Camshaft wear question, 72 Mercedes 250 w114

[Jub]

Hi everyone,

My camshaft lobe on #12 (exhaust) valve is worn down. PO must not have checked valve clearance as necessary.

Car runs, but stumbles at idle (compression test #12 at 95; rest of valves ~140).

This would mean that exhaust isn’t opening all the way.

Question: rather than replacing camshaft, can’t I just adjust valve to compensate (ie, move clearance from .007 to .006-ish, theoretically)? Also, is the camshaft for the Mercedes 280 same as one for 250 (I ask b/c lot of 280 camshafts, can’t find one for 250)?

Thanks,

Jub

[Sugar Bear]

In theory yes a tighter clearance would help but only in a very very almost immeasurable small amount. The downside is it will increase the chances of burning the exhaust valve, so I would not recommend doing it.

The low compression isn't from the exhaust valve not opening all the way, rather not closing all the way/sealing. The low compression is from a leaking intake and/or exhaust valve and/or rings. My guess is a leaking exhaust valve from not having enough clearance over a long period.

Try putting air into the cylinder with both valves closed on that cylinder and see where it leaks out. Also try adding a 1/2 teaspoon of oil and redo the compression test on that cylinder. If it goes up a lot bad rings, if not a major increase it is a leaking valve.

Good luck!!!

[Jub]

Valves not closing: either valve seal leak, or rings...makes sense (thnx SB). Now I just replaced my head gasket and had aluminum head leveled/cleaned and pressure tested. So either the shop messed up, or engines toast. Jeez.

Any other thoughts SB, others?

[Jub]

Update: added oil, pressure went from 95 to 100, thinking valves (exhaust or intake)?

[Sugar Bear]

Has it been run since this work? How long? Try a wet compression test first or air in the cylinder. It will run decent with one cylinder at 95 PSI. The camshaft is the bigger issue at the moment. If the car is clean don't give up on it but if it is rings that can get costly.

Good luck!!!

[Jub]

Was running for past two weeks (~2hrs/day). Stumbled a bit. Last week reset dwell, timing, Weber’s...still something off. Next week (today), compression tested cylinders. In the back of my mind I spotted camshaft lobe when I put it back in, so was chasing the camshaft angle (thus this post). Strange coincidence that cylinder and lobe have independent issues, thought the two were connected SB.

[Jub]

Jeez, more I think of it more I think may be rings. If exhaust valves don’t fully open...air backs up in chamber and gets forced back and past piston? Piston rings (or valve seals?).

[Sugar Bear]

That is good news...95 up to 100 indicates rings ok. If you can put air in the cylinder you'll know if it is intake or exhaust. To correct this the head has to come off and the valves ground but it is waaaay less than a ring job. Did the shop leak test the valves when resurfacing the head???

About the cam, someone on this forum probably has the exact one or one that would work. Someone that knows interchangeability hopefully jumps in on this question. Replace the rocker arms too from a matched used rocker camshaft set-up.

[Sugar Bear]

Compression isn't valve seal related. Valve seals seal the valve stem at the top of the valve guide to prevent oil from inside the valve cover going down the valve stem past the guide and getting into the cylinder.

The air pressure/compression on the cylinder side is sealed by the rings and valves.

If you want to give it a try...there might, might, might be a piece of carbon stuck on a valve face causing low compression. Try turning/rotating both valves multiple times to possibly clean the sealing area and redo the compression test on that cylinder. It isn't too likely to work but it only takes a few minutes.

If that works I'd ask for a beer and you should buy a lottery ticket ASAP

[Jub]

Shop said they “compression tested it”, thought that’s what that was. If it’s a valve, my moneys on the #12 exhaust valve SB.

Gonna try an old Russian trick (worked for me once before). Hook up the valve to a drill and bit (via a rubber hose). Spin valve around in seat a few hundred revolutions. If it’s debris, should shake loose.

[Sugar Bear]

If they didn't have the whole motor I doubt they compression tested. They may have and probably did vacuum test it though.

Keep us posted!!!

[Jub]

Thnx SB, will post f/u.

Thanks again,

Jub

[Sugar Bear]

Position the piston on that cylinder slightly below TDC so that you can work the valve up and down/on and off its seat but not low enough that the valve can fall out of the guide into the cylinder. Also keep the valve seal oiled during the process.
Hope it works!!!

[rumb]

You definitely need a new camshaft. make sure the oiler tube is allowing oil to flow on that lobe.
Did you put new valve stem seals in?
did they check if valve guides were worn?
If the shop skimmed bottom of the head did you check to see if the top surface is still parallel and flat?

[Sugar Bear]

Excellent suggestion on checking the oiler tube for restrictions, it is CRITICAL!