BimmersInGA

The deets: 2002 Boxster 2.7, 5-speed manual, 99K miles, spotty maintenance history.

I bought this car about 4 months ago and know very little about its history other than a clutch replacement (around 60K miles) and a new alternator at around 80K miles. So, as a precaution, I went over it from stem to stern and did all the remedial maintenance I thought it would need. This included a new serpentine belt, air filter, cabin filter, oil/filter change, coolant flush, and a few other odds and ends. I've been spending money on BMWs for the last 15 years and if I've learned anything besides how expensive they are, it's that they gleefully punish those who neglect them.

Anyhoo, this Boxster has been running great and is a ton of fun. I really like it! I took it on the first longish trip since I've had it (about 500 miles round trip). While I was at my destination, I was cruising in traffic (about 30MPH in 3rd gear), when the CEL flashed a few times. I was able to cancel the light by using the stalk on the steering column. I stopped, switched off the ignition, restarted it, and the light stayed out. Then, a mile or so later it flashed again and stayed on steady.

The car has been running fine. No hesitation, stuttering, rough idle, or poor acceleration. In other words, there have been no symptoms of any kind (except for the light, of course).

Since I was away from home and my diagnostic tools, I stopped at an Advance Auto Parts and had the codes read. They are P0300, P0304, P0305, and P0306 - all related to misfires. From my BMW experience, this is almost always caused by bad ignition coils (or spark plugs). However, there are usually also driveability issues. But before I just throw that money at it, I wanted to see if there were any other theories. I have read here and on other forums that it could also be a bad cam solenoid or oxygen sensor.

Since all three misfires are on the same side of the engine and all happened at the same time, I'm wondering if it could be the O2 sensor? Is there a definitive test I can perform to single out the problem?

Lastly, which bank is which? Is 1-3 on the driver (left) side and 4-6 on the passenger (right)?

Macster

Did any of that remedial maintenance include per chance changing the spark plugs? If so last thing touched.. which includes not only the plugs but the coils which could have been damaged by rough handling or not reinstalled correctly, at the plug tip but also the connector at the harness.

Cyls 1-3 (aka bank 1) are on the passenger side, and of course cyls 4-6 (aka bank 2) are on the driver's side.

O2 sensors are very critical to the proper operation of the engine. In some respects this is not what you think but simply ensuring the engine is fed an air fuel mixture that results in exhaust gases with a make up that falls into a very narrow range which is most efficiently/thoroughly processed by the converters.

As a result the DME checks the O2 sensors's function often and quite thoroughly and would if it found anything amiss log one or more O2 sensor error codes.

Misfires are also critical for this can result in the converters getting fed raw gasoline and this can damage them in no time. Since converters are expensive enough to fall under the emissions warranty automakers take great pains to flag any problem that puts these at risk.

Be that as it may I have not come across anything that suggests the Boxster DME is misfire trigger happy. (Some cars have misfire trigger happy DMEs' though.)

So, a long way to making this point that the misfires are probably not O2 sensor related.

I do not know where you live or more importantly where the car has lived, spent the time before it came into your hands.

If this has been in a humid area, one that receives a lot of rain, if the car has been driven in the winter, IOWS, the coils subjected to harsher environmental conditions than those on a car that spent its life in a drier clime, I would guess "coils".

Even if you tell me the car has spent its lifetime in AZ and has never seen rain since day one, I'd still guess "coils".

You can swap the "bad" coils for the "good" coils and see if the misfires folllow the coils.

This assumes the misfires are not a sign of something deeper something more serious is going on. This is rare but not unknown.

To this end I assume the oil level is good, the coolant level is good and there are no signs of any coolant/oil intermix?

A problem with swapping coils is they are old (likely original) and while they can sometimes last a long time (the coils in my 02 Boxster are OE and have covered 256K miles) and this handling can send a marginal coil over the edge.

What could happen is the "good" coils become "bad" from the handling and one or more misfires occur at the same cyls with the "good" coils. This can lead you on a wild goose chase, well make your diagnosis harder.

Would I replace the coils outright?

Probably not right away. If you do though and if the plugs are of unknown age/type you should consider replacing these. Be sure you remove each one carefully and note its condition. If you find one or more (say all on bank 2) different from those of the other bank...

Couple of questions. Any chance on your trip you filled up the car at an out of the way station, perhaps filled it up with some brand of gasoline that the engine has been been exposed to before? I assume it was not wet/raining during the drive? That the car didn't even get driven through any water say on the road from lawn spriklers?

