Tech help-burnt valves--300SD - PeachParts Mercedes-Benz Forum

CHAPP · #1 05-04-2002, 06:12 AM

I need some help... my car has run great for 45,000 miles and now has a leak (after running a leak down test) in #2 exhaust valve (maybe burnt). I am trying to find out why?? I did a search for rough idle, which is the problem that brought this to my attention...but did not cover my problem...Thanks for any help.

Chapp

engatwork · #2 05-04-2002, 07:36 AM

Had the valves ever been adjusted during those 45k miles. They need adjusting about every 15k to keep them seating the way they should.

CHAPP · #3 05-04-2002, 07:52 AM

I had a broken timing chain (5000 mi.) and replaced lots of parts and had the valves re-adjusted after 2000 miles. It has run great the first 5000 miles but justed started with a rough idle.

Chapp

tcane · #4 05-04-2002, 02:10 PM

You did not say how much #2 ex. valve leaked. M-B specs allow for a certain amount of leakage during pressure testing of a cylinder. I assume you adjusted the valves before the leak down test was done.

Try this test, check the clearance for ex. valve #2, then turn #2 ex. valve a bit and check the clearance, turn the valve a bit more and re-check the clearance, do the same till you've checked the clearnace at 4-5 positions in the valve's 360 degree rotation. A valve that is seating well should vary by .001" or so and hopefully less, more can indicate poor valve face to valve seal contact (even after a good valve job, there will be some variance in valve clearance when doing this test, and there is the chance wear on the valve train can cause clearance differences but wear can be seen and accounted for).

A burned valve and/or damaged seat can be damaged in one place and the "turn the valve check the clearance" test won't help. A burned valve/seat or damaged contact area can be caused by several things: cracked head in the seat area or a cracked seat (overheated engine, bent valves when the timing chain broke may have damaged the seat area and was not found when repaired, etc.), or burned from too lean mixture (fuel filters clogged, injection pump start of delivery is too lean, low injector nozzle pressure causing too little fuel injected, algae in the fuel system clogging fuel flow, etc.) or a bad valve spring, or debris entering the intake tract and damaging a particular valve and seat, or a piece from a broken piston damaging the valve and seat, etc., etc. The possible damage to your #2 ex. valve and the reason why it is damaged can only be found doing a TDI (tear down and inspection), unless you can get a cylinder inspection tool (like fiber optic cable with an inspection lens and light inserted into the cyliner after removing the injection nozzl and pre-chamber).

M-B has a device that turns the valves (rotocap is the name I recall) and this device greatly helps to prevent burned valves, and prevents valves and seats wearing in one place because the valve does not turn.

The problems listed above can also cause a rough idle. A rough idle can also be caused by air entering the fuel system, water/dirt in the fuel system, low fuel pump pressure, check valves in fuel pump leak and/or the springs are collapsed, the IP check valve spring on the return fuel side has collapsed and needs to be stretched (M-B repair & stretch to 25-27 mm) this causes low fuel pressure in the IP, IP delivery is not equal, uneven cylinder compression, valves not adjusted correctly, turbo not adjusted and/or clogged, idle too low, engine mounts or limit strap bad or in a bind, gravel in the trans mount, EGR malfunction, etc., etc.

Good Luck!
Tom