Exhaust valve for #4 cylinder sticks open about a 1/4 inch in 1987 MB 560 SEL engine
 

Exhaust valve for #4 cylinder sticks open about a 1/4 inch in 1987 MB 560 SEL engine.

Some info leading up to problem, my 560 overheated after a 3-4 mile drive home from work and began KNOCKING loudly. The noise was so loud I figured it had to be coming from bottom end. I pulled valve cover and found a bad hydraulic valve clearance compensating element (ball stud, lifter) for exhaust valve for #4 cylinder. So I replaced it and checked it with the Go-No-Go tool (it’s the right height). I removed and measured both valve springs and both are correct length. I can spin and move the valve up and down freely and I can see only a small amount of carbon on bottom of valve (1-2 mm thick) while looking through the spark plug hole. So does anyone have any suggestions on what the problem is or what I need to do?

Thanks,

Justin

No suggestions??!

When it's sticking open, the hockey puck is tight against the rocker, and the rocker tight against the heel of the cam lobe, correct?

Is the new ball stud the same height as the others?

Did you re-use the hockey puck?

Chuck,

When valve is sticking open with "hockey puck" tight against the rocker and the rocker tight against heel of cam lobe the 1/4 inch gap will now be between the ball stud with correct height and the rocker arm.

I did measure the new and old ball studs and they're within .01 mm.

I did re-use "hockey puck", because I need to figure out what or why the valve is sticking open about a 1/4 inch.

I am able to see some of the valve through the spark plug hole when it should be closed and the valve spring height of the stuck valve is about a 1/4 inch shorter than other valve springs in closed position.

Thanks for your suggestions!

Justin

I hate to say it but it might be bent. ....and your engine might have jumped timing.

Tom W

OK, so what you're saying is that the cam/rocker/puck are not holding the valve open, rather you have a gap when it should be fully closed.

When you pull the valve up, does it stop suddenly and still spin freely,or does it sort of bind and get difficult to spin? The former makes me think that (maybe) the guide has dropped down into the cylinder, although this would be highly unusual and I'm not sure that it's even possible. The latter would indcate a bent valve or maybe heat damage to the guide.

Does the guide stick up enoungh so the you can compare it to another guide?

Tom,

It is possible the valve is bent, but I'm able to move the valve about an inch up/down and spin it without any binding or excessive rubbing with valve guide. I'm pretty sure it didn't jump time because I can rotate engine manually without valve/piston contact even with #4 exhaust valve sticking open about a 1/4 inch.

Chuck,

Yes, there is a gap between rocker arm/puck when the valve should be closed and cam/rocker/puck are not holding valve open.

The valve stops suddenly and spins freely without any binding.

The valve guide appears to be straight and is inserted into the cylinder head up to the circlip that prevents guide from going into head further (I believe this indicates the proper depth when installing valve guide).

I will compare valve guides hopefully later today.

Thanks for your suggestions!

Justin

If the guide hasn't moved, then I'm out of ideas. My only other thought is that the valve seat has come loose.

Please keep us posted ...

Chuck,

I agree with you that valve seat is probly damaged or dislodged and preventing valve from closing/seating properly.

Sure wish I had a Borescope to look in the cylinder at valve seat rather than having to pull the head to look at the valve seat.

Thanks for your suggestions!

Justin

Hmm ...

Why don't you pull the exhaust manifold? You might be able to see something, and you need to remove it anyway to get the head off.

Chuck,

Sounds like a good idea, I will let you know if I'm able to see anything.

Thanks for your suggestions!

Justin

I have a bent exhaust valve from an M117 on my desk. It behaved exactly as you describe yours. It sounds like it's bent.

Tom,

What caused your bent exhaust valve? I borrowed a Borescope today and will look at valve and valve seat tomorrow.

Thanks for your suggestions!

Justin

Pulled heads from a junker with the cams still on it. That exhaust valve was fully opened and I banged it against the other head when putting it in the back of my Jeep. Didn't take much force to bend it, and it's not noticeably out of round, but it won't close fully!

Hi Justin,

Did you ever figure out what the problem was with this ? Was it a bent valve ? Also, what did the spark plug look like for that cylinder ? I have a 1987 560SL with a very similar problem. It runs roughly and has a tapping noise.....I haven't yet removed the head but cylinder #4 has a spark plug with lots of black deposits on it...the others look good.

Also, assuming that it was a bent valve, did you determine the cause of this ? My vehicle has only 94k orig. miles, the timing chain, guides, tensioner, etc. were all replaced < 10k miles ago. I am thinking that the engine is still in time, as it does turn over just fine.

Thanks in advance,

Brian