diagnostics may find many of the problems, but swapping components may be the only way to solve some.
So far this problem has cost me 3 months. And since I have been agonizing over this, it feels like dog-time (7*3=21 months).
Any thoughts would be greatly appreciated.

Ted928

CORRECTION: 32 VALVE

Ted928

In addition, I checked the fuel system. Pressure and leak down rate are both within spec.

John Speake

Hi Ted,

It does sound like the MAF is a strong possibilty. The MAF can appear to give the right kind of outputs, but it can still be out of limits. The output voltage is very critical, plus or minus about 3%.

The only way you can check a MAF is with a calibrated airflow jig.(I have built one)

For you, I agree that swapping a known good MAF from a friend's car is the quickest way to determine the fault. I think your car uses the idle pot on the MAF, so I would set that to about 500 ohms(pin 6 to the body of the MAF), before you fit it to your car. That won't mess up your friends car when he refits it, becasue his car doesn't have that pot connected to anything.

The same model MAF was used on all LH equipped 928's.

Let us know how you get on !

Ted928

John,

SUCCESS!!!!!!!!!!

I tried my MAF in my friends 87S4 and it blew rich smoke. For the first time, I was very happy to see such a poorly running car. After 3 long months, I now know what component is bad. Before I replace it I plan to open it up and take a look for anything obvious. Since you have built your own bench test, you must have some knowledge on its operation. Any thoughts on what to look for?? I have a Bosche injection book which shows an electrical schematic of the MAF but it doesn't show the terminals. You are certainly right, crude measurements are not enough to know if it is working properly.

John Speake

Since you have built your own bench test, you must have some knowledge on its operation.

>>Yes, I spent much time on this !

I have a Bosch injection book which shows an electrical schematic of the MAF but it doesn't show the terminals.

>> Well, the pin numbers are shown on the connector on the MAF body. Pin 1 is for the burn of function (+5v to activate) Pin2 is +12v in. Pin3 is ground (bat -ve) Pin 4 is ground Pin 5 is the output to the LH brain. Pin 6 is the mixture pot.

Any thoughts on what to look for??
>>You already have done the basic functional check of the output, when blowing through the MAF. They just age and go out of calibration. As I said before the margin for error is very small. There's not much else you can do at home. If you are tempted to take the black cover off, take care especially with the bond wires going to the power transistor........

Do you intend to buy a new or exchange unit ?

I might be able to help you out........

Ted928

John,

Thanks. I popped the cover but didn't find any obvious problem. I ordered a rebuilt unit. Rebuilt prices vary from $250 to $500. I went cheap - we will see.

I'll let you know when life is good again.

John Speake

Hi Ted,

I'm really pleased you have now found the fault with your car.

Is the unit you are getting an exchange unit ?

If not, I would buy your bad MAF from you for investigation.

Where are you thinking of getting the refurb unit from ?

John