Intermittent Rough Operation
I would appreciate suggestions as to next diagnostic steps with which to pinpoint a problem I am having with intermittent rough engine operation on my 1983 240D with manual transmission (204K miles).
Most of the time it runs fine with its typical nice muffled diesel engine sound and smooth performance. However, about 15% of the time, it will idle and run rough and louder from initial starting and will continue that way, with poorer performance, throughout the entire time the engine is operating until it is shut down. I have observed that a change of state, i.e. from good to bad or the reverse, never occurs during the time the engine is operating and also that once the condition occurs during a start and operation, it is likely to exhibit the same condition during subsequent starts and operations during that same day. The next days initial start is most likely OK (~85%). I have verified that the intake manifold butterfly valve is operating smoothly and is not sticking closed or open. All of the mechanical throttle/linkage seems to be operating freely. I have recently renewed the fuel pre-filter and filter and do not notice any fuel flow starving at the pre-filter. Does the intermittent occurrence and the nature of the problem suggest a vacuum issue -- pump, valve, lines, etc.?