[Torbjorn968]

I have my car in for a remapping at a well credited place. Everything is fine the first runs, then when warm it missfires on the 2nd cylinder over 5000 RPM.



The ingintion cables, rotor and distributor has been changes, spark plugs and compression checked. Variocam cam shaft position indicatior and flywheel indicator checked.



Any ideas? A broken valve spring has been one suggestion.



From last year the main changes is a 996 GT 3 flywheel modified for the car by a Polish member of this forum.



//TL

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[flash]

did you check the codes?

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[94SilverCab]

Rod bearing? Usually #2 goes first...



Jay

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[tamathumper]

Why would a bad rod bearing cause a misfire?



I understand a misfire hammering a rod bearing, but I don't get the other way 'round.

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[craigawoodman]

Valve spring - higher rpm - it does not close properly. Lower rpm, dynamic loads and speed are not enough to cause a problem. Could be the valve itself, but there would have to be a reason it was only one valve (i.e. something injested and went through one cylinder only). If a valve spring has partially failed, then you might be getting unburnt fuel that you could pick up from an exhaust sensor (would read rich over 5,000 rpm). Blocked injector (able to deliver fuel up to a certain point and then not at full load?).



Is the mis-fire a knock (pre-ignition) or a mis-fire (i.e. incorrect ignition)?



Could the rings in #2 be poor (have you done a compression test and leak down test)?



Have you checked the coil? Is it only #2 cylinder?



When trouble shooting diesel engines in mining gear and locomotives, we used to measure the exhaust manifold with a heat gun. The coolest outlet was the cylinder with the problem (i.e. no ignition).



Good luck, let us know what you find.

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[Cloud9...68]

You say it's fine when cold, then gets worse as it warms up. Do you think it's more dependent on engine temperature, or ambient? You say you checked the plug wires, but how? I had a very similar problem about five years ago, and it turned out to be the plug wires. Run the engine after dark (make sure it is really dark), and look around the plug wires, coil, and distributor cap for sparks. Mine was fine at first, but after letting it warm up for a good ten minutes, I started seeing sparks all around several of the plug wires. But I also agree to run a blink test as well.

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[RS Barn]

TL,

Could be lots of things--But you need some way to diagnose ignition/electrical vs mechanical problem.

There should be some sort of code stored. You can access with blink codes or Bosch hammer or Durametric.

Due to temp sensitivity I believe it's possibly oil temp sensor giving bad signal to DME. The variocam function is temp sensitive.

If there is a problem with temp sensor ECU will go into safe mode and switch to batch firing. This will cut power and rev limit to 6k

Once ECU senses problem, code needs to be cleared or problem will continue to happen. There is a 6 minute timer in ECU

Pete

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[RS Barn]

We've also seen misfiring from bad crank sensor or incorrect gap to flywheel trigger gear. change in flywheel could be possible cause also.

You need to check crank sensor signal with ociliscope to see if in range.

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[Torbjorn968]

[quote name='craigawoodman' timestamp='1306498602' post='110343']

Valve spring - higher rpm - it does not close properly.



>>Now checked, springs look good accordint the the mechanic, will check regarding the valve and other suggestions.





Lower rpm, dynamic loads and speed are not enough to cause a problem. Could be the valve itself, but there would have to be a reason it was only one valve (i.e. something injested and went through one cylinder only). If a valve spring has partially failed, then you might be getting unburnt fuel that you could pick up from an exhaust sensor (would read rich over 5,000 rpm). Blocked injector (able to deliver fuel up to a certain point and then not at full load?).



Is the mis-fire a knock (pre-ignition) or a mis-fire (i.e. incorrect ignition)?



>>To mme the engine was harsh picked up from the mechanic having it during the winter with a lack of power, since then it has lived a company doing remaps of race cars, they have labeled this as missfiring and have seen more of it than me, I have not driven it hard sine it was reported as running lean (that was mainly a lambda sensor that had become loose causing this...)





Could the rings in #2 be poor (have you done a compression test and leak down test)?



>>Done, good compression reported.





Have you checked the coil? Is it only #2 cylinder?



