rkellison

I have a 02 996 TT with 62K miles on it. The car runs good and strong with WOT boost in the 1.1 range. It has thrown a code P1508 Porsche fault code 408 Function monitoring torque comparison.... 3 times in the last month. Sometimes under heavy load and sometimes under normal driving. When it happens, I have to shut the engine down and restart to clear it. I have read the posts and some suggest MAF although I do not see the connection and a bad DME. If it were a bad DME would it not do it all the time and not randomly? I do not get any other codes with it and have no other problems.

Macster

The MAF comes into question because its input is used to calculate engine load and if the input is bad and the calculated engine load is out of some acceptable range this can trigger an error.

This is not sufficient grounds to condemn the MAF only to let you know there is a connection.

rkellison

Understandable. Since the MAF is common to both banks I would think its effect would be equal between them. Do I understand the P1508 error as telling me that there is an unacceptable difference between the torque produced in the right bank as compared to the left bank?

Macster

AFAIK cylinder bank differences are not monitored. Cylinders are monitored on an individual basis for insufficient (or too much) acceleration of the flywheel during each cylinder's power stroke. Any out of range readings triggers a misfire error for the weak (or strong) cylinders.

P1508 - Torque comparison function monitor -- signal implausible.

My WAG is "torque comparison function monitor" is German speak for the DME function that compares torque demanded (signaled by the e-Gas system) vs. torque produced (calculated engine load).

From the 996 Turbo factory manual:

Possible fault cause: DME control module faulty.

Replace DME control module. [ Note there is no electrical testing involved. This suggests the functionality is solely in the software (DME software). ]

Perform adaptation.

Switch on the ignition. Wait one minute. Do not press accelerator. Switch off ignition for at least 10 seconds. Read out fault memory.

While the above has the DME at fault IIRC mod'd cars are hard on MAFs.

Under hard acceleration the mod'd engine is consuming air at a rate far higher than the MAF was intended to measure. It can measure this but suffers from accelerated (no pun...) degradation.

So I would consider the MAF before the DME. But how to confirm the MAF is at fault I can't say. Maybe data logging the intake temp and air rate numbers before, during, and after the appearance of a P1508 error code might capture anomalous readings that point the finger of guilt at the MAF?

rmc1148

I would suspect the maf also.

rkellison

I suspect the car is mod'd since I get boost pressures easily in the 1.0 to 1.1 range. As far as I can tell, the only way to find out is to have the ECU mod'd again with a known version. It does have the K&N filter too but does not appear to be an oiled filter. I have seen posts on the destructive nature of these oiled filters on the MAF. I did clean the MAF but that only removes soluble deposits. If it is a plated on deposit, I am not sure electronics cleaner will help. I will inspect it again and try another cleaning. I could grab some data with the Durametric as a baseline but catching it in the act will require constant recording of the data since I do not know when it will happen nor can I cause it to happen.

"02996ttx50

tuned cars have a higher propensity toward "frying" mafs than do stock cars. if you see 1.0-1.1 then it's safe to assume your car has been flashed.

cleaning the maf will not help if the diode is fried. there are two aspects of this wonderfully idiosyncratic part that can and do "fail". your k&n may not have been olied? but once i finally eliminated my bmc oiled fillter from the equation, so i now only need a new maf when i have fried it from voltage spikes ( rare but "known" ) associated with higher hp as opposed to it simply getting "dirty" ( common ). given the highly questionable *increase* in hp ( breathing ) and ultimately performance of an oiled element filter.. measured against these cars propensity toward MAF failure,.. it made more sense for me to go back to the paper filter. on balance, i think it's the more intelligent choice. why add to potential maf failures, i say.

hope that helps also.

rkellison

The logic supports your assumptions. Is there a high performance paper filter available?

Macster

An oiled air filter can cause problems with MAF. An unoiled filter can cause engine problems. It passes abrasive particulate matter that causes engine wear.

"02996ttx50

the paper mahle ( oem ) is perfect and like 20 bucks. although my airbox is altered ( a la 997tt ) it is no faster for it. of this i am sure lol.

i will add, the car SOUNDS better ( esp on boost ) with the less restrictive aftermarket filters..but again, not worth what i see as a trade off with no *measurable* upside. really just a dirty MAF downside. GL w it.

rkellison

So why not just use an oiled filter and clean the MAF at every oil change? It is a little more work but better filtering. Does the oil cause physical damage or just leave a film on the MAF?

Macster

The problem is the MAF runs with oil on it during the meantime. There is much debate on the merits of an oiled filter. From what I read I would never use one. The dry filter is a very good filter and requires nothing but a replacement every once in a while. I have driven my Boxster hundreds of thousands of miles without having to clean the MAF and except for an approx 60K mile interval when it was out of the car (long story) the original MAF is still working just fine.

The Turbo has 111K miles on its MAF and I don't even know where the MAF is located.

"02996ttx50

your questions are based upon the assumption that the oiled element is somehow "better", when i'm not sure it is ( unless the white pages on a k&n website are to be taken at face value ) particularly ..IF.. any HP gains that might potentially be achieved by running the oiled bmc/kn are offset by a higher propensity toward maf failures or intermittent maf coded cels. then what would be the point?

if the gains were significant and even measurable ( which they are not ) i could understand using them.. but they're not. folks use bmc's all the time without issues. i was one of them.but i had ( periodic ) maf issues ( as do most owners of the 996t at one time or another ) i have less issues now, than when i ran the oiled filter. so i realize this isn't a scientific study, and it is purely anecdotal in nature.. but i'm all about causal relationships, and i believe i have noted for you one that can be detrimental to the consistently high performance we have justifiably come to expect from these cars.

namely an oiled filter in a tuned tt. also, cleaning a maf is a stopgap measure. they are not designed to be "cleaned", so any improvement in them after spraying what is essentially electrical cleaner on a diode is dubious at best, and will be short lived. trust me on that, at least

rmc1148

I have tried the oiled filters and personally believe its mostly hype and not worth changing out/cleaning the maf more frequently and as said the oem filter does the job. I would buy a new maf go with a stock filter= done.