Thread: V8 Breathing Heavily and Misfire on LPG

Originally Posted by PLR

G`day Edward ,

for the miss you need to try to narrow it down to a cylinder , when it misses spend some time removing plug leads and see if you can locate it , if it`s that type of thing .

To me is still seems like two problems or maybe three problems .

The lower comp of #2 maybe valve .
The crankcase pressure may be firetraps etc .
The miss may be ignition .

The oil staying clean tells you that the combustion gasses are mostly going where they should , out of the engine and not into the crankcase .

So the flame traps etc would be worth a look .

The colour of the plugs is a good tell also but i`d suggest you look at them closely for anything not uniform .

I found a recent carb diaphragm failure by looking at new plugs with less than 200km on them burning lpg , four of the plugs were a very slightly whiter colour on the insulator and these plugs mated to the cylinders the left hand carb feeds .

Compression tests done cold relate .

Compression tests are normally done when the engine is at running or near temp because that is where it is mostly used and the tolerances are where they should be best .

Adding oil to a cylinder if cold or hot when compression testing is mostly helping to seal the compression ring .

If a large increase in either case is seen on the gauge the condition of the compression ring comes into question .

I`d suggest you do a hot compression test on all cylinders and a without and with oil on cylinder #2 .

If you get the same or similar outcome as with the cold test you have done then it is pointing to a valve problem .

This however may have nothing to do with the miss .

The norm would be for the exhaust valve to be the one to look at and for carbon to build up on the seat and back of the valve which causes the valve to not seal as it should .

Or the carbon builds up inside the valve guide which impedes the valve stem movement which also causes the valve to not seal as it should .

I have a fitting which screws into the sparkplug hole and used with an air compressor it can be used to hold valves closed if there is reason to remove valve springs without removing the head and it will also allow the loss of air to be heard if a valve is leaking etc .

Generally valve problems will be either constant or worsen when the engine warms up .

Your is not acting this way so it seems to me that an ignition fault for the miss is still possible .

Making all these different suggestions may not seem overly helpful but until you are able to narrow some things down the correct direction is awkward to find .

Thank you PLR. I'll start checking these things first thing in the morning, including the flame traps. I will start pulling the leads off one a time. The Landy does not really miss at idle, it's only when i apply load. So i guess i'll start disconnecting leads (starting with cylinder 2) on my way to work tomorrow and see if i can isolate which cylinder is the problem.

Regarding a possible valve issue, the miss is very intermittent, and certainly not regular (like i would imagine a valve miss to be like). The needle on the vacuum gauge also does not indicate a valve issue.

Edward

I pulled plug lead 2 off this morning. The Landy felt smooth (well, you know what i mean - didn't do its usual miss). Put the lead back on expecting it to start missing again, but it was perfectly smooth all the way to work. I'll keep chasing...

Originally Posted by I Love My Landy!

I pulled plug lead 2 off this morning. The Landy felt smooth (well, you know what i mean - didn't do its usual miss). Put the lead back on expecting it to start missing again, but it was perfectly smooth all the way to work. I'll keep chasing...

Is it a fighting type miss , one that can be felt like a stutter or hiccup type thing not a miss your just aware of one that tells you it`s there ?

I understand that`s a hard question to answer but if it`s doing anything like i ask i think you`ll understand .

Excuse me if you will Edward , if i`m asked something i think it good manners to answer .

Originally Posted by PLR

I wouldn`t remove the push rods as suggested unless you take the inlet manifold off first or are prepared to do so .

Originally Posted by Sitec

Just curious as to why you say that...

If the compression is leaking into the valley , in many circumstances the inlet gasket can be seen to breath , go up and down with the leak . If not seen to move they will often crumple out of shape or distort and nothing really needs to be dismantled to be aware of it .

The fact that only a little of the ball end of the pushrod can be held and positioned makes it an awkward job and i wouldn`t suggest removing the pushrods with out warning of dropping and maybe having to remove the inlet manifold .

I agree the lifters most likely won`t come all the way out with the pushrod partly because in a worn engine the lifters will possibly be mushroomed and if not a twist with angle will normally dislodge the pushrod but this same twist or holding the pushrod needs to be done when removing the arms so they don`t lift and drop the pushrod .

