Boxster Misfire - suggestions? - 986 Forum - The Community for Porsche Boxster & Cayman Owners

Legba · 10-03-2012, 01:54 AM

Can anyone advise on the following please?

Boxster 2.5 1998 63000 miles.
Serviced 800 miles ago including new spark plugs and air filter. It had been misfiring prior to this and it improved it but it is now worse than ever.
The car is not used often and when it is used it is usually for short runs. However, I did take it for a long run last week and since then the problem has been worse.
I have not identified any air leaks.
I have tried disconnecting the MAF and it made no difference.
I have cleaned the MAF with electrical contract cleaner. Prior to cleaning it looked a bit dirty and is now clean. This made no difference.
As this problem is getting worse and the weather is getting colder could there be a connection?

Symptoms:
Cold start idle is fine. Hot idle is fine.
On revving above about 2000 rpm there is a misfire and revs drop to idle speed.
If I run it and rev it long enough for the engine to reach operating temperature the engine runs ok.
For some time prior to this the engine occasionally misfired about 4000rpm but this would eventually clear.
I borrowed a fault code reader and it showed misfire on 123, I cleared it and ran the car again and this time it showed misfire on 456. No other faults shown.

mikefocke · 10-03-2012, 04:36 AM

Change the AOS ever on the car? Oil in the intake tube? Any smoke?

Ever changed the 14 year old coil packs?

Worst kind of driving you can do to the car is infrequent short runs.

Legba · 10-03-2012, 08:27 AM

I have read of AOS problems so that might be a possibility, and I have never charged the coils so thanks for those suggestions.

It does smoke a bit on start up occasionally but there is no apparent oil use and I thought this was just one of the Boxster characteristics.

The car was MOTd a couple of weeks ago and emissions were fine.

I warmed the car up this morning and it seemed to be running well so I took it for a fast run. It cruised smoothly at 85 for a couple of miles then started to misfire and lose power. OBD showed misfire on 456 so my thought that this was only a cold running problem was wrong, although it is much worse when the engine is cold.

Thanks for the suggestions so far, any other suggestions are very welcome.

The Radium King · 10-03-2012, 08:42 AM

i agree with coil packs as an issue. it is odd, however, that it is swinging from one bank to the other. bad spark could be due to fouling of plugs, but then one would expect sign of oil in exhaust. perhaps also bad spark/misfire due to leaking injectors, but codes should indicate mix problems. similarly, symptoms also indicate potential fuel delivery problems (plugged filter, bad pump or pressure regulator) but again codes should indicate mix problems.

Legba · 10-04-2012, 01:30 AM

Thanks for the suggestions, I have removed and checked the plugs and there is no sign of fouling and there are no other fault codes.

I think I will replace the coils.

Can anyone suggest anything else before I spent lots of money?

The Radium King · 10-04-2012, 08:10 AM

sniff your oil; if it smells like gas your injectors might be leaking. a cheap test, but still sounds like coils.

Legba · 10-04-2012, 11:56 AM

Thanks Radium King, I'll try that.

I borrowed a fault code reader and used live data. I found the following but don't know what it means. Can anyone help?
At the time I had these readings I had a fault code showing misfire on 456.
Fuel Sys1: CL
Fuel Sys2: OL Fault
STFT B1 25%
LTFT B1 0%
STFT B2 0%
LTFT B2 0%

At other times I noticed that the STFT and LTFT for B1 and B2 mirrored each other so I suspect that the STFT B1 is probably indicating a fault.

shadrach74 · 10-04-2012, 12:16 PM

Quote:

Originally Posted by Legba

Thanks Radium King, I'll try that.

I borrowed a fault code reader and used live data. I found the following but don't know what it means. Can anyone help?
At the time I had these readings I had a fault code showing misfire on 456.
Fuel Sys1: CL
Fuel Sys2: OL Fault
STFT B1 25%
LTFT B1 0%
STFT B2 0%
LTFT B2 0%

At other times I noticed that the STFT and LTFT for B1 and B2 mirrored each other so I suspect that the STFT B1 is probably indicating a fault.

You've a fuel system issue.

STFT = Short term Fuel Trim
LTFL = Long Term Fuel Trim
B1 = Bank 1
B2 = Bank 2

0% means that you fuel system is operating in open loop, which is to say that it is not adjusting mixture based on input from the 02 sensors.

Your car is running based on the factory preset fuel program. it is metering fuel based on throttle position for the most part with some input from the MAF and coolant temp.

Something is certainly arrive and I don't think it's the coil pack (at least not as far as the codes provided.

Legba · 10-08-2012, 02:05 AM

Sorry if this is a bit long but.....

I am trying to interpret some data to track down a misfire and need some help to interpret live OBD data on my 1998 2.5 Boxster (UK spec).

Can anyone tell me what the following means?

STFT (Short Term Fuel Trim) and
LTFT (Long Term Fuel Trim)

At times I have seen STFT vary from 0 to 25%. Presumably the readings for B1 and B2 should mirror each other.

I also have readings for:
SHRTFT
SHRTFT 12
SHRTFT 22

What do they mean? What is the difference between STFT and SHRTFT?

The readings obviously vary depending on driving conditions but a snapshot I got was:

STFT B1 18.75%
LTFT B1 0.78%
STFT B2 18.75%
LTFT B2 0.78%

SHRTFT = 0%
SHRTFT 12 = 99.21%
SHRTFT 22 = 99.21%

I also have readings for O2S which I assume is the voltage of the O2 sensors.

What does the following mean?

O2SL 11 12 21 22
O2S11 = 0
O2S12 = 0.435
O2S 21 = 0
O2S 22 = 0.435

Grateful for any help.

Bruce Wayne · 10-09-2012, 09:40 AM

i would look at the coil packs too, where they sit, they get a lot of heat and corrosion.

shadrach74 · 10-09-2012, 10:46 AM

Quote:

Originally Posted by Bruce Wayne

i would look at the coil packs too, where they sit, they get a lot of heat and corrosion.

Why would coil packs cause an issue with fuel trim?

Bruce Wayne · 10-09-2012, 10:59 AM

apologies, dog tired, didnt scroll down to the fuel trim posts.

shadrach74 · 10-09-2012, 11:51 AM

Quote:

Originally Posted by Bruce Wayne

apologies, dog tired, didnt scroll down to the fuel trim posts.

It's possible you may be correct. It's not inconceivable that the fuel trim issue is separate from the misfire. I'd go after the known malfunction first though.

shadrach74 · 10-09-2012, 12:06 PM

Legba,

I was thinking you might check the cats. I have heard of instances where a plugged exhaust can cause the symptoms you describe. Bring the car up to operating temp gently, then hit both the front and the back of the cat with an infrared thermometer. Exit temp of the cat should be much hotter than the entrance. I would also give the cats a vey light blows with a mallet and listen for loose particulate.

Keep us posted!

Legba · 10-10-2012, 04:14 AM

Some very useful responses. Many thanks.

Things have changed!

I now have both Fuel Sys 1 and 2 showing OL (open loop i believe).
STFT and LTFT all read 0% and do not change with load.
O2 sensors (pre-cat, my car does not have post cat) read 0.435v and do not change with load.

No fault codes are showing.

It was suggested to me that the car is now running on its basic mapping without any input from sensors. I have disconnected that battery for 24 hours to see it anything re-sets but without any result.