Macster

In the case of the OP's car, he posted reading misfire error codes from the car's DME. (He didn't post the specific error codes but they are: P0300 - random/multiple cylinder misfires detected; P0301 to P0306 misfires detected for specific cylinders. Might add the DME constantly monitors the flywheel's acceleration from each cylinder's power stroke and if it detects a low (or high) acceleration over a span of N number of power strokes the DME will set the misfire code(s) and turn on the CEL. If the misfire is the type that can expose the converters to raw gasoline the CEL flashes signaling the converters are at risk of damage.)

My policy is to only call a misfire a misfire when it is backed up by one or more misfire error codes.

Anything else is a hesitation, stumble, bog, rough running, etc. Now one may feel the hesitation/bog/stumble/etc and shortly thereafter the check engine light may come on and the error code(s) may be misfire error codes, but until that happens saying the engine is misfiring is a bit misleading.

Absent a check engine light and one or more error codes doesn't mean one can't trouble-shoot a hesitation, stumble, etc., but the task is made a bit more difficult because the engine controller has not yet id'd anything connected to the engine's odd or abnormal behavior. As far as the engine controller is concerned the engine and its population of sensors are working just fine.

This in itself is important because it does suggest the engine's behavior is not due to what usually causes real misfires: bad plug(s); coils; to name two of the most common causes.

But oftentimes eliminating that a plug or coil -- just in case -- is the cause of the engine's behavior is cheaper/easier to do than eliminating other possible causes. Always, though, absent any other signs of a problem that could be linked to the engine's behavior.

I mean if the engine was stumbling, hesitating, etc, and oil smoke was pouring out of the exhaust that's not likely due to a bad a plug or a coil, but more likely due to a bad AOS, and then to the AOS is where one's diagnostic focus should be.

Sincerely,

Macster.

s329

Thanks Macster. Your reply confirms my thoughts that a misfire isn't an obvious error that can be identified easily nor with 100% accuracy. Having said that, most Porsche owners on this board are hyper sensitive to their cars' behavior (and rightly so) that any "new" symptom no matter how big or small will bother them. I know because I'm that way...

Macster

Well, a 'misfire' backed up with one or more error DTC misfire error codes does have a diagnostics path to follow. While the root cause may not be obvious various tests are provided for and working through these will (should) have the root cause known.

But a stumble, hesitation, and so on again absent any CEL and DTCs is a bit harder to diagnose.

One important key is to note under what conditons/circumstances the symptoms appear. The symptom could come from not the engine per se but if it occurs at about the same point in the accelerator pedal's action it could come from a faulty or binding e-Gas pedal's mechanism, as one example.

We are all to some extent a bit hyper about our cars. I in fact am/have been hyper about any car. I rely quite a bit on my cars and expect/demand reliable transportation. I make an effort to give my cars good service, treat them rather well, and in turn I want (demand) relability, dependability. There is however a limit to what I'll spend to ensure the car's reliable. I will not spend huge sums of money in order to keep my cars reliable and on the road.

Sincerely,

Macster.

s2Rick

OK.,Maybe 6000 miles on an engine that went through an ims bearing failure. New chains,L&N updated shaft and bearing new gaskets etc. Everything drove fine for about 5000 miles. In the process of putting on a fixed wing the car sat in my garage for 3or 4 wks.During that time we received 8 inches of rain in a weeks time,but the car stayed dry.Started mis firing when started driving after this wing addition Also purchased the wiring harness to by-pass the power from the original spoiler. Thought maybe moisture and ran additives to dry any possible water in fuel.Then put in new plugs,then new coils,then a new maf sensor no change in mis fire and new air filter. It shows up on the scanner in different cylinders and # 1 cy. seems to also misfire on a regular basis as mis fires on low rpm's especially when first starting off in 1st gear but can run in 4th gear and slow speed to 30 or 31mph and it will mis fire. at higher speeds it seems to run great. It does'nt do this every single time but the cel will flash when the problem is worse than other times.I did not put on the cold air take till after this problem started. Do not use any kind of lube for any air filter. My indie thinks it may be fuel related as stated before. He wants me to leave it with him for a few days to check out. Also sprayed some on the engine while running to check for vacuum leaks. any ideas? Thanks

mattmotos

I had a similar issue. Porsche eventually tracked it down to flywheel vibration (after they did all coils, etc). After replacing the flywheel I haven't had any issues.

s2Rick

How did they determine it was the fly wheel?