You might want to carefully remove the oil filter housing and dump its oil out into a *clean* drain pan along with the filter element. Examine the oil closely. If you see scary bits of metal that's not coils....

Last there have been a few (well at least one) case where a bad MAF caused misfires yet only on one side of the engine.

If you are feeling up to it, you can disconnect the MAF at the wiring harness clear the error codes which in this case resets the long term fuel trims to their defaults (learned from info supplied by in this case a suspected MAF) then drive the car as you were the time the misfires happened.

I might point out in some way you can eliminate the sensors from the list of suspects by carefully swapping sensors from side to side. Do not swap bank 1 sensor 1 (sensor before the converter) with bank 2 sensor 2 (sensor after the converter) (the leads may not fit/reach) but instead swap bank 1 sensor 1 with bank 2 sensor 1 and bank 1 sensor 2 with bank 2 sensor 2. Clear as mud eh?

Drive the car.

Caution: If at any time the engine starts making any extraordinary noises, it become rougher running, starts smoking, or just acting up in general shut off the engine ASAP. Your time diagnosing the engine's problems just expired. A professional tech's attention is needed.

Sincerely,

Macster.

BimmersInGA

Macster:

Thanks for the info!

AFAIK, this car has spent most of it's life in the southeastern US. I am in the middle of Georgia. Very hot, very humid in summertime. Little to no snow in winter.

I have not touched the plugs or coils (yet), and if I were a bettin' man, I'd say they are original. I have been under the car a few times and the coils are definitely not new and don't look as if any have been replaced recently.

During a very recent oil change I took a good look at the filter and the interior of the filter housing and did not see anything scary.

I did fill up before heading out on my trip (to northern Florida - no rain encountered), and put about 1/3 can Seafoam in the tank. I routinely do that with BMWs before a long trip. Maybe it's not a good idea for a Porsche?

On the return trip with the car seemingly running fine, I averaged 27MPG on mostly open highway and the interstate. That is the best mileage I've gotten so far with this car.

All the error codes were misfires, there weren't any O2 sensor errors.

I don't mind spending the money to replace the coils and plugs, but obviously don't want to do it if I don't have to. But that does seem like the most likely cause.

I cleared the codes and after commuting to work for a couple of days they have not returned. I'm watching that little orange light like a hawk, though. Just waiting for it to come back on. I even embarrassed myself by stalling it at a stoplight this afternoon because I was so focused on that light that I didn't see the other light change to green! I haven't stalled a car like that since I was teenager (and NASA was still sending men to the moon when I was born).

Macster

The car has spent its life in a humid place and even though there is no snow there is still plenty of salt around and road splash and humidty day after day will degrade the coils.

Up to you but if the misfires come back without any other symptoms/noisies new coils and plugs I think should be given some consideration.

There is no need to use any additive in your Porsche's gas or oil on a regular basis. Thus I would advise you to stop using Seafoam.

If you suspect the car was fed a subpar gasoline or subjected to short trips or just not driven with enthusiasm you can try a bottle or two of Techron mixed according to directions. When I bought a used (10K miles) car I used at least one bottle of Techron according to directions just because I didn't know how the car was driven by the previous owner but given what I learned about the car -- before I bought it -- and based on its superb condition I suspect the majority of those 10K miles were put on the car driving out and back into the garage to wash and wipe the car down.

After you have driven the car enough that the last tank of gas with Techron added is down say 1/3rd or 1/4th of a tank change the oil/filter.

Then use a top tier gasoline, but from a busy station, and use the car.

I have to point out -- I have seen too much of this -- these cars are not tractors or diesel big rigs. They can stand and in fact benefit from operating at a higher rpm range. No need to drive on the fwy at speed in 2nd gear to keep the rpms at redline but avoid like I watched one owner as he drove away he had the tranny in 4th gear in just a few car lengths after starting from a dead stop.

Keep your eyes and ears open and at the first sign of any thing amiss shut down the engine. I do not expect you'll see or hear anything amiss but when an engine suddenly gets noisy it never helps the engine to keep running it in the hope it will somehow fix itself.

It won't.

And I did I mention to not use Seafoam?

Sincerely,

Macster.

BimmersInGA

From now on, I will refrain from using Seafoam

FWIW, I did buy a Porsche and not a Buick, and treat it as such. The tach needle sees the upper half of the dial on a regular basis. As it was meant to!

I will keep a close eye on things and throw a set of plugs and coils on it if it acts up.

Thanks again for the advice!