>>English failing me, coil?



When trouble shooting diesel engines in mining gear and locomotives, we used to measure the exhaust manifold with a heat gun. The coolest outlet was the cylinder with the problem (i.e. no ignition).



Good luck, let us know what you find.

[/quote]





Thanks,will pass your suggestions on

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[Torbjorn968]

[quote name='Cloud9...68' timestamp='1306536533' post='110367']

You say it's fine when cold, then gets worse as it warms up. Do you think it's more dependent on engine temperature, or ambient? You say you checked the plug wires, but how? I had a very similar problem about five years ago, and it turned out to be the plug wires. Run the engine after dark (make sure it is really dark), and look around the plug wires, coil, and distributor cap for sparks. Mine was fine at first, but after letting it warm up for a good ten minutes, I started seeing sparks all around several of the plug wires. But I also agree to run a blink test as well.

[/quote]



Plug wires was changed for new ones as was the plugs, cap, rotor and "ignition coil" if I remember my English correctly. The spark signal was checked with an ocilloscope.



Thanks for the suggestions!



//TL

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[Torbjorn968]

[quote name='RS Barn' timestamp='1306630649' post='110399']

TL,

Could be lots of things--But you need some way to diagnose ignition/electrical vs mechanical problem.

There should be some sort of code stored. You can access with blink codes or Bosch hammer or Durametric.

Due to temp sensitivity I believe it's possibly oil temp sensor giving bad signal to DME. The variocam function is temp sensitive.

If there is a problem with temp sensor ECU will go into safe mode and switch to batch firing. This will cut power and rev limit to 6k

Once ECU senses problem, code needs to be cleared or problem will continue to happen. There is a 6 minute timer in ECU

Pete

[/quote]



Thanks, interresting lead, but there is no 6 K limit. Perhaps switch back to the older flywheel.



//TL

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[flash]

if you're using the flywheel i sent you, there was no problem with it all the way to 7800 on my car

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[Torbjorn968]

Not you flywheel, goes into another car. This was a Porsche 996 GT3 clutch/flywheel adapted to the 968 by a Polish member of the forum, Tomasz, who seems to have a very good handle on his cars.



Things changed since last season:



- flywheel

- header

- race cat

- Odyssey 680 battery



//TL

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[craigawoodman]

Have you checked the spark plugs or changed them? It could be a fault in the insulation on the plug which gets worse when hot and at high rpm/load. You could simply swap the plug and see if it moves, or change the plugs. The problem may not be the insulator itself, but where it joins (if it was dropped, or bumped, etc).



If it was the insulator - you might see arcing and discharge when dark, but given it is so low in the head and the leads seal it - you will most likely not.



Coil - ignition coil.

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[Torbjorn968]

Good lead, we tried with different spark plugs.



//TL

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[Torbjorn968]

Had a try myself with the car today. Seven or so clean runs to the rew limit, the pop, spark and crackle over 3000 rpm. Uneven idle oscilating between 800 and 1200 rpm with intervals of a couple of seconds.



Test drive with a new motronic box and original mapping (car has RS Barn sport cams, larger injectors, header and exaust-



Could it be the change of the flywheel? (see above)¨



Or the new fuel preassure regulatpr (2nd hand standdard)



//TL

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[RS Barn]

TL,

Try lowering idle screw on throttle body. If anyone has raised it -Idle will cycle up and then seem to die-then up and seem to die. Similar to what you described.

Stock chip will be way too rich at wot between 5500 and 6500 with big injectors.

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[Torbjorn968]

Tomorrow all ignition items will be swopped for new ones.



Then the flywheel I would think.



Fuel preassure regulator is a replacement one, it has been tested to good values, any chance this can work 7 times and then create a problem like this?



Nothing like this experienced the previous season!



All the best!



//TL

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[flash]

i'm wondering if it's a lifter

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[Torbjorn968]

Was either the distributor cap/rotor or the sparkplug wiring. Changed for another set and it worked. But during testing at the workshop s*** happened, happy that things worked I saw smoke in the mirror. Then flames....

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