V6 missing at idle and beyond

[sudso]

I'm trying to track down the cause of my 3.2 missing at idle.

Its the rear drivers side cylinder and I've already ruled out plugs and coil packs by swapping them around and disconnecting them one by one with the motor running to check the difference.

The motor shakes a bit at idle and I can hear it missing at the exhaust end but it seems to smooth out more as the rpms increases.


I'm thinking that it's 1 of 5 things:

valve(s) not seating in that cylinder,
tappet shim worn,
unmetered air leak into that cylinder,
head gasket or,
cracked head

There's no signs of oil contamination in the cooling system or water contamination in the engine oil.
No water dripping from the exhaust at full temp either.

It misses no matter what the engine temperature is too and the "Check Engine" light hasn't come on either.

What I want to know is there any 'do it yourself at home' ways of checking for all those 5 things without pulling the thing to bits?

Oh yer, it's a 98 Rodeo EFI 3.2 quad cam, no dizzy

cheers, sudso

[DamTriton]

Stuck/blocked injector??

Depressurize the fuel rail and see if you can swap that cyl's injector from one bank to the other, and see if the miss is still on that rear cylinder.

[CWBYUP]

Burnt valve, stuck valve, hole in piston ( dont laugh it happens with out of tune LPG ) Pull the rocker covers off and see if the lifter / valve is moving properly.

Nick

[sudso]

Stuck/blocked injector??

Depressurize the fuel rail and see if you can swap that cyl's injector from one bank to the other, and see if the miss is still on that rear cylinder.

Nup, does it on gas too.
I'm wondering if power is actually getting to that coil pack though. Might pull that and the plug out to see.

[sudso]

Burnt valve, stuck valve, hole in piston ( dont laugh it happens with out of tune LPG ) Pull the rocker covers off and see if the lifter / valve is moving properly.

Nick

Not what I wanted to hear Nick. I hate you
But your'e right I know. It is on LPG too and the symptom is the same on both fuels so it cant be the injectors and the sparkplug does fire.

Oh yeah I forgot to mention that it started doing it after I accidently redlined it while climbing granite and chasing rabbits in paddocks.

The heads were done about 70,000 kms ago with harder seats so I hope it's not that.
Like I said there's no bubbles in the radiator, no oil or water contanimation anywhere so I dont think it's a hole in the piston.

Probably is something to do with the valves or an air leak there somewhere.
Will have to take off the cover like you said
cheers, dave

[DamTriton]

Stuck/blocked injector??

Depressurize the fuel rail and see if you can swap that cyl's injector from one bank to the other, and see if the miss is still on that rear cylinder.

Nup, does it on gas too.
I'm wondering if power is actually getting to that coil pack though. Might pull that and the plug out to see.

Be a bit careful when you try to measure anything on the coil, even the primary side of the coil can give you a rather uncomfortable "tickle" on the HO EFI systems

[sudso]

Thanks. I want to test ECM to see how much voltage is actually coming from it while the engine is running but I'm not sure which multimeter probes go to what terminals in the coil pack connector.
Might have to ask this one in Electrical.
cheers

[LUX90]

Check the intake manifold gasket as well, they commonly crack as they ars a composite plastic and metal gasket. The plastic cracks and creates a vacum leak

[Hoppy11]

Vacume leak????, the commo V6 in the MK/Hummer, had a leak in one of the hoses and made it idle like shit
Hoppy

[lay80n]

This will sound stupid, but i used to see it heaps when i was working for holden. Moisture builds up in the plug holes, and causes misses. Try blowing the whole area out with compressed air (take lead out and blow out lead connection and plug hole). Long shot, but you would be supprised how many mystery missfire issues this has fixed. Esp if you have been hooning through water, or have large temp/humidity changes causeing condensation.

layto....

[ricky1970]

For vacume leeks, CAREFULLY spray aerostart, carb cleaner, even WD40 in small squirts in suspect spots with engine running, if leaks sucks it in, idle will smooth out. Is a bit dangerous if the prob is a earthing plug lead, so a fire extinguisher handy isnt a bad idea!

[V8Patrol]

Stuck/blocked injector??

Depressurize the fuel rail and see if you can swap that cyl's injector from one bank to the other, and see if the miss is still on that rear cylinder.

X2

[Goatse.AJ]

Stuck/blocked injector??

Depressurize the fuel rail and see if you can swap that cyl's injector from one bank to the other, and see if the miss is still on that rear cylinder.

X2

X3

[sudso]

Burnt valve, stuck valve, hole in piston ( dont laugh it happens with out of tune LPG ) Pull the rocker covers off and see if the lifter / valve is moving properly.

Nick

Thanks for all the tips guys, they will all come in handy one day but today the winner is Nick.

I did a compression test on that pot and it was only 50psi
So I got the piston up to top dead on compression and connect my air comp. to a fitting for the plug hole.
Air blew straight out the exhaust pipe.

So it looks like an exhaust valve problem. It's a quad cam too so I might have 2 burnt ones
Guess what I'm stuck doing this weekend

[Goatse.AJ]

Bummer mate.

[CWBYUP]

I'd pull the rocker covers off and just see if its stuck or burnt. If its just stuck you might be able to get away with just freeing it up for a quick fix.

Did it on my VS and it lasted 4 months before it did it again.

Nick

[Ruffy]

On post 1950 engines valves don't "Stick" unless there is a problem.. Don't be fooled into thinking that you can "Free" a stuck valve and have your problem solved...
Unless you have a good understanding of the workings of the valve train on an internal combustion engine you wont be aware of the miriad of faults that can seem like a "stuck" valve.
Good luck pulling your heads off!...

[CWBYUP]

Sorry Sudso If it is a stuck valve / lifter by freeing it you will only fix it for a short period if time until it does it again.

Basically your valve dosent fully reseat itself and you lose hose power and it runs rough.

Nick

[V8Patrol]

2 questions ........

I did a compression test on that pot and it was only 50psi

So what did the others come in @ ?????????????

So I got the piston up to top dead on compression and connect my air comp. to a fitting for the plug hole.
Air blew straight out the exhaust pipe.

Something doesnt sound right here......
50psi on a compression test shouldnt = "air blew straight out the exhaust pipe"

Are you 100% sure it was on TDC on the firing stroke and not TDC on the zorst stroke ???

Guess it comes down to how much air was rushing out the zorst

Kingy

[sudso]

2 questions ........

I did a compression test on that pot and it was only 50psi

So what did the others come in @ ?????????????

So I got the piston up to top dead on compression and connect my air comp. to a fitting for the plug hole.
Air blew straight out the exhaust pipe.

Something doesnt sound right here......
50psi on a compression test shouldnt = "air blew straight out the exhaust pipe"

Are you 100% sure it was on TDC on the firing stroke and not TDC on the zorst stroke ???

Guess it comes down to how much air was rushing out the zorst

Kingy

It's a V6 and I only tested the other 2 pots on the same bank. They were 145psi which is within the specs.

Yes it was on TDC of compression stroke, all the valves on that pot were closed (visibly)
So when I pressurised the chamber with my air compressor you could hear and feel the air coming out the exhaust pipe which indicates to me one or both of the exhaust valves in that cylinder is not seating.
That tells me it's probably a burnt valve.
I have to take the RH head off and have a looksy.

Worst of all this is a long weekend here and next weekend is the Riverland 4x4 challenge which I wanted to compete in