Halp! ....No. 3 Cylinder kaput

Drove over to pick up a later year car oil filter head to convert to spin on and on the way back lost power suddenly to my # 3 cylinder about 4 miles from home. Checked it quickly but all wires attached so drove it on home....er, limped really.

What I have done so far:
1. Switched wires to be sure it was getting fire....still #3 was the one...when I would pull it off...no change to engine running vs pulling the other cylinder wires which would make it run even rougher immediately.
2. Switched plugs - no difference
3. Checked compression all cylinders:
#1 145 lbs #2 130 lbs #3 5 lbs #4 140 lbs

So damn near zero compression in #3

When I pulled the plug on #3 it was very black. #2 was a little black but mostly light brown and #'s 1 and 4 were light brown/tan.

No external leaks though I thought I saw a minute bit of coolant at the base of spark plug #2 but it hasn't re-occurred while testing engine.

What are the suspects and the diagnosis path?

Head gasket?
Cracked head?
Piston cracked?
Piston ring broken/cracked?

69 MGB

Could be either a burnt valve, stuck valve, or a piece of carbon between valve and seat. I would suspect something amist with the exhause valve at #3.The first thing to do is check the valve ajustment. Also you might fire her up with the valve cover off and watch #3 rockers.

Thanks, Tom. I'll check that next. Seems like last time I had a drop in a cylinder pressure like this it was a bit of carbon jammed and I ran some Sea Foam through and it cleaned it out. Will pull the valve cover first and check that as you say. Appreciate your reply.

Pulled the valve cover and watched it as it ran. Seems like the front valve on #3 is opening and closing less than the front valve on the other cylinders. Suppose I should go ahead and set the valve clearances on that cylinder?

Tom is on top of it, but with this new revelation I would more seriously measure the lift on several of the valves in order to have a base line to compare with that bad boy on #3. It sounds like you have a badly worn cam there but it is unlikely that such a condition would occur rapidly to the point of the car running well one minute and poorly the next.

Also while you have the valve cover off look REALLY closely at the valve SPRINGS for #3. It could be that you broke a spring and the valve is not closing completely because of that, perhaps more prone with single springs than concentric dual springs.

Also, did the business end of the #3 plug look like it had been bashed by anything. I'm thinking of the possibility of something having found it's way into the combustion chamber and crashing around doing damage to a valve or the piston crown.

If you can pressurize the cylinder as in doing a leak down test that will tell you a lot too.

Keep at this and you will get to the bottom of it in short order.

Jack

Good point Jack and I should have checked that spring. Have to buy some fittings to pressurize it but I do have a compressor. If you have any references to that procedure for dummies like me, it would be great. I ain't stupid just ignorant.

You'll figure it out John. Take you time--you have some good help here. Sounds like you have caught it real early. Hopefully the fix isnt going to be that bad.

With your compression that low, you probably don't need any fitting to do a leak down test. Just turn the engine until both valves on that cylinder are closed and stick your blow gun in the plug hole - maybe wrapped with some tape for a decent seal. It should be pretty easy to hear if the air is escaping via the intake, exhaust or crankcase...

That sounds like a practical approach, Joe. Thanks. So...if it's the crankcase it's a broken ring or burnt piston? Assume, either way you just pull the head and get a look. Does it make sense to just go ahead and pull it? Guess I better not short the diag process though since you would have to pull the piston out and look to really know and if the head is cracked I will see bubbling in the radiator. Nothing to do but try it and it see. Will report later today.

I once had a sudden loss of a cylinder whilst driving -- turned out to be this:


With the plugs out and someone turning the engine over, it was obvious looking down the plug hole that the piston crown in that cylinder was not moving. Remarkably, there was NO damage to the cylinder nor head!

The main reason I'd do the quick check with a blow gun is to determine if it was a valve or a piston/ring problem. If it's just a valve, you know you'll just be pulling the head. If it's a piston or ring problem you're probably gonna be pulling the engine.

Make your own John.

Bash out the porcelain parts of a spark plug and braze on a male air fitting, the fitting thingie that screws into your air tools and fits into your air line. Then go to the parts house and buy an air pressure regulator and jump through what ever hoops you need too in order to put it between your compressed air source and the newly made fitting.

OR

In a pinch for gross testing like you need to do you can do without the gauge set and bend the air line enough to pinch off the supply before making the connection to the fitting. Then you can slowly release some of the pinching until you get significant flow/pressure in the cylinder to see/feel/hear it escaping from where ever it might be leaking. If it is going to go out the exhaust valve you might need an assistant to run to the back of the car to listen at the tail pipe. You ought to be able to hear the leaks your self if it is coming back through the intake or down by some wasted rings.

I any event the piston should be at TDC with the valves of the mirror cylinder in the firing order just rocking. Keep your hands away from the fan just to be safe.

Jack

Went to my favorite NAPA store yesterday afternoon where previously I have been able to get brass air fittings to fit my oil gauge sender hole so I could fit to pressure gauge to test oil pressure (so I know they will have a proper fitting) but the guy who does all that left 45 mins earlier so I will head by there today and do the test tonight, hopefully. I was assuming you put car in gear and put other plugs in so it stays where I have it set with valves close.

Is there any problem getting it hooked up and leaving air connected and blowing while I run around and check tail pipe, radiator, etc.?

If it's blowing by the rings/piston how to you know that?

JTBZ

"Is there any problem getting it hooked up and leaving air connected and blowing while I run around and check tail pipe, radiator, etc.?"
-Nope....it's not going anywhere!

'If it's blowing by the rings/piston how to you know that?'
-You should have air coming out the dipstick hole.

Thanks, Joe. Smart-assed AND helpful. How good can it get !!!

Finally found the fitting to go in the spark plug hole. Checker...nope, Autozone..nope, Advance....nope, NAPA by my work...nope....NAPA near home....yep. It was in a bubble wrap and was described as what you use for changing a broken spring on a cylinder without having to take the head off. More or less in the compression testing section of their catalog. Page 631 to 634 somewhere. Had a friend use the same technique in 1974 when I broke a valve spring on the way to Colorado from Florida so it ain't exactly new but had to find a guy who knew what he was doing to locate it.

Thanks for all the help guys. Got the head off tonight and there is a hole completely through the exhaust valve about the size of the bolt that a half inch nut fits on. Will start a new post about head work. Thanks again.

A little JB Weld oughta fix that right up! :D

You mentioned that the valves on the number 3 are moving slower than the others?

This is usually casued by oil starvation to the valve and valve guide. The valve stem is bonding itself to the valve guide.

Is there a lot of oil up on top?

They only get oil from the little weep holes that drip down in there.

Sometimes a piece of carbon or crud from the oil will get lodged in there and cause a hole to clog, stopping oil from leaking down to the guides.

Of course, if they are actually moving the same speed then it can be all of the above things that were mentioned.

Brian