986.2 misfire at high rpm

[liquidfunk]

Hello all!

I've currently been trying to hunt down the cause of the misfires on my 2003 Boxster S with 84k miles. It started off as a Bank 1 Knock sensor, which was replaced. After changing the fault for the censor went away but the misfire still persisted. The codes are P0300, P0301, and P0304. Afterwards I read on forums and replaced the spark plugs, and coil packs. Even after replacing those, the codes still persist. It's an awkward case since the car doesn't misfire till it hits 5,000 rpm and pulls hard up until then. I've switched around the spark plugs and coil packs with no luck. Has anyone else encountered something of this nature before? The air filter I've replaced, and the IMS was replaced at 79,000 miles. I'm currently at a loss on what it could be. I don't see any bore scoring, compression is great on all cylinders. I'm stuck on what could be the cause of the high rpm misfires.

I greatly appreciate anyone's help! Thank you!

[ike84]

Not sure about the codes, but my initial thought is fueling issue. It could be a flow restriction for the entire system (filter) or for a single cylinder (injector). Low fuel will run lean and knock.

If the codes are bank specific, start with the injectors. The easiest thing to do there would be swap the injector from bank 1 to bank 2 and vice versa if the codes then switch banks, you have your issue

If they codes are both banks, check the fuel pump flow volumes. If it's low, either the pump or the filter are likely culprits (I'd start with the filter)


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[tcoradeschi]

Originally Posted by ike84

Not sure about the codes, but my initial thought is fueling issue. It could be a flow restriction for the entire system (filter) or for a single cylinder (injector). Low fuel will run lean and knock.

If the codes are bank specific, start with the injectors. The easiest thing to do there would be swap the injector from bank 1 to bank 2 and vice versa if the codes then switch banks, you have your issue

If they codes are both banks, check the fuel pump flow volumes. If it's low, either the pump or the filter are likely culprits (I'd start with the filter)


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Those codes are for misfires in cylinders 1 & 4.

[ike84]

1 and 4 are the closest cylinders to the front firewall on either side. If 5 and 6 arent knocking, the system is likely getting enough fuel as a whole (5 and 6 are the last cylinders to receive fuel along the lines).

Id start with moving injectors around and see if the knock follows.

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[liquidfunk]

Originally Posted by ike84

1 and 4 are the closest cylinders to the front firewall on either side. If 5 and 6 arent knocking, the system is likely getting enough fuel as a whole (5 and 6 are the last cylinders to receive fuel along the lines).

Id start with moving injectors around and see if the knock follows.

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It's been awhile due to work but as an update. I got a durametric diag tool. Fuel trim, and everything is reading properly. I'll take your advice here and move the injectors to see if it causes any issues. I did notice that on Bank 2, there's no cam deviation it's stuck at 0.00 while bank 1 reads 1.20. When I look at the actual angle readings, it reads just fine and updates as rpm increases. So is there an issue here or it's something to not gawk at whatsoever? I know that it's supposed to read but I'm unsure what's going on.

[JZeke]

I wonder if the variocam solenoid is failing here, as ~5000 is when it kicks over to the high RPM profile. At least being a 986.2 it's easier to replace than the earlier models (it can be removed without removing the valve cover in a 986.2.)

[PLP]

Any update on it?\

Mine behaves similarly. It starts smooth, drives just fine at all ranges of RPM until I push it hard to 6k RPM range. Then it will flash CEL and store pending misfire code for multiple cylinders.

How to verify fuel starvation at high RPM?

I do have Durametric.

Also, I was planning on changing spark plugs this season, so anything specific to look for?

[Newsguy]

I'm surprised your miss wasn't cured with new coil packs. That is what it took on my 99 and it's been fine since and that's been about 7 years. You know, just because they are new doesn't mean they can't be bad! I once had two new bad thermostats in a row.
And as for deviation, I know the 2.5s didn't show bank two cam deviation, but I don't know if that is true for the 2003.

[maz8062]

I'm surprised no one mentioned MAF sensor.

If you have Durametric you can datalog the MAF at idle for Hot start voltage and readings. If out of spec swap it out and it should be fine.