Thread: Mystery misfire

IMHO it is unlikely to be a lifter as it would tend to empty of oil when not used.

I would be thinking about a sticking exhaust valve .

If it were only on petrol I would suspect an injector or a break in the injector loom..

BTW in my experience you don't notice a miss in a V8 once the revs are up a bit. It may still be missing.
Maybe you could try to see the inside of the cylinder with a borescope and look after it has warmed up.

Another suggestion is to look at the combustion with a Colortune.
Regards Philip A

Slight wear on the distributor cap contact perhaps..

Can't make a suggestion on the misfire (although Tombie might be on the right path with the dizzy cap, or perhaps the plug lead) but you should be aware that these engines do not like synthetic oils. They need a high zinc mineral oil (see recent posts on this subject) and you will find very high wear on cam lobes if you use synthetic.

Thanks for the input guys.

I have new leads, plugs, cap and button. The cap and button are Lucas. All changed after the misfire started.

I switched to synthetic as I had it there left over from my last car and thought I might as well use it.

I keep going over the list of things that could cause a single cylinder misfire, but when I compare them with the list of things that may occur when the engine is not started for a day or two the only thing that seems possible is oil and lifters. Failing ignition systems generally either only get worse or are moisture sensitive. LPG removes any possible ECU or fuelling issue. How would a day off fix mechanical problems like compression and sticky valves?

Could the oil pressure be slightly higher after a day of rest? Maybe sedimentation in the filter falls off during the day off and the oil pressure increases for a few kms? The normal oil pressure is relatively low at idle, so maybe mine is a little lower and I have a leaky lifter that can't perform its job with the slightly lower oil pressure. The oil light goes out straight after starting, so I assume the oil pressure is ok.

I have had the synthetic and filter in there for a few thousand kms, I might just dump it and try good old 20W50 mineral oil.

G`day ,


If you have or know someone that has a Vacuum gauge because you are able to read it when it is missing and not , it may help in your quest .

I`d also suggest when it is missing remove the plug and check its colour before the miss goes which in turn may clean the plug to be the same colour as the others .

Philips valve idea has merit in its self but also because your 93 ( factory ) has the later stem seals but the earlier valve type stems which is a common mix for carbon build up , specially on short distance runners .

Thanks PLR.

I don't have a vacuum gauge. On the topic of vacuum. I have noticed that on the drivers side of the plenum chamber right next to where the #1 cylinder port starts there is a vacuum hose that goes to the vacuum reservoir (big round ball) that is used for the fresh/recycle air vent actuator. An air leak in that system could cause #1 to be slightly more lean. I will have to investigate that.

I have also removed the plug while misfiring and not. It looks just like the others. I have swapped plugs around with no change.

Try nipping up the inlet manifold bolts, I've often seen them come loose. Not sure how sitting around would change things other than from the gasket swelling slightly with ambient moisture.

Originally Posted by Fity

Thanks PLR.

I don't have a vacuum gauge. On the topic of vacuum. I have noticed that on the drivers side of the plenum chamber right next to where the #1 cylinder port starts there is a vacuum hose that goes to the vacuum reservoir (big round ball) that is used for the fresh/recycle air vent actuator. An air leak in that system could cause #1 to be slightly more lean. I will have to investigate that.

I have also removed the plug while misfiring and not. It looks just like the others. I have swapped plugs around with no change.

G`day ,

i could be wrong but i think the runner your looking at is for coolant .

The vac for the ball should come off the drive side just below the top cover from the manifold or similar .

I`m not sure how your finding the miss , can you hear it?

Does the engine rock and can you feel it by touch ?

Can you feel the miss at the tailpipe by putting you fingers over and covering it loosely ?

Is the norm that it goes after reaching running temp ?

Have you noted the miss from cold to running temp at idle while stationary ?

Do you still have the old plugs or do you remember the colours they gave ?

If it were lean the plug colour will tell , even short term it shows and with either fuel .

On fairly new plugs with a couple of hundred Ks on them the white will be whiter on the lean ones , they need to be looked at closely but then it is obvious .