PeachParts Mercedes-Benz Forum - View Single Post

tcane · #5 01-24-2002, 01:16 PM

Jerry:

I looked at my CD-ROM 300D engine manual and found that your turbo engine does have the rotating device for the valves. M-B calls the device a Rotorcap.

With luck, the valves may work OK so you don't have to R&R the head to do a valve job. Sounds like you adjusted them slightly loose and that will help a bit, but if they were tight without any clearance for a long time then who knows. The Rotorcaps should work better now that there is valve clearance and that should help to regain valve to seat sealing as the Rotorcaps rotate the valves.

Peter's suggestion to use really large valve clearances (up to 3 times intake valve clearance and nearly twice for the exhaust) to let the valves pound themselves into the seats to regain sealing integrity could be an option. However, you have already adjusted the valves with more clearance and I would allow some time to pass (several hundreds of miles) with 2-3 or more valve adjustments to see if they are OK. On racing motorcycles and some cars with solid lifters there were cams made to operate with greater than normal valve clearances, these engines all had two valve springs per valve to control the valve when accelerating off the valve seat when opening and deaccelerating when closing - and these were racing engines using camshafts designed for greater valve clearances.

The M-B diesel engines have one spring per valve to control them (this same spring is also used on some M-B gas engines as the outer spring in a two spring per valve set-up to control the valve). The suggestion to use much greater than normal valve clearances means the valves will be accelerating and deaccelerating much faster than normal without the camshaft helping to control the valves during a good portion of the valve's travel. The lack of valve control will mean the valve will accelerate much faster than normal and I am concerned about uncontrolled acceleration causing the single valve spring to momentarily lose control of the valve's speed resulting in possible contact with the top of the piston (there are really tight piston to head clearances to achieve the high compression ratio of 21.5 to 1 in your turbo engine - about 0.50 to 0.90mm). Similarly, on deacceleration the camshaft will not control the valve for a longer time and the valve will have greater speed than normal when it contacts the seat. The valve spring will probably allow the head of the valve to bounce off the valve seat upon initial contact (perhaps more than one bounce). At low engine RPM's these valve control concerns are less, but as engine RPM's increase these control concerns will increase - even if the engine is only operated for a short time or driven a short distance.

I would do a compression test before using much greater than normal valve clearances (and after runing this engine for several hundred miles and doing several valve adjustments). With the engine at normal operating temperature, compression should be 350 to 400 psi (24 atu or more) with a minimum of about 300 to 325psi for decent engine operation (M-B info and comments by Steve Brotherton the moderator for the Tech's Only forum). Compression reading may be affected by the compression tester used, so some leeway should be considered. If the compression is low, then a pressure/leak-down test can be done to see if the problem is with the valves (listen for air movement when listening at the intake manifold/air cleaner and exhaust pipe, rings listen at the oil filler opening, head gasket leak into the coolant jackets listen at the radiator filler). If you have low compression and/or the pressure test shows that the valves are not sealing, you could then try the much greater than normal valve clearances to try and get them to seal by pounding in an attempt to avoid taking the head off to do a valve job. You should realize that more damage could occur if you use much greater than normal valve clearances. So, you will need to weigh the risks vs. benefits vs. costs.

Hopefully, running the engine with slightly loose valves with several valve adjustments in the next few hundred miles will restore valve to seat sealing and the above info will not have to be considered and/or done. However, you will not know until you put some miles on this engine and see what happens.

My $0.02 worth!
Good Luck!!
Tom