Macster

If misfires started after wing installation while I'm not familiar with the job one has to suspect something disturbed during this operation. An non-spoiler related section of the wiring harness possibly damaged, one that perhaps connects to a camshaft positon sensor or VarioCam Plus camshaft solenoid.

Or a fuel pressure regulator vacuum line/connector/wiring harness got damaged. A fuel pressure check out seems called for. (More on this below.)

That you have replaced the plugs, coils, MAF you have eliminated the more common sources of misfires which suggests the failure resides elsewhere.

The sitting around weeks unused then the symptom appearing does by itself have some significance. It does add considerable weight to your tech's opinion the symptom may be fuel related.

If the fuel filter is a servicable item it may be worth doing this. But before I did this I'd consider draining the gas tank and refilling it with fresh fuel. It may be worth while, worth cost to open the fuel system as close to the injectors as possible and at the same time a fuel system pressure and delivery test performed.

Another check out worth considering would be removing the oil filter housing and carefully dumping its content of oil and filter into a clean drain pan and looking for any evidence of debris of a sinster nature.

That the engine experienced a IMS bearing failure and was resurrected could mean -- it depends somewhat upon the rep of the shop who did the engine -- that some IMS bearing trash ended up in one or both VarioCam Plus actuators.

With the proper diagnostics computer the tech can control both the camshaft advance/retard feature and the variable lift feature and observe the various sensor activity (O2 mainly) to know that this system is working properly.

Briefly, while this is from a Turbo reference I think it may of some value: a Cycle of 1000 crank revs is evaluated. For misfiring that puts the TWC (3-way converters) at risk, 200 revs.

If the misfire rate is greater than the threshold value a fault is recorded.

The CEL is turned on and stays on when the misfire rate stays above the threhsold value at which emissions limits are exceeded during 2 consecutive driving cycles..

If the misfire rate may lead to TWC damage the CEL flashes. If the misfire rate is not reached during the first trip the MIL goes out. If the rate is reached during the 2nd trip the CEL flashes. If this misfire rate is subsequently no longer reached the CEL changes to a steady on state.

Misfiires are not recorded if the fuel tank is nearly empty.

In the event of a short circuit to B+ or ground in the O2 sensors ahead of the TWC, teh mixture becomes too lean or too rich. This can cause misfiring. If an O2 sensor fault is stored in DTC memory the cause of the O2 sensor fault needs to addressed first the faults cleared and the vehicle road tested.

Ok misfire possible causes...

Ingition system fault.

Injection system fault.

Flat-base tappets (valve lift fault).

Mixture too rich.

Mixture too lean.

Valve lifter chattering. This is caused by dirt in the lifter. One might hear this chattering during the misfiring. (To hear this over the noise a misfiring engine makes almost certainly requires listening to the engine camshaft covers with a mechanic's stethescope to detect valve noises.) The mixture adaptation values [long term fuel trims I think] are normal.

Now this chattering, if (big if) caused by dirt, this dirt might come from debris from the IMS bearing having gotten into the valve lifters. (My info is that when a Porsche tech evaluates an engine that has experienced an IMS bearing failure he checks for IMS metal/debris on the 'clean' and high pressure side of the oiling system, for instance checking scavage pumps in the heads. If any metal debris found in these then the metal debris has gotten into the lifters, lifter bores, etc. and the engine is not a candidate for rebuilding.)

However, the manual does go on to say remove the lifter bores [lifters I think] and check for damage and blow out the oil passages. Replace all lifters. During test drive listen for lifter noises.

Another possibility is the camshaft control times. No chattering noises occur. The DME signals misfiring for bank 1 or 2. The mixture adaptation values in the idle speed range differ from bank 1 to bank 2.

(I observed this in my Boxster while its VarioCam solenoid/actuator were acting up.)

The mixture ranges in the upwer and lower load ranges were generally normal.

(I observed this in my Boxster while its VarioCam solenoid/actuator were acting up.)

There is more, serveral pages worth (at least) that cover testing the VarioCam timing/lift action and fuel pressure/flow and so on. Best for you and me if you want the info PM me with your email address and I'll make a PDF copy and send it to you.

Sincerely,

Macster.

mattmotos

Trial and error. Ended up sending computer results to PCNA (or whomever who the Porsche dealership calls when they can't figure it out). It took them three times of me bringing it back. CEL kept going off. Plugs, coils, MAFs... everything done before they looked for help outside the dealership. They said it was the vibrations that kept tripping the light. Out of balance perhaps.

s2Rick

But did the vib in the flywheel cause it to mis-fire or just cause the cel to show up? Thanks

mattmotos

Just the CEL to show up. Everything else was fine after replacement.