Lean Misfires at 2700-2800 rpm

[ProwlerGT]

Is it possible to have a bad valve or piston and compression still tests good or ok?

I'm asking now cause I see in the first post to this thread that compression tested good.

[markoverclock]

Originally Posted by ProwlerGT

Are you saying thats what fixed yours?

Just for an FYI for me.... whats the difference in forged pistons vs stock ones? What does it mean when people say "forged" ahead of something regarding engines etc... mechanical stuff.. Yes, I know thats a newbie kinda question, but I don't have forged pistons so I don't know. lol

im saying that my engine is a fresh rebuild with forged internals. and im haveing the problem.

foged means that the materials used in making the pistons is top of the line quality they mix alloy metals to make them stronger. so they dont break like the stock ones they handle lots of abuse reliably.

[ProwlerGT]

Originally Posted by markoverclock

im saying that my engine is a fresh rebuild with forged internals. and im haveing the problem.

forged means that the materials used in making the pistons is top of the line quality they mix alloy metals to make them stronger. so they dont break like the stock ones they handle lots of abuse reliably.

Oh that sucks ballz.

and

Thanks for the explanation on "forged". Quite a simple concept yet effective too.

[DoctorDex]

Originally Posted by ProwlerGT

Is it possible to have a bad valve or piston and compression still tests good or ok?

The only scenarios I can think of where something related to a valve would show good compression and leak down would be:

1) Collapsed lifter - valve would stay closed all the time. If this was the case, it should make a horrible racket, especially at idle and it may get better with rpm as oil pressure increases??

2) If there was a broken valve spring (broken on either end). The broken spring would have enough force to open/close the valve while cranking during a compression test, but it might not allow the valve to close fully every time while. If this was the case, you'd assume it would get worse with rpm because the valve would be floating??

3) If the valve guide was worn, which would allow the valve to oscillate and occasionally it would not seat properly. If this was the case, one would assume the valve guide seals would be bad as well??

This list is not comprehensive, its just the only scenarios I can dream up at the moment. Please feel free to correct or add on to my list.

Otherwise, a bad valve should show up as poor compression and/or leak down.

[DoctorDex]

Update post #1. Just wondering if anybody has noticed this:

"9) When I rev the car in neutral, while keeping the throttle position constant, the rpms gradually increase until it gets to ~ 2700 rpm. Then it stumbles, the rpms drop to 2500 rpm and then the cycle repeats itself. If I give it enough gas, it'll eventually make it over the hiccup, until it reaches 3000 rpm and then the same thing happens again. It also wants to do it at 3500 rpm. Its only once I get over 4000 rpm that it revs gradually and smoothly."

[DoctorDex]

Just throwing an idea out.

I've noticed in my logs that while it is missing the graph of my rpm starts to oscillate. I realize this could be caused by the missing, but it could it be the other way around and could it be the cause of the missing?

[badass3000]

ive been wondering that too^

Originally Posted by ProwlerGT

You guys may want to check this out. This dude may have inadvertently solved this problem. Would be good if so... Have ya'll inspected your pistons thoroughly already? I haven't checked your list in a while.

I just got done installing a brand new dealership shortblock and it made no difference. and my heads have just been rebuilt by DR.
Nice try but no beans.

driving on 5 cylinders is a totaly different feeling, ive even driven on 4 cylinders in my VR4. My ECU went crazy, gave a constant trigger to #6 injector, the fuel backed up SOO bad it found its way to the nearby #4 cylinder and filled it with fuel too.

[DoctorDex]

Just throwing out more random thoughts:

1) I find it strange while it is missing the O2 sensor voltages of both the front and rear banks are pretty much identical. This makes me wonder if the missing is occuring on both banks?

2) Is it possible to check for a "dirty" rpm signal? Could one tap into the PTU rpm signal and then again at the ECU and compare the two using an oscilloscope? Maybe it might be an idea to run a seperate shielded wire from the PTU and/or CAS to the ECU and see if it makes a difference?

3) With regards to the methanol test, I don't know if it was entirely accurate because there was an extra variable I didn't consider previously. While spraying the meth, I used the MAFTranslator "trigger wire" to pull the maximum amount of fuel while spraying. This would have fooled the ECU into thinking there was significantly less load (air) than there actually was and as result it increased timing to a maximum. In addition, the size of the nozzles I was using were way overkill for that test (2x375ml @ 220psi). I was literally flooding the motor with methanol. This would have made the engine conditions very similar to what it sees during cold start (low load & +++ fuel).

I think I may try this test again, using a tiny nozzle (ie. 60 cc/min @ 220 psi) and hopefully this way I won't have to pull fuel to compensate. The only thing stopping me is that I need a 60 cc/min nozzle and after the last time running 100% meth, I fried all the seals in my quick connect fittings & check valves. I ended up having to re-plumb my entire alchy system.

[DoctorDex]

Bob: Do you have any ideas of how one would test for excesive EMI interfering with rpm/injector/ptu signals? Would temporarily disconnecting the alternator temporarily reduce the amount of EMI (for testing purposes), or would one have to remove the belt entirely?

[markoverclock]

Originally Posted by DoctorDex

Bob: Do you have any ideas of how one would test for excesive EMI interfering with rpm/injector/ptu signals? Would temporarily disconnecting the alternator temporarily reduce the amount of EMI (for testing purposes), or would one have to remove the belt entirely?

remove the regulator plug that would be a quick test. it wont make a mag feild then (hense the no charging)