Low compression

[Neil Thompson]

My 280 sl manual is running rough on idle, it pulls hard when driving but is a little off on idle. The weakest cylinder is number 5, so after swapping plugs, swapping leads around etc no'5 cylinder was weak still. I have carried out a cylinder leakage test and all cylinders except no'5 show low leakage and are well balanced at around 15%. No'5 cylinder however is leaking heavily at 50% I think I can hear gas escaping at the tailpipe but am not totally sure. The car does smoke a bit after being on tickover so I am hoping an exhaust valve and a set of valve stem oil seals may cure both problems but what I am asking the knowledge of the forum for is how can I eliminate piston/ring problems? One idea I have is to pour a little oil in the plug hole and repeat the leakage test, if the result is the same I think it will confirm valve problems, am I right or are there any better options?

Thanks, Neil

[garymand]

You know what you are doing!  Yes a few squirts of oil plus I would rotate the crank to spread the oil a bit. 

But, if its rings just pull the dipstick and listen for air passing into the pan.  It will be much louder on 5 that the others for 50%.  Listen with a hose at the intake and the tail pipes.  50% is plenyt to hear the air escapiing past the valves.  You will hear the exhaust leak clearly at the tail pipe when you use a hose to the pipe and your ear. 

I've got 250k on a 560.  I can easily hear the intake leaking at 40% on 3 and a 45% exhaust valve leak on 4.  It will run like that for a long time with only an idle miss to give it away.  I had a 380 with the same problem.  It took me 4 years to learn about the leak down test to find out why it had a miss at idle.  I even took it to 2 mechanics who assumed it was lifters or injectors or electrical.  In the mean time the family put 60K on the motor.

[ja17]

What is the condition of the spark plug in #5 cylinder?  If it stays clean, it may be the exhaust valve guide. If you have low compression, and your valves are all adjusted, then removal of the cylinder head would be my first line of action. If you find significant problems with the head and no obvious problems below, just recondition the head and re-install it.

[Neil Thompson]

Thanks Gary and Joe.

Joe the plugs are all clean and the same colour which I believe also points to the valve am I right? I will just add a little oil to the cylinder later today and hope that the leakage is the same. the valve clearance is a little tight, around .005" so I will just slacken it a little before my final check

Neil

[stickandrudderman]

With a leakage of 50% you're going to be taking the head off anyway so why not do that and then measure the bores? It's no good just looking at them for a ridge, you need to check for ovality with an internal micrometer.

[Neil Thompson]

thanks Colin I realise the head needs to come off but i'd rather be sure that it is just valves rather than pistons prior to removal and I haven't got the knowledge or equipment to test some of these measurements so I am going down the process of elimination route with the help of members here. I poured some oil in the bore and carried out another leakage test which reduced the leakage by approx 5% steadily increasing to where it was previously as the oil slowly seeped down the bores under pressure which to me makes me think it is definately the exhaust valve, which also had a .005" clearance and being tight could help to 'burn'. The car is low mileage and I would hope the bores/pistons to be good but I think I'm ready now for the head off.

thanks again all

Neil