Originally Posted by PLR

Is it a fighting type miss , one that can be felt like a stutter or hiccup type thing not a miss your just aware of one that tells you it`s there ?

I understand that`s a hard question to answer but if it`s doing anything like i ask i think you`ll understand .

I think i know what you mean. The miss is like an intermittent hiccup or stutter. Id say of a morning it will stutter up to 10 - 12 times randomly within a minute (it gets better as the engine warms up). It is not a regular type miss/lope. The miss feels 'electrical' to me, or possibly vacuum related. In fact i have just blocked the vacuum line to the diff lock in case there is a leak in the hose near the gearbox that i cannot see. I'll see how it goes.

I also pulled the lead off cylinder 2 again this morning, and it did it's usual miss. Good news i thought - the miss may not be related to the low compression of that cylinder. I will keep trying to isolate the problem cylinder over the next few days.

I also checked the flame traps - all clear.

Thank you,

Edward

What colour is no.2 plug compared to the others? A bit of coolant could be seeping in via a weak head gasket. Shim steel head gaskets can leak a bit without catastrophic failure.

Originally Posted by I Love My Landy!

I think i know what you mean. The miss is like an intermittent hiccup or stutter. Id say of a morning it will stutter up to 10 - 12 times randomly within a minute (it gets better as the engine warms up). It is not a regular type miss/lope. The miss feels 'electrical' to me, or possibly vacuum related. In fact i have just blocked the vacuum line to the diff lock in case there is a leak in the hose near the gearbox that i cannot see. I'll see how it goes.

I also pulled the lead off cylinder 2 again this morning, and it did it's usual miss. Good news i thought - the miss may not be related to the low compression of that cylinder. I will keep trying to isolate the problem cylinder over the next few days.

I also checked the flame traps - all clear.

Thank you,

Edward

G`day Edward ,

it should have a filter for the crankcase breather .

The pipe connects on the right side high at the back of the engine block above the bell housing , it`s just an open T that only uses the L part of the T , if you didn`t know it was there it would be hard to find , EFI engines don`t use it .

If yours has a charcoal canister the filter will be on the center pipe , if not it will be on that pipe that comes from the back of the engine which is right at the corner of and between the engine and firewall .

All that to get to , i`d suggest because you`ve found the traps are clear that it would be the next thing to check for the crankcase pressure .


The miss as i described could be a crossfire not one where the cylinder is back firing to the extent of a noise but one where it is causing a spark at the wrong time in the wrong cylinder and the firing is causing the miss .

I have had this sort of thing after fitting new plug leads , i had only fitted new leads as a precautionary measure the engine was running fine .

It would stumble hiccup when moving off , sometimes or not there was no knowing when or if .

It could drive for 100km and do nothing or it could drive for 10km and stumble at some point like up a hill and then another 60 km and be fine .

The plug leads were the only thing that had changed so i put an ohm meter on them all and they checked out .

I then pulled on each lead in turn and one broken in two pieces , by replacing this one lead everything was good again .

I learn`t this about wires over fourty years ago when i did an apprenticeship mostly working on motorcycles and tractors a good one you can pull on and do no harm a bad one will come apart or stretch .

The break inside the lead allowed the cylinder to run ok but sometimes under load it would leak across to another lead and cause that cylinder to fire when unwanted but it was only sometimes and not consistent.

Eventually that lead would have leaked all the time and been easily found but as with yours it is a bit hard to put up with for too long .

I would imagine you have your leads spaced as much as possible though some generally have to come pretty close if not touch .

Originally Posted by Roverlord off road spares

I noticed you also said your coil was replaced, was it aftermarket ? we had a lot of trouble with aftermarket coils years ago so we only sell the lucas ones now . If you have another coil try a swap and see if the problem goes away.

Same here,fitted a non Lucas because Lucas are crap and the thing ran like a dog,fitted a second hand Lucas and it ran like a dream.I also had a brand new set of leads fitted that had a buggered coil lead,I only found out after pulling it out changing the coil and the end came off.LPG likes compression and a strong spark. Pat