Okay RL - Now I'm looking for some help in diagnosing my engine misfire problem.

I've noticed recently that my engine seems to idle roughly (more than the usual). But it only occurs once the engine is warm and it is not consistent.

But today after I pulled the car into the garage I notice the same rough idle again. When I put my PST-2 on it read as a problem with only on right bank for cylinders 4, 5, 6 as codes 53, 54, and 55 (P0304, P0305 and P0306). When I turned off each injector using the PST-2, the idle became noticeably rougher. Any ideas where to look? Bad injectors, ignition coil? Any thing else I should run on my PST-2 to narrow down the problem?

I brought the car to a local Porsche shop last week who could not reproduce the codes (I admit the car, although it was warm, ran smoothly when they tested it.) They said it needed to "act up" in order for them to diagnose and they didn't have an answer as to why it was intermittent.


Marc G.

 

Misfires are often hard to find as you know. What about some other parameters, like when were the plugs last done, caps/rotors? Were the injectors every cleaned? What is your mileage now? Every one the old compression/leakdown to check the health of your valves?

I noticed on your screens you managed to get to the Operational Duration -> of 2562hrs. I thought that the 993 ECU did not record how many hours on the engine. The pictures are not high resolution, what screen are you on?

Cheers,

Mike

 

Hey guys. Thanks for all your comments. Let me fill you in on some details which I failed to provide with my initial post so as to not become too wordy.

I changed the distributor caps / rotors, belts (all Porsche OEM), and spark plug wires (Beru OEM) all within the last 6 months. I even just purchased a new idler valve (Bosch OEM) just 2 months ago. The injectors were flushed about 1 1/2 year ago. The plugs have about 20-25K on them. When I run the car on one distributor or turn off each cylinder by injector (using PST-2 software) I get a noticeable change in smoothness of idle.

When I drive the car, there doesn't seem to be any loss of power. But, after the car is warm, the car intermittently idles rough. It's not always consistent. Sometimes it's warm and runs smooth, other times it does not. When it idles rough, it seems to consistently misfire.

I've gotten so used to working on these 993's that I immediately noticed a slight rough idle which was confirmed with a misfire from my PST-2 about a month or two ago.

Could the coils be losing their effectively when warm? Perhaps I should change the plugs? What's the significance of an engine temp sensor that always seems to read >256 F?

Marc G.

 

It is telling you that it is dead, change it, erase all error codes and you will be fine
Operation duration is not on ECU, but on PST2.
Operation duration of a job started on PST2, but apparently, never stopped.
regards

 

That's the upper limit of the 8-bit A/D converter. And it doesn't take very long to reach that, at which point the Motronic ECU basically ignores it. Unless you're seeing a fault code for the CHT?

 

Thanks George! I have never used that variable, so that is why the confusion. I seem to remember that operation hours is only available on the 996 and up, correct?

Cheers,

Mike

 

Yes Mike, you are right, my best regards

 

Most 993 mis-fire codes result from aftermarket fan belts.
Then, a lean condition the result of a MAF sensor or low fuel pressure may be an issue.
A simple test to help eliminate ignition problems is to just run with the upper or lower
ignitions, i.e. by disconnecting either of the two ignition modules.

Check here for some 993 running data under 'Porsche 993 MAF & DME System Data';
http://www.systemsc.com/tests.htm

 

Marc,
How is your car running? Resolution on this?

 

Marc,
You had a whole laundry list of things you've had done recently including the changing of the belts, but I did not see where you stated your mileage.

The reason I am stating this because I am wondering if the renewing of the belts included the distributor belt?

I was reading not to long ago, that a wearing distributor belt can manifest itself in rough idle. I suspect however, that the rough idle would be more of a consistent indicator in a wearing dist belt, while your issue description appears more inconsistent?

Sorry, I hadn't realized this was such an old post...Current status/resolution?

 

All guessing so I will add my own. Intake leak. Changes as leak changes due to warm-up changing parts fit where it is leaking. Make sure the clamp holding the MAF Sensor's pipe in is tight - it is next to and behind the idle regulator on the top of the engine. Not sure about other leak potential spots.\
Andy

 

Okay guys (and gals) ... I've been driving my targa again. Especially now that it has PSS10's.

But the misfire codes came back again. But this time even worse. That is to say, they are throwing a CEL in addition to multiple misfire codes. They are also not isolated to just one bank, but now they are EVERYWHERE. That is, every cylinder will read a misfire at some point.

And now the engine does seem to idle more rough, at times. Other times, however, it idles smooth. Seems to be worse after she gets hot, but can be random.

Then I took another look at my PST-2 and noticed something very unusual, but also revealing. I went to a local Porsche shop and they agree.

Here's the pic. Can anyone take a guess at what we found?

 

[QUOTE=Lorenfb;10199008]Most 993 mis-fire codes result from aftermarket fan belts.QUOTE]

How do fan belts cause a mis-fire?
Andy

 

Your engine appears to be running above redline.
RPM sensor issue?

 

[QUOTE=pp000830;10539566]Other RL'ers, feel free to chime in on this one. But the way I understand it ... it has been shown that if you do NOT use the Porsche belts, you may get a misfire. Apparently, the belt thicknesses are very unique to the way Porsche makes them. The way the 993 DME determines a misfire is by determining the acceleration / deceleration of the crank sensor with each rotation. A non-OEM belt can cause the RPM sensor to misread the crank rotations as a misfire because other belts do not turn as smoothly on a V-belt tensioner system.

Actually, I was told that the sensor rarely goes bad. Rather, a "phantom" misfire is caused by the dual-mass flywheel separating. This vibration, like the belts, can effect the spinning rotation of the crank. A really high RPM like 10,000 is really impossible with this motor, but the failing flywheel can cause these apparently non-sensible readings. Also, the fact that my engine misfire jumps around from every cylinder is another indication that it's not really a true cylinder misfire, but rather a "phantom" misfire.

The car does idle rough, but only randomly as I believe the failing DMF is tricking the DME to make adjustments to A/F ratio. Other RL'ers may better explain this.

Also, when I start and turn off the car, as well as accelerate / decelerate quickly in neutral, I can hear the rattling of the DMF. I've had that rattle for a while, but it's been progressively worsening, and is now creating a CEL for misfires. But my understanding is I can still drive the car until I need to repeat my smog since a failing DMF doesn't really do much more than that idle rough at times.

I can try to record the rattling and rough idle that's become quite apparent in my car for others to witness if there's interest to hear this.

Any experts on this, please feel free to chime on this to confirm, deny, or add to these comments the way I understand it.