mirousse

Hello all,

I'm just coming out of the Porsche dealer. It looks like I have a MAF calibration problem / eventual failure.

Any of you has ever tried to cablibrate the MAF with good results?

I'm at ease with electronics and mechanics. Is it worth a try? If so, what is the method?

If I need a MAF rebuilt, with whom should I do business with? At what cost?

Regards.

M.Tremblay
1989 928 S4 ROW

Hilton

Hi Mirousse,

Does your car have catalytic converters? If your car has an O2 sensor and cats, then you need a new MAF, or possibly O2 sensor, or both.

if not, like my ex-UK '89 S4, you can try calibrating the MAF using the MAF signal adjuster screw just above the electric panel. You'll need to go to a local garage to plug into their exhaust gas analyser and then turn the screw to 1% CO at idle (clockwise enriches the mixture, anti-clockwise leans it).

The MAF on my car was original, and I was able to adjust it to 0.9% CO screwed all the way in. I've since fitted a MAF rebuilt by John Speake and have it adjusted to 172ohms which gives me 1.02% CO at idle with a lamda of 1.301.

Hilton.

mirousse

Thanks Hilton.

The mechanics told me that even if it is not written in the shop manuals, they used to calibrate the MAF with an omhmeter in order to match a certain resistance value.

Knowing that my car was equipped with catalytic converters, he suggested the method but warned me that even with a proper resistance reading, it could end up with worst perfomance.

What do you think about that?

Regards.

Michel.

Hilton

I don't have cats or an O2 sensor, but my understanding is that the ECU will adapt the MAF signal on its own to maintain correct AFR, and that the MAF only becomes an issue when it no longer provides the full voltage range that it should.

In your shoes, I'd put the money towards a rebuilt MAF rather than a mechanic's time that will likely not result in any benefit and still require the new MAF.

Someone with more knowledge of cat-equipped cars will hopefully post a more meaningful response.

I intend to fit an O2 sensor when I fit an X-pipe to my car - for now I don't have anywhere to screw an O2 sensor into, as my car has the factory mid muffler and not cats.

Bill Ball

This doesn't sound right. The MAFs cannot be calbrated externally with an Ohm meter. How do they propose to calibrate it? John Speake does this by using a variety of known airflow rates in a special jig adjusting the internal electronics until the voltage output is correct.

Hilton

Bill, you can adjust the MAF signal on all the LH maf's - there's a potentiometer built into them all (hides behind a plastic plug that can be drilled and pulled out). This is used to adjust idle speed on early L-jet cars I think?

Non-cat S4's like mine have an external potentiometer that's attached above the CEL in the passenger footwell and is used to adjust the MAF signal to a determined value so that the LH/EZ-F have a known baseline to work from in the absence of an O2 sensor.

However, calibration of the MAF itself in terms of voltage range can't be done - only the relative limit of one end of the range. The range itself is determined by the length of the platinum wire AFAIK, hence John Speake uses his test-jig to correctly calibrate the MAF's during rebuild.

John Speake

There is a misunderstanding here. Bill is correct.....

The pot on the side of the MAF is NOT connected to any electronics inside the MAF. On Euro S2(84-86) and US 32v 85.86 it is connected to a pin on the LH2.2 ECU. This then adjusts the idle pulse width.

On the non cat S4's and later cars, the pot on the MAF is no ponger accessible, so a remote pot was fitted above the fuse/relay panel.

On cat equipped cars, the O2 loop sets the idle CO, so the pot is not fitted.

So on S4 and later, adjusting the pot on the MAF has no effect at all.

Hilton

Interesting thanks John, my mistake. I hadn't realised that the pot in the MAF wasn't actually part of the MAF sensor.

Wonder why they didn't just make the screw separately?

Daniel Dudley

IIRC, a tired MAF has a tired hot wire, leading to power loss and reduced throttle response.

I replaced mine at 70,000. There was an increase of 10 HP on the dyno, but there was also a better throttle response. If you think that your car isn't running quite right, this is a prime suspect. They often degrade slowly over time, and often display no symptoms of poor running, just a decrease in performance. O2 sensor replacement is also a good idea, if you have never done it, and checking for vaccume leaks with a smoke generator is also worth doing now. Knock sensors are also a non service item that tend to degrade performance with no symptoms, but I have never had to replace them, and do not have them on my current shark.

As I have been told numerous times, many people go ahead and do the X pipe, exhaust, etc, but overlook the basic services that will ensure that the car can actually benefit from the enhancements. A stock 928 is a pretty good performer right out of the box.

PorKen

The wire/thermistor combo are (re)calibrated internally. Still, each MAF has a slightly different output. (For maximum results, you almost have to match the LH chip to a particular MAF.)

Here's what a rebuilt 928 MAF (not JDS) looks like under the cover:

icsmike

I think Ken is the original creator of the Flux Capacitor.

Lizard928

M. Tremblay, where in Canada are you located? If on the west coast, we do have a few locals who you could probably swap a MAF from another car to try.

whitefox

Wait a second, so to rebuild my MAF all I would need to do is solder in a resistor and make a cut on the board? Or am I just being naive here...

PorKen

AFAIK, after a new platinum wire is brazed(?) on, the MAF has to be recalibrated on a flowbench, hence the resistor.

whitefox

I knew there had to be more to it...