Steve Weiner-Rennsport Systems

Failing DMF's have been a major cause of random misfire issues for many years now and thats why we always replace them as part of a clutch job.

In many cases, we see the misfires long before the owner hears noises,....

mgianzero

Well, that explains it. I noticed the random rough idling about a year or two ago. I guess the DMF vibration is throwing the DME off and making adjustments to the compensate? But I didn't think much of it, until I got my PST-2 that read these misfires. The rattle become more obvious lately.

Thanks for chiming in Steve. As one of the guru 993 mechanics on the site, did my explanations sound correct to you? Anything to add?

BesideTheBox

Yes! I would like to hear what your idle and the rattling sound like.
My car rattles a little bit when switching the car off.
The clutch, pressure plate, and flywheel were supposedly replaced at 43K miles. 78K miles on the car now.

mgianzero

BesideTheBox - If you have the records it should show a flywheel replaced or not. My car had a clutch replaced by shop BEFORE I owned it and the flywheel was left original. I know that from the records.

I'll see if I can get a good sound byte of the engine rattle with a friend of mine here in a little.

Tom 328

My '95 has an occasional rough idle and misfire/hesitation when accelerating, especially in the first few mins of driving when the car is cold. I don't hear any rattling. Do these symptoms sound like possibly the DMF as well? The car has 36k and the original clutch as far as I know.

Kika

failing DMF is probably 3rd or 4th on the list of things to check first.

Not sure if this is complete, but:

ISV valve, make sure it is clean
Rotors, caps: when was the last time they were replaced?
Spark plugs and wires? Lower wires on 5 and 6 are exposed to exhaust and tend to dry and crack

Mileage is really difficult to use as a measuring stick. For example, I have 95k on my original clutch. Not sure, how or exactly why, but my experience is that I am pretty easy on clutches.

mgianzero

The most methodical way of diagnosing this problem would be to first have the OBD codes read by a Porsche-specific reader. This would probably limit you (if you do not have one yourself) to shops which specialize in Porsches. It will tell you if the misfire is real and tell you where if it is particular to a specific cylinder, etc.

The other things to try are mostly included in Kika's list. It is usually good practice to not just replace parts until you find the problem as that can be costly, and time consuming. However, most of these items that Kika mentioned, if original on your car, should be replaced any ways. Check your records to see what's been replaced if you have that resource.

There are many DIY projects you can do to clean the ISV valve - "idle valve", change distributor caps/rotors, plugs, wires if you wish to try yourself. If you get stuck, just ask us for help as many have done these projects.

Vorsicht

Marc, What's the mileage on the car? Is the DMF the original?

BesideTheBox

I do have the records, but they are not very detailed. The top end and clutch work were done at 43K miles by the dealer that sold the car to the previous owner. This also happens to be the dealer that sold the car to me.

The top end work was paid for by the "sales department". It looks like the previous owner paid for the clutch at that time.

See Scanned Document004.pdf. 1st page shows top end work and 2nd page shows previous owner payment for clutch work.

Marc - Since you have a PST-2, can you report your TRA/FRA? I also have the misfiring at idle and my TRA/FRA are 4.0/.90.

If I understand TRA correctly, 4.0 is off the charts. It should be near zero. An FRA of .9 is close to the desired value of 1.0.

I think whatever is affecting the TRA number is also reflected in the long term fuel trim (LTFT) OBDII parameter. MY LTFTs are at -3.9 (bank1) and -6.3 (bank2). I believe these should also be near zero. Negative LTFTs tell me that the DME thinks that the engine should be running with more fuel than what the O2 sensors are telling the DME to do in order to run stoichiometrically.

Sorry for the sidetrack, but I've had misfire CEL, and still have misfires at idle and rattling when I shut off my engine. I've noticed that my TRA and LTFTs are not where they should be and I'm wondering what yours are like.

Tom 328

Thanks Kika and Marc. And sorry for hijacking your thread a little bit, Marc...just coincidental that I have similar symptoms and didn't have a clue where to start.

My car, which I've only had about 6 months, had the distributor caps, rotors, and spark plug wires replaced about 2500 miles ago in 2011 with OEM parts. I will start with cleaning the ISV, which hopefully will help the intermittently unstable idle when cold, infrequent stalling when letting out the clutch, intermittent shaking of the car when idling. Would the ISV, however, have anything to do with the misfire that happens upon acceleration at lower engine speeds when the car is cold?

mgianzero

My car has about 74,000 miles on it - 1996 model year. Yes, the DMF is original. Although they changed the clutch at about 43,000 miles, they did NOT change the flywheel (done before I owned the car.)

