Faulty valve seating?

re: 81 300D

Have any of you mechanics seen cases where a valve (or valves)take 'spells' not seating tightly/snugly during idle, causing miss, rough idle and puffing heavy smoke?

As I've posted before, my mb will idle perfectly one moment, and get rough the next. This is ONLY during idle, does fine when I accelerate. Also, it starts beutifully! cranks right off, no problem a'tall.

I talked to a local MB mechanic over weekend and he tells me it could be a valve not seating properly at times, thus causing these "spells" loosing compression and causing the smoke.

This would make sense, but why would a valve seat tightly one time on idle, and not the next? Seems to me it would be a consistant problem if the case. Thanks for any 2 cents, Phill

[Benzmac]

I have seen this, but when I did, I took apart the intake and found a large screw that had entered the intake somehow. I removed it and the problem was resolved...I'd look at injection nozzles.

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Benzmac:
Donnie Drummonds
1981 280GE SWB
1987 16V
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM

[jeffsr]

Weak or broken springs, damaged cam or lifters, burned or damaged valves, worn out valve guides, to mention a few. Check the compression. If the valves are damaged, the evidence of this is generally pretty consistent. Back when I used to build big block chevy racing motors, we use roller tip aluminum rocker arms which were offset from the center of the valve stem and would impart a turning moment on the valve when opening, giving the valve a small "spin" and keeping the seats (triple angle grind) from getting pounded flat quickly (big heavy springs). Don't know if this goes on inside MBZ motors. In fact, now that I think about it, I have no idea why I mentioned it at all. DUHH!! Anyway all that other stuff I said at the beginning is what you need to think about.

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Jeff L
1987 300e
1989 300e
1987 BMW 325

Thanks Benzmac and Jeff.

I have checked the injection nozzles. Had a specialist check them and he said they were working fine. I also broke loose each injection line and checked it that way. I could hear a marked roughness on EACH line even while it was taking those "spells"

Next, I'm going to check compression AND
fuel pump, make sure both working ok. Do you feel it would probably more likely a compression problem or Pump problem? Which would you check out first?

I can't for the life of me understand why it takes rough idling spells intermittently??? Why not consistant?
Thanks, Phill

[jeffsr]

Go ahead and check the compression. It's easy to do and will get you in the ballpark as to where to go next. If it's OK and I mean no big differences between cylinders, then you need to start looking at your fuel supply system.

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Jeff L
1987 300e
1989 300e
1987 BMW 325

Will do Jeff.

One more question however. IF indeed the engine is taking 'spells' intermittently loosing compression can this affect false reading in gauge compression? If valves are seating fine one time, and not the next?

[jeffsr]

OK, here I go again. Phil, valves generally seat or don't seat for whatever reason.. If the valves or seats are damaged, there is no way I know of that they can have "spells" of getting better. If you can't get decent compression, then it's the valves or rings. Tech Tip: If you get a low reading that bleeds down slowly->most likely rings.. If you don't get any build of pressure or rapid bleed down->most likely valves.. Good Luck.