PeachParts Mercedes-Benz Forum - View Single Post - Hard starting, knocking 240D: Still stumped. Getting warmer?

TedG · #3 02-20-2005, 04:52 PM

Thank you, Hunter.
I figured Frank knew what he's talking about. I can tell by visiting his shop that he knows his stuff.
Anyway, I will answer your question, then can you please answer mine? Thanks.
Maybe I shouldn't have the valve job done; I would like to know how much the car is worth when it can barely be started, runs rough, puffs smoke, and makes a loud knocking sound, vs. running great with a fresh valve job (assuming that's all it needs).
The car is a US-model 240D automatic, has hardly any rust, very nice original powder blue paint, immaculate interior w/dark blue MB Tex, and no other mechanical problems. It shows 120,000 miles. You indicated when I spoke to you on the phone that it could have an odometer lapse, as valve trouble would be unusual at 120k, but I'm assuming correct mileage since that is what was notarized on the title when I bought the car.
The work was going to be ~$1000 with Frank pulling the head, but he said I could save "quite a bit" if I R&Rd the head, so I'm guessing $650 or $750.
Anyway, I would still like an answer to my questions:
b) Could low compression cause a knock? It seems like it would just be responsible for the rough running, hard starting, and puffs of black smoke. The apparent location of the knock near #2 would seem to indicate another problem causing the knock, since 3 & 4 were named the culprits for the low compression. Maybe this other problem is tha cause of the rough running, smoke, and hard starting also?
c) The car started and ran great for the first five years I owned it (bought it in '98 with indicated 70k miles - now has indicated 120k), but it always needed full fuel to start. What might cause that? Injection pump? Timing? Glow plugs? Bad injectors?
As an update, I think I answered question (a) by adjusting the valves spot-on and still encountering very hard starting, noise, and roughness.
I also may be on my way to answering (b) because I did a cylinder cut-out test today and it seemed like the noise only went away completely when I cut out no. 4, which is the one with way low compression. Maybe low compression can give combustion conditions resulting in a knock? Or could it be the ball thing in the pre-chamber broken loose, in addition to the low compression? Note again that the knock is heard during idle and acceleration only, so I think (?) that rules out rod knock.
Maybe I'm closing in on an answer to (c) as well? I'm thinking that maybe the compression has been less than healthy ever since I got the car, causing the full-fuel requirement for start-up, and it merely went from bad to worse recently?
Let me know what you think.
Thanks again.