My 02 996 is throwing a CEL and the codes say it misfiring on cylinders 2, 4, and 6. I got a lot of good scoop from forum members on another site about possible rodent damage, coil packs, and such. It seems to me that there is some common item failure that would control the coils for cylinders 2, 4, and 6. Anybody have any ideas? Im on the road for the next couple of weeks and only home on weekends, so i'm trying to trouble shoot as much as I can on the road and if I need any parts, have them at the ready when I start to tear into the car this weekend.


Thanks in advance for any advice.


John

 

One common item failure that could affect coils on both banks (but not all coils on both banks) is the wiring harness.


There is also the DME.


Another is the MAF. In this case all cylinders could eventually misfire but once some misfires appear the driver changes how he drives the car or stops driving the car. Either is understandable.


Yet another possible source of misfires is a malfunctioning VarioCam Plus system. But there are two systems, one for each bank, and I believe each is isolated from the other to some extent.


Not sure how (or if it is even possible) a failure on one bank could cause a misfire on a cylinder on another bank other than the misfire of the cylinder on the other otherwise unaffected bank is being caused by the (severity?) of the misfires of the cylinders on the other bank.


Misfire is just saying the output of the cylinder is not within spec. The output is measured by the acceleration imparted to the flywheel during each cylinder's power stroke. If two cylinders were misfiring, for whatever reason, on one bank this could possibly upset intake air flow into a cylinder on the other bank, or perhaps cause the cylinder to start out with a handicap of having an already sick engine and even though the cylinder is ok it can't overcome the handicap. IOWS, does the DME judge each cylinder on relative or absolute terms. I'm not sure if the misfire algorithm is that sophisticated. Misfires are naturally rare and point to something pardon the expression, amiss. The misfire algorithm has done its job by id'ing the worst offenders, turning on the CEL and storing the associated error codes. It is then up to the tech to deal with the major offenders then retest the engine. If the misfires at other cylinder reappear then he'll know. If not then the assumption is the misfires at other cylinders were collateral damage, so to speak, arising from the real misfiring cylinders.


Let's see. Fuel quality and supply. While I have encountered stale, very stale, fuel -- the fuel sat a year in the gas tank of a family member's car while she was in the hospital -- the engine never misfired. It proved to be down on power which the engine's transformation after a fill up with fresh gasoline made quite obvious.


If the fuel level is too low -- I'm not sure if the low fuel warning light has to be on or the fuel level down to say 1/4 a tank -- misfires are not flagged. So low fuel caused misfires won't have the car in the shop needlessly.


But if there is a fuel supply problem at the injectors yet with an adequate amount of fuel in the tank...


There is a fuel filter and a rather small one. Another gotcha is the Turbo's fuel supply is exceptionally complex, using siphon jets one on each side of the split tank to siphon fuel out of this basin and keep the fuel pump adequately supplied with sufficient fuel.


In some cases if one of the siphon lines fails and the engine dies, runs out of fuel, with about a half a tank showing. All the remaining fuel has collected in the basin with the non-functioning siphon jet.


If the problem was just not quite bad enough the fuel pump maybe getting fuel with too much air in it and the fuel pressure could be varying. The DME expects a consistent fuel pressure as it opens each fuel injector. If the pressure were too low, or too high, the injector could inject too little or too much fuel. Misfires could occur. The misfires might not be evenly distributed across the population of cylinders either.

An under performing alternator can also cause misfires. However, in the few cases I have indirectly experienced this as a cause there were other signs of a power supply problem: other vehicle subsystems had lit warning lights along with warning messages displayed on the dash.

Even if one of the above proves to be the cause, or even if someone puts forth the correct diagnosis, the engine will not fix itself.

You, or someone, is going to have to get his hands dirty.

When faced with a problem like this I like to start eliminating possible/reasonable explanations that do not require tons of labor or expensive parts. I mean, I would not be real happy with myself to pay for a plug/coil R&R only to find out afterwards when the misfires returned that the problem was with a bad fuel filter.

On this note, generally, unless there are compelling reasons, I prefer to leave fuel system pressure/flow testing to the last. I mean I'd replace a fuel filter without any real qualms but I would not dive in to a pressure/flow test right out of the starting, not without a smoking gun so to speak, clearly indicating a fuel supply/pressure problem.

 

Thanks Macster,


I ordered 6 plugs and coils yesterday. The plugs aren't very old, but might as well replace them when I have it apart replacing the coils. At this point, I'm leaning towards those coils 3 coils being cracked and got wet when I washed the car. Either way, they are 12 years old and will fail at one point anyway. Should be in by Friday and ill be installing on sunday. Also, checking the wiring harnesses. Ill let you know what I find.
Thx!
john

 

Could be coils. My 03 Turbo misfired the other day when I started it. It has been a bit humid here of late. The engine fired right up but was running rough enough -- but not that rough -- that I suspected what was going to happen and sure enough the CEL came on.


Later I checked the error codes and they were P0303, P0360 and P0300.


I'm getting the car in week after next to get the coils replaced. It is time, I think, at least for my Turbo: A 2003 with 124K miles on the original coils.


(I'm spoiled: My 02 Boxster with 281K miles has its original coils and has only misfired just once and this was after the car sat a few days and it turned cool and more humid.)


One point I have to make. The idle did smooth out and there was no untoward behavior from the engine. After I read the error codes I cleared the errors and extinguished the CEL and the engine ran just fine and didn't misfire again. 'course, it hasn't been subjected to the humidity we had here a few days ago.