Generation IV Internal Engine - Cylinder #7 misfire at idle only!! TRIED IT ALL!! help please

had this lil misfire problem around ever since i melted the plug in #7 on the
N2O last year but, changed the plugs from the TR6's to the NGK BR7EF (2 heat range colder) gapped at .035 mind you im boosting now....

I have read all the post it feels like that is related to ALL the possible reasons for misfires etc....I have tried just about everything; including consulting several professionals. Among the ranks would be Mike Norris and some other repitable LSx folks. Lots of good info but, only speculation at this point becuase of everthing I have tried as far as testing comes back in speck!

So....here I am...so your familiar with the car...

It is an LS4(5.3L) step child LOL!! FWD I KNOW I KNOW...BLAH BLAH lets get that out of the way LOL...It has a MP T70 Rear mounted...only engine modifications are a 160* tstat and 2 heat range colder plugs....the car is transversely mounted obviously and is on manifolds that come into a single downpipe with no CAT but a race pipe, no ressy and straight into the turbo...running 69lb injects with a walbro 255 in the tank and running a catch can...

car is not a kit car from STS...we bought some of thier solutions to make our parts and design work...so this is the world's first RMT LS4...

Now, it has an oil pump return system just as the STS cars do as well....returns the valve cover...
Gotta tranny cooler and thats about it.....

it is an N2O car with an FJO on her...but not running N2O just yet with the turbo...but did run it last year...hints the 12 sec club LOL...

so what ive done so far:

Changed plugs........not it

regapped...............not it

switch wires...........not it

switch coil packs.....not it

switch injectors.......not it

running comp test....not it: all cylinders have compression

Leak down test........not it: all in specks


VC pull/valves/sprng..not it: all seated and looked good, no apparent breaks

cranking comp test...SOMETHING!!: had a lil leak bye from the spark plug hole on my number 7 but, went away as they threaded plug in tighter?

I think i cross threaded the plug sometime back last year with all the misfire problems after the N2O and with all the plug pulling etc....but the plug still threads properly....NOW..i did notice I have a Oily/black residue coming out of the spark plug hole in the head....

we did get some flashing SES's on the dyno....misfires are isolated on #7 and only occure at idle....and sometimes NONE AT ALL!! its very strange...none under load at all...

I read somewhere that some guys have cross threaded the plug or plugs and got misfires and put thread loc and high temp sealant on the plug and it cured their ills due to the plug not getting full contact with the head and sealing assuring contact...this was the theory atleast and it worked!! for THEM!!

Yes, i tried it...why not...well...misfires went even more crazy and melted the crap right off!! LOL...i know..idiot me...hey, give me a break...im trying everything!!

Plug looks good in porselan but, the base ring is DARK DARK DARK which I feel and some others say rich....and yes the car is boosting a lil rich right now but, the other plugs dont look quite as dark...

car is on 8psi running 11.5afr and 15-18* of timing throughout the tune.....

no cam!! factory rods, springs, pistons, connectors, lifters...etc...all OE except for what I have listed....

Was going to take the next step and have the head helicoiled from a shop that might be able to do it on the car....my #7 is the front-driver side if looking at the car...so atleast easy to get to!!

I know...should take the head off...well, doing the best with what i have....trying to avoid that at all cost until the end...thats when Mike Norris is probably going to take a look...I live in a small apt with no tools or a garage...im limited to my back seat and trunk....this is a sad reminder of what happens when a guy that is an ethusiast that doesnt have all the resources to do it right at hand but, can make it happen LOL....

Cars getting a street/strip tranny rebuild currently to allow my FWD ass to slam it 1000 plus whp if I desired....goaling for 500 for right now...

so with the long post...just wanted no stick unturned here....Mike is saying could be a valve guide sticking the exhaust valve or holding it up a lil bit....

but really wont know until the helicoil or the head has to come off i guess....

car idle is good...power is good...not cut out or anything...VERY WEIRD...and if you do a search for residue coming out of a spark plug hole head...or use any of those words together...you only get a few post to come up...the top on is my build thread LOL!!

Help please...if you can...

i wouldnt hesitate to pull the head. you have exhausted all your resources with the head on. the oily residue leads me to either a cracked head or something wrong with the guide/seal.

If it is not a electrical gremlin then its the head itself... goop you mentioned makes me agree with it being the head itself. Good luck!

Thanks guys....keep it coming if anyone knows anyone else with this similar problem....from what i have read so fare past the misfire process of elimination, that this is the first of the goop coming out of the head....so maybe making some history here LOL....

You said it is not the coil, plug wire, spark plug or injector. Did you swap these parts from another cylinder? I had a spark plug with a hair line crack in the porcelain one time that periodically caused a mis fire. Did you try a noid light?

Because it's number 7, I'd say that you have a cracked ring-land. Pull the heads and inspect.

You probably have only a partial leak of compression, a possible valve seating issue or even the broken head land scenario on the piston. I say that only because you mentioned you melted the plug. So that cylinder has been very hot!

