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my mechanics have run out of idea's and i'm running out of money…

my '96 100hp yanmar diesel, 4JH2H-UTE vibrates….only in forward gear, only after reaching operating temp, and only between 700-800 rpm's. as perplexing as that is, listen to this.. once the shaking (which is not violent) begins, it will usually just stop within a minute or so even though nothing else has changed including the rpms. this condition has been around for 7-years and has never gotten worse or better. it is a nuisance to me, but apparently a big deal to a possible buyer.

here's what's been tried:
1. acqua drive /cv joint rebuilt
2. transmission pulled/inspected..meets specs
3. timing advanced/retarded..no change
4. fuel files/lines inspected..all clear
5. injectors checked
6. fuel filter leading to injector pump bypassed..no change

HELP
 

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In considering your dilemma, some things to note. The relationship between fuel delivery and load in forward and reverse are different and therefore the conditions are not matched at the same rpm, so the question is, what occurrences are there which could cause your problem which also correct themselves? Considering that the VE pump is the one component which is constantly transitioning itself between springs, hydraulic pressures and centrifugal forces, all of which are much more likely to show their flaw at lower rpms, my money is on your fuel pump. Pump characteristics change as they warm up and sometimes they change for the better, and sometimes not. My advice would be to have the pump removed and tested/reclalibrated specifying that you have this problem. Any fuel tech worth his 10 coffees will be able to sort this out for you.
 
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