cylinder 6 not firing - but spark, fuel, compression seem ok (solved)

[eddy6241]

Strange issue would appreciate some help.


car developed misfire, stopping and starting the engine fixed it a couple times for a few days, but now makes no difference constant misfire in cyl 6. it starts and runs on 5 cylinders. it drivable, but poorly.

pulling the connector plug off on coilpack 6 only makes no diff to the misfire, therefore it must be cylinder 6. (pulling plug on other coilpacks makes misfire worse.)

however removing coilpack and plug from #6 and stiiting it on manifold (with a spare plug back in plughole 6) and sparks fine! i did the same procedure on cyl 1 to compare spark strength , and spark is about the same.. so why misfire?

the car is dual fuel, and same issue on gas or petrol, i flicked between the two when warm and made no difference. and spark plug comes out wet when cold (starts on petrol) , so there seems to be fuel.

compression on #6 with engine running does seems low, reving it a bit on a cold engine = about 100psi, but surely this is enough psi for combustion? cylinder 1 was 140psi . i havent checked others.

i thought maybe it was ECU, since the problem cleared that time by restarting the car couple times. plugs and coils were all done a few months ago with the same symptom, it didnt go away initially, but then did after fiddling around it came good for a few months, now im wondering if the coils and plugs were ever the issue..

any help or ideas would be greatly appreciated..

ive read a stretched timing chain or head gasket issue could theoretically cause this, but why did it come good after a stop/restart those couple times.. and i dont want to go down that path to find it was much simpler... the engine 235000kms. no mods.

 

[DavesSV6Tonner]

I feel your pain. Twice I have had the same issue as yours (I don't have dual fuel though) and both times the #6 miss fire was caused by a faulty ECU.
Replacement refurbished ECU approx $1000.00 installed.

I should add that spark is not the issue, the ECU isn't opening #6 injector.
There is a possibility it is simply a fuel injector failure which can be checked by swapping injectors over ie swap injector #6 with #3 and see if the problem moves to cylinder #3. If the miss fire stays at #6 then the ECU is the likely culprit.

 

[Fu Manchu]

Or a loose wire in the connector. When you remove the plug and have it out it is getting contact again?

 

[eddy6241]

bugger, i sent off the ECU to injectronics (along with the key, PIM and BCM module) for a repair/remanufacture, but they tested and it was fine (that cost about $350 once i got it back via repco , injectronics dont deal direct)

so i took off plenum and properly did the compressions warm etc and cylinder 6 is 25 psi (the rest 155 to 160) ill put up another post to see what people think might have caused that..

 

[eddy6241]

hi , VZ exec wagon, 175kw dual fuel 235000kms

i have constant misfire on cyl 6, and CEL, thought it was plugs or coils or ECU had all that checked/replaced (see my other post)

carefully did a compression test, all cylinders are 155 to 160psi EXCEPT # 6 is 25psi

thats bad. seems thats the issue.

adding capful of oil into # 6 doesnt help, maybe 5 or 10 more psi, so i assume rings are ok.

I took off rocker cover and all springs, rocker arms etc all seem ok on # 6 with a tiny bit of clearance on rocker arms on compression stroke, nothing broken, all operating the same as # 5 and # 4 , cant identify or feel anything wrong from looking on top there fiddling around.

blowing air back through the plug hole (by mouth) with all valves closed on # 6, there is no air leak back out the inlet side valves, (i sit a piece of paper over the inlet hole and it doesnt move at all with air movement) , so i am thinking the air is escaping past the exhaust valves. so is that carbon, or a damaged valve seat? they dont seem to be riding open, but i am not an expert working that out.

has anyone come across this and can advise where to next? how hard is it to pull and reinstall a head , ive never done one. do all 3 timing chains have to come off and is it a big job, is there anyway to potentially solve this problem without removing a head..

the inlet valves on # 6 did seem much blacker than all the others, looking down the inlet channels, but maybe thats because i was running the car for 30 mins or so on 5 cylinders , so that is not a root cause, just a side effect of no combustion for 30 mins driving? im not sure..

any help on where i should go from here, i really dont want to pull the head off...

 

[Skylarking]

Such low compression could be due to failed piston rings, hole in the piston, burnt valve seats or failed head gasket or very worn cylinder walls. If the spark plug looks OK and you don't remember the engine pinging lots and you haven't played with the tune (ECU), then i'd think a hole burnt into the piston would be less likley.

One thing you can do is to get a cheap borescope inspection camera (for the cost of a few coffee's from ebay). These are cheap and connects to your phone where you can see what you can see. Stick the borescope camera down the #6 spark plug hole to have a look at the pistone top. Then remove the camera and rotate the engine so #6 piston is at the bottom of the stroke. Again insert the camera and look at the cylinder walls. If the piston top and walls look OK then it's likely the issue is burnt valve guides or head gasket. You can check inlet valve seats by taking off manifolds and rotating #6 until inlet vales are open then sticking camera down the inlet port. Same can be done with exhaust valve seats. If valve seat looks OK, then it's likely herad gasket. Also look at where the #6 spark plug sits in the head and check where it seals against as sometimes this can burn out if plug not installed correctly.

When compression is so low, if piston or cylinder walls, or valve seats, or spark plug seat are stuffed, then it should be rather apparent when looking at such.

Other than that, i dont have any other suggestions.

If you do such checks, it's still a bit of work but nowhere near as bad as pulling a head off, post pics

 

[Immortality]

Sounds like the head needs to come off.

Seems you already have a thread on this subject so I dont really see the need to start another?

Merged threads.

 

[eddy6241]

yes, i am awaiting a ebay borescope to see what that shows, but i think the passenger side head will need to come off regardless... but i cant find a diy on alloytech head removal on the forum here, if anyone can point me to one that be great.. i am wondering also can i just remove a single camchain to get 1 head off?

 

[shane_3800]

Could be lifters. Pull the VCT solenoids and see if there is much gunk on the filter screens.

 

[shane_3800]

You could possibly take the chain off.one side but the issue is you time the engine off the chain markings. So I would advise against it.