M104 - Camshaft/Crankshaft Position Discrepancy??
 

Having a problem with camshaft/crankshaft timing on a '95 E320 (124.032; 104.992; 75K miles) ...

Head gasket starting leaking oil into engine coolant following a water pump failure that resulted in an episode of overheating.

Cylinder head was removed, inspected and found to have a slight warp. (VMI reveiew reveals a prior head gasket failure, while under warranty, at 7K miles). Head was disassembled, pressure tested and milled by a machine shop.


During reinstallation of the cylinder head, basic camshaft positions were checked and adjusted per WIS procedures 05-2230 and 05-2240:

1. Position cylinder 1 to TDC by rotating crankshaft till TDC marking on vibration dampner and TDC pointer are aligned.

2. Check/correct position of camshafts so that 4 mm holes in camshaft flanges are aligned with the top edge of the cylinder head; intake cam adjuster is in retarded position.

With both the cams and crankshaft at TDC, the front cover and top guide rail are installed, followed by the chain tensioner....


But, after reassembly, when the crankshaft is rotated (in the direction of engine rotation) by hand to recheck timing, the crank always ends up at approx 3 - 4 degrees AFTER TDC before the holes in the cam flanges are aligned with the cylinder head. The 3-4 degees discrepancy appears less than the adjustment that would be achieved by repositioning by the cams by one sprocket tooth.


If allowed to run in this configuration (cams at TDC, crank at 3-4 degrees after), will the valves contact the piston crowns. Or is this too small a deviation to be concerned with?

How can the factory specified cam and crank timing of TDC be reachieved on this motor? Is this the result of having the head milled? The machine shop assured that the warpage was mild and little material required removal. If the head had also been milled by the dealer when it was removed at 7K, woud two minor resurfacings result in enough material loss to achieve this current discrepancy?

Would setting the cams at TDC and the crank at approx. 4 degrees BEFORE TDC result in the proper setup?

Sounds like classic chain stretch to me. At least that's the diagnosis I'd make on a diesel engine I'm more familiar with. I know offset keys are available for OM617 engines. Perhaps they are available for M104 engines, also.

I wouldn't worry about it. Back on motors that had offset keys for adjusting such, four degrees was the first level of adjustment.

The adjustment was 4,6,8, and 10 degrees with one tooth being 18 degrees. Thus any value could be achieved by either advancing or retarding the cams in these increments or jumping the chain.

Proper evaluation in this area can not be done with the marks. It must be done with a dial indicator on the valve in question using the TDC specs found in the TDM (tech data manual).