85 300td. It's always had a hiccup at idle when cold. Recently it has begun to hiccup even when warm and it's getting worse. It idles around 650-750 rpm's approximately. It doesn't sound like a miss. The engine runs smoothly for a short period of time, then it hiccups. Runs smoothly again, then hiccups. It smooths out completely at 900rpms. It's gotten to the point where it almost stalls on the hiccup when cold.
I've recently adjusted the valves and purged. New fuel filters. Still doing the same thing.
Any good hypotheses out there as to the cause?

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

I'd have it at a warm idle and crack open one injector line slightly at the injector. Much like pulling a plug wire to isolate a miss. The engine will run a little rougher but I think if the hiccup remained, that cylinder would be proven ok. If you crack another and the hiccup quits, I would suspect a dripping injector.

__________________
81 300CD (sold) 1972 280 SEL 4.5 (sold) 1966 250 S 4 spd (sold) 1974 450 SL (sold) 86 BMW 325ES (sold), 1973 280C (sold) 1988 300 SE.

I'll add that the hiccup seemed to disappear during the time I was running the diesel purge but returned when running on diesel.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

i have the same thing happening now as well. however mine seemed to start AFTER i ran a can of diesel purge. the problem with trying to crack some injector lines, is that i barely have enough time before it clears up. its a pretty harsh hiccup too though. its all disheartening, while at the same time, im trying to track down either nailing, or some bearing noise. or maybe rod knock. let us know if you fix it.

__________________
1980 300D

Can you elaborate some more on what exactly happens with a dripping injector?

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

An injector is much like a spraying sprinkler head in your yard! When you turn on the water its supposed to spray in a fine mist, then when you shut it off, it stops. If, when its shut off it doesn't fully seat, it will drip water. On an injector not seating it will drip fuel and cause a miss or hiccup upsetting the atomization of the fuel and air. On a gas engine you, in most cases, can pull the entire fuel rail with injectors attached and cycle the key to spray once, then with the key off you can watch for a dripper. Not really possible/practical on a diesel with hard non flexable lines. So if you isolate the hiccup to a cylinder you can remove that injector and have it tested for opening pressure and spray pattern and/or dripping. Also I've seen in rare cases sticky valves that, because of carbon, bind in their guides which can cause a hiccup. Various fuel treatments can eventually help remove this carbon.

__________________
81 300CD (sold) 1972 280 SEL 4.5 (sold) 1966 250 S 4 spd (sold) 1974 450 SL (sold) 86 BMW 325ES (sold), 1973 280C (sold) 1988 300 SE.