I run into these problems from time to time on regular old run of the mill cars at our shop. I just did a 2006 Hummer H3 with a 3.5 last week. It had a very similar problem to what you describe, misfire at idle, and then goes away off idle. Long story short it was a leaking intake valve. When I say 'leaking' I am only talking a small amount. This was not a DEAD cylinder misfire. This truck had been to the dealer as well as several other shops in the area. They all missed it. There was only a difference of 14 psi between cylinders on a cranking compression test. A cylinder leak test was performed and did not show the problem, more than likely because the procedure was not done correctly. Piston position in the cylinder is critical when doing this test to find small abnormalities.
Also, valve sealing issues can come and go, especially when dealing with carbon concerns on the valve face or seat, or both,

At our shop we use electronic methods to find mechanical engine concerns, it is much more accurate. Often times guys who use the age old mechanical methods perform the procedure with error and it skews the results. Procedure is everything when doing mechanical testing with mechanical methods, either leak down or cranking compression or even running compression.

The methods I am posting are common diagnostic methods used by many today in the trade, so my point is you may need to seek a 'pro' to put your finger on the problem. But you will most likely be pulling the head for starters.

Major compression leaks are easy, partial leaks are a bitch. The Misfire Monitor on today's cars can pick up even the slightest irregularity in crankshaft rotational velocity, from dirty injectors to slight losses in compression.

With it being #7 (the common problem on these engines) I would think you hurt something inside the engine.

Here is one of the simpler tests that can be performed with a high amp clamp around the battery cable and cranking of the engine with the ignition disabled. If all the cylinders pump the same compression the starter amp draw will be equal. Here can see the #5 cyl on this Hummer was slightly low. Clue #1!

http://i160.photobucket.com/albums/t162/2helicoil1/Shop/EagleVisioncamretardproject004.jpg

http://i160.photobucket.com/albums/t162/2helicoil1/Shop/HummerCrankingCurrent.jpg

With an in cylinder pressure transducer in place of the sparkplug you can do a cranking test as well as a running test. Coupled with a scope you get a picture as to what is going on within the cylinder.

The running test is usually more informative. Thing like valve timing and valve sealing can be seen here. (Yes, even with no combustion). The benefits of a running test is the pressure goes up in the hole, so a leak path will become more pronounced.

http://i160.photobucket.com/albums/t162/2helicoil1/Shop/EagleVisioncamretardproject015.jpg

http://i160.photobucket.com/albums/t162/2helicoil1/Shop/EagleVisioncamretardproject020.jpg

http://i160.photobucket.com/albums/t162/2helicoil1/Shop/CompressionWaveform.jpg

http://i160.photobucket.com/albums/t162/2helicoil1/Shop/CompressionWaveformevery90.jpg

http://i160.photobucket.com/albums/t162/2helicoil1/Shop/CompressionMisfirezoomed.jpg

http://i160.photobucket.com/albums/t162/2helicoil1/Shop/CarbonedValvesTopTierGas.jpg

Obviously from what you described you are working with limited equipment, sometimes it is just easier to seek out someone with the tools to the job. If it was always easy then guys like me wouldn't need to spend thousands on equipment. Good luck with it, but I would pull it down, you have exhausted all possibilities outside the cylinder, except for a wiring concern.

i had the same thing a couple miles down the road #7 said bye bye and blew up the ringland cracked

so how would you explain the residue coming out of the plug hole past the threads??? this is what points me to the cross threading....and funny thing is, when i put that sealant and tephlon tape on the threads....the misfire got EXTREMELY MORE PRONOUNCED!!!! then went away when i cleaned it off....and its funny too...the misfires sometimes dont even show their faces....only once in a great while...and then, for no reason...back at it again!!

and a counterpart of mine with a front mount LS4 with meth....said that they were eating up ringlands left and right in 5 and 7 due to lean conditions in the engine....and they kept putting more and more gas in it to compensate...which i think was the culprate right there in combo with meth....but this was never a problem much more until before we started booting when i cross threaded the plug....

and to answer Gadgetized question, yes...i used parts from another cylinder...to see if it moved...and im familiar with the cracked porselon...i actually did that when i cross threaded the plug.....

i have run into the same situation before. 5.3L, dead miss on #3 at idle, bring rpms up and misfire ceases. did it all too, swapped plugs, coils, wires, no change. performed active fuel injector test and found injectors working normally. pulled valve cover and found valve spring and all in tact. found normal compression and cylinder leakdown. pulled the head and didnt really find anything abnormal, sent the head off to machine shop to be worked. re-installed the head and found misfire gone. i would venture to guess you have a head issue

If at all possible, find someone with a Borescope, maybe a friend, local shop, get them to look into that cylinder and it might give you some clues.

This is all being pushed through guys!! thank you thank you!! ill post up when I find the issue....Im going to start slow first....get the helicoil done...see if we can do it on the car...these guys have been doing the local stuff for quite some time now and are very very skilled and very careful...i know the risk...going to try that first....especially since my misfires get more pronounced with some tape on the plug threads....and that residue on the head out of the plug hole in the head....and it showing a little leakage out of that hole during comp test....idk which one though...running or cranking...

Thanks again though guys...going to probably rent a scope and take a look-see as well...

A very leaky valve seal will cause your problem. At idle the oil getting by (sucked in from the valve cover oil mist) will cause a mifire....but when rpm's rise you'll be ok.

No?


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A very leaky valve seal will cause your problem. At idle the oil getting by (sucked in from the valve cover oil mist) will cause a mifire....but when rpm's rise you'll be ok.

No?


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Sounds probable....i found a used head just in case...it came off of a car functioning well!

A very leaky valve seal will cause your problem. At idle the oil getting by (sucked in from the valve cover oil mist) will cause a mifire....but when rpm's rise you'll be ok.

No?


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possibly...it would still pull more in the higher the revs go, just like it pulls more air thru the intake as the revs go up.