Engine problem: M50 dies on quick throttle change

[dcapn@hotmail.c]

Hi,
I recently bought a 1986 E38-200 with an M50 diesel that dies whenever I make a too-quick throttle change (up or down). I have replace the (gunged up) filters, and run a 1/2 gallon 50/50 clean diesel/Seafoam mixture through it, but still have the same problem. The engine idles ok, and runs fine at all speeds, and is ok with slow-easy throttle changes. I'm thinking it might be exhaust blockage. Any ideas?

 

[Geoff Johnson]

I had the same problem a number of years ago. I turned out that the flapper valve just after the exhaust riser (which prevents backflow into the engine) had become dislodged and whenever power was applied it would block the exhaust, causing it to stall.

 

[eknebel]

Any smoke colors? white, blue black?
Problems starting?
any other changes, speed, wide open throttle rpm? (general background idea fishing)
Generally a exhaust restriction will cause black smoke and loss of high rpm power and overheating, especially after several minutes at full throttle(black smoke can also be bad injectors
My first inclination is to go the poor fuel delivery route, since you are going thru filters. It doesn't logically explain why decreasing throttle causes the issue, but this is from the "fix first what you know is wrong" theory of troubleshooting. Have you changed/replaced the filter that is in most fuel pumps on the inlet side? Verified the fuel pickup tube is clear, and fuel system is well bleed? I know these are basics but easier to do yourself than pay a mechanic $100/hour to do(at least for me). Before you start further expensive repair/replacements, you definitely want to make sure your tank is cleaned of sediment, and the rest of the fuel system is verifiably squeaky clean. My scary thought is there is a problem with the fuel rack assembly(often in the jerk pump), which is what ultimately controls the amount of fuel delivered to the jerk pumps for the cylinders. It is my understanding that governors are pretty reliable, alas dirty fuel can clog permanently the fine tolerances involved with diesel fuel jerk pump(machined to 0.0004 of an inch, ouch!).
Best of luck, keep us informed,

 

[dcapn@hotmail.c]

Update on my M50 engine problem

Any smoke colors? white, blue black?
Problems starting?
any other changes, speed, wide open throttle rpm? (general background idea fishing)
Generally a exhaust restriction will cause black smoke and loss of high rpm power and overheating, especially after several minutes at full throttle(black smoke can also be bad injectors
My first inclination is to go the poor fuel delivery route, since you are going thru filters. It doesn't logically explain why decreasing throttle causes the issue, but this is from the "fix first what you know is wrong" theory of troubleshooting. Have you changed/replaced the filter that is in most fuel pumps on the inlet side? Verified the fuel pickup tube is clear, and fuel system is well bleed? I know these are basics but easier to do yourself than pay a mechanic $100/hour to do(at least for me). Before you start further expensive repair/replacements, you definitely want to make sure your tank is cleaned of sediment, and the rest of the fuel system is verifiably squeaky clean. My scary thought is there is a problem with the fuel rack assembly(often in the jerk pump), which is what ultimately controls the amount of fuel delivered to the jerk pumps for the cylinders. It is my understanding that governors are pretty reliable, alas dirty fuel can clog permanently the fine tolerances involved with diesel fuel jerk pump(machined to 0.0004 of an inch, ouch!).
Best of luck, keep us informed,

10-15-16: I've run a coupe cans of SeaFoam through the system; replaced both fuel filters, and completed a compression test (390,390, 390, 410 (front to back)*). Engine runs, starts, and idles better, but problem remains. I will be trying to do a back-pressure test next and searching for the mysterious "flapper valve" mentioned above. Will report any findings here.
* Anybody know if these compression readings are ok or indicate low compression for an M50?