John Speake

You need a flow jig accurate to calibrate to better than 0.2%. The maximum acceptable error due to ageing is only 3%.

I use 01.% resistors. 2 per MAF for best accuracy.

mirousse

Very interesting discussion here!

I live in Montréal. Thanks for the West coast offer. Much appreciated.

First I should have told you that when the car is moderatly pushed, I experience some sort of power lagging from idle to about 2000 rpm where the all power suddently emerge. This does not happen during mild driving. This is why I consulted the Porsche dealer. My goal was to have an experienced and specialized mechanic to give me hints as to where I should look for that problem. I also had the LH / EZK brain re-adapted with the diagnosis tool. The carbon traces at the exhaust end tells the mechanic that the car may run rich.

I started troubleshooting the car using the shop manuals figures and just noticed that the idle speed contact works fine but the full load contact is defective (display a closed valve value even with the throttle pushed at more than 2/3 of its travel).

Now, do you think that a defective full load contact could cause the engine to run rich and/or cause a lack of power during moderate driving?

Thanks again for all your inputs.

Michel.

John Speake

The closed throttle switch won't come into play (when working) until 2/3 throttle travel.

If it is not working, then the full load enrichment will not be activated, and the LH will not switch out of closed O2 loop operation. So the car should not run rich whgen pushed, with a faulty WOT switch.

In my email to you I gave you the details of my agent in Canada, who is in Toronto. I am sure he could help you out.

If you car is high mileage (>80k) the MAF could cause lack of throttle response. But there are plenty of other possible reasons, which I assume you have addressed ?

Bertrand Daoust

Don't want to hi jacked this thread but...

After reading Michel's #17 post, It reminds me that my car has traces of carbon at the end of the exhaust too and the car has some sort of power lagging between 1000 and 1500 WHEN COLD. After only 1 minute or two, everything is fine.

My car as 82000 km.
Can this be related to a MAF going bad?

Thank you.

John Speake

It would be unusual for the MAF to be aged enough at that mileage for it to caused a lean out hesitation when the engine is cold, and the car is running closed loop (when it is critically dependant on MAF calibration).

Has your car been used for mainly short journeys all its life ? i.e less than 10 miles. This will accelerate MAF ageing which is dependant on the number of burn off cycles at each engine switch off.

Bertrand Daoust

Thank you for the quick response John.
Yes I do mostly short trips with my car but does not use it very often. 1 or 2 times a week (summer only). As for the 2 POs, I can't tell.

Does a bad MAF would make the car to run a bit to rich?

Thank you.

John Speake

The car may well run a little rich when in warm up, as the engine needs an over-rich mixture to run at that time. You have to be careful with trying to determine engine mixture at the tail pipe.

There is a failure mode with some MAFs which gives an over-rich condition under all conditions, but this is usually quite severe.

Your car seems to run OK in closed loop. The most common issue is the slowly increasing lean out of the mixture under acceleration and WOT conditions (when the LH goes open loop, and fuelling is solely dependant on MAF calibration).

If you are concerned about the black look of the tail pipe, if possible do a high speed run with engine hot. Then re-check the tail pipe, but also pull a spark plug and check its colour. That is a far more accurate test than the tail pipe.

Bertrand Daoust

Thank you John.
I will check the spark plugs later this week.

mj1pate

My auto doesn't have much power when "punching it" while cruising and below 2k RPM. Its not that it hesitates or lags. The engine's power kicks in closer to (and above) 3k RPM, as the owners manual power curve suggests. The auto likes to shift into 4'th by 50 mph. I've found that in city driving and in traffic, that selecting "3" versus "drive" on the auto selector works the best for power. On the other hand, punching the accelerator from dead stop on a fairly level surface while in "drive" produces mid-sixes on 0-60, as the factory suggests it should, as the engine spends more time on the cam. Every other basic tuning having been already done, having the MAF rebuilt/calibrated is high on the short list of costly things that will be done with 100k on the odo. This of course, as mad-money accumulates. I'm not expecting too much to be noticeable, based on the existing close-to-factory 80's car performance. On the hand, waiting too long can be disadvantageous, in case people like John decide they want to make a successful living instead of supporting our hobbies ;-)

mirousse

Thanks everyone for all the valuable information.

I just finished running the LH diagnosis/troubleshooting of the shop manual. Except for the WOT that appears to be defective (my diagnosis could be wrong since the Porsche diagnosis computer did not display an error code for this) everthing is within the manufacturer's figures for the MAF, idle switch, temp II sensor and wiring.

On my way towards the idle switch, I found a broken rubber hose (I guess an oil vapor return) that connects in the "Y" just below the MAF. I replaced it and testdrove the car.

I can't tell if the rubber hose was the cause but the car now behaves as it should. Acceleration are crisp and steady. No lagging or hesitation.

Do you think that this 3$ fix could have solve my problem (air leak below the MAF)? Almost too good to be true!

Regards.
Michel
928 S4 1989 ROW, 125 000 km/78000mi