No, it appears that your flywheel had NOT been replaced, only the clutch. I find that many shops do NOT change the flywheel in these 993's. More conservative shops tend to consider changing your flywheel as the dual-mass tends to degrade over time and is a concern in anything over 10 years old. (See Steve Weiner's comments above.)


Yes, your FRA numbers seem really high. From my understanding, your TRA numbers should be less than 0.3 and FRA numbers should be around 1.0. These numbers, when they reach a threshold (not sure what that is on the 993), can trigger a CEL which tells operator the engine is running poorly enough to warrant a service check.


Here goes my scientific explanation of fuel trim. So car experts, please feel free to chime in here if I'm off base on something ...


You are correct is saying the TRA are fuel adaption by the DME at idle and FRA is fuel adaption under load. The way I remember it is - "FRA is like freeway" which is driving car under load, and TRA is therefore for idle. Fuel trim are calculated as the percentage change in fuel over time that the DME is adding or subtracting from the baseline air/fuel mix in order to optimize the burning of fuel (14.7:1 is ideal air fuel ratio).

The air/fuel ratio (A/F) is measured using oxygen sensors in the exhaust. When the gas is burned completely, the oxygen content is low. When the fuel is left unburned, the oxygen content measure is high. If the oxygen sensors read a lean mixture, it's response is to tell the engines computer (DME) to add fuel. One way to do this is by lengthening injector pulse. In a rich condition, the O2 sensors tell the DME to reduce the fuel to be burned, thereby shortening the injector pulse.

In addition to TRA and FRA values, there are STFT's and LTFT's which stand for short term and long term fuel trim values. Short term FT's refers to the immediate changes in fuel occurring several times per second. The long term FT's refer to changes in STFT's but over a longer period of time. It's a similar concept to measuring "instantaneous" versus "averaged" mile-per-gallon measurements that we can sometimes read on our more modern day cars. My understanding is these measurements for STFT's can be negative or positive, but should all be around or less than 5%. An older or more inefficient car would read higher readings like 8 or 9%.

The reason why fuel trim is measured and monitored by mechanics is because the DME adjusts for normal variations in an engines environment such as RPM, engine load, altitude, intake temperature, etc. However, there are other factors that cause these fuel trim numbers to go out of range that should be fixed since the engine is NOT functioning in optimum range, and, thus, is more inefficient and produce more pollutants.

Some things I know that can cause these abnormal fuel trim numbers (please feel free to add to the list) are vacuum leaks and leaky injectors. A vacuum leak can cause a lean mixture by sucking in more air than usual and cause the DME to make the fuel mixture more rich, thereby wasting gas. If your injectors are dirty, they can leak by not close completely, adding more fuel than needed and at the wrong times which causes a rich fuel source and the DME tries to correct for this with a leaner mixture, but cannot do this adequately. As a result, your car simply does not run right smoothly and also wastes gas and more pollutants.

Hope I'm on track with these comments.

BesideTheBox

Marc - Thanks for all the information. I think you are on track w.r.t. FRA/TRA and STFT/LTFT. I understand that there aren't absolutes. Numbers will vary based on how the car is driven. I've also been told that for our cars, the LTFT should be closer to zero.

And your numbers?

mgianzero

A quick look with my PST-2 showed TRA for bank 1 = -0.19 and bank 2 = -0.11.

My FRA for both banks 1 and 2 were 1.02.

I have to remember where the LTFT numbers are in the PST-2. They are not on the same screen.

Lorenfb

"MY LTFTs are at -3.9 (bank1) and -6.3 (bank2)."

The DME ECM is attempting to correct for a rich fuel mixture condition the
result of, e.g. high fuel pressure, MAF sensor, marginal O2 sensors, or cylinder
head sensor. It's correct to state that a normal running engine should have
long trims at about zero and are percentage changes in the mixture, i.e. the
injection time percent change. The short term trims are actual injection time
changes and indicate how the DME ECM is responding to the values received
from the narrow band front O2 sensors of an OBDII 993.

For more info check here under FRA & TRA; http://www.systemsc.com/glossary.htm

Jesus Aguirre

2011 cayenne engine codes po300 po305 note five plugs were black except for one one injector tip had oil.