rough idle again - PeachParts Mercedes-Benz Forum

shadetree77 · #1 11-22-2011, 01:36 PM

hi,

sorry to bring up another rough idle issue: here's what i got

83 300SD (about 500,000 miles on the chassis but only 60,000 miles on the engine/trans)

rough idle when hot, not as bad when cold but still rough

here's what i checked/adjusted:

valves adjusted (3 times with italian tuneup inbetween)
ran with fuel cap off to rule out vaccum in tank issue
rack damper (gold) adjusted- pretty much all the way in; no problem starting cold or hot
cracked each injector, each time i opened one the idle got worse
new fuel filters (2x replaced in the last 1000 miles), no black specs in primary fuel filter, also checked tank screen few months back and all is clean
checked fuel pressure at line from pressure relief valve, sits right at 19 psi
no visible smoke at idle
basically 0 blow-by (engine only has 60,000 miles)
checked timing chain stretch, about 2 degrees (checked via harmonic balancer method)
diesel purge (2X)-made no difference
motor mounts new-geniune MB

rough idle goes away right around 900 rpm, ran with the idle set at that speed for about a day but engine will try to accelerate downhill since it's almost out of the governed idle speed. when i let it idle around 750 rpm the exhaust sounds like a gas engine that has an air intake leak (puff puff puff..lol)

the car was sitting for 5 years before i rescued it,so i'm thinking it's either the IP or the injectors, since the car was sitting so long there must be something gunked up inside the IP or the injectors. the injectors look very rusty; i'm thinking maybe the old ones were installed in the rebuild engine (the IP is rebuilt as well)

i was going to replace the injectors with new ones with monark nozzles (been hearing good things about those). the injectors that are on there now do look very rusty-i'm thinking maybe the original injectors from the old engine were put into this one, but they could just be so rusty from sitting for 5 years.

so...before i go spent good money on those injectors i was wondering if anyone has any other suggestions; i've read somewhere about filling the IP fuel passages with acetone
to remove any gunk; not really sure if that's safe on the IP

any suggestions will be highly appreciated

andrewjtx · #2 11-22-2011, 03:37 PM

you said the rack damper bolt was adjusted all the way in, that sounds to me like the internal spring is worn on it... How does the tip of the bolt look. If it's been flattened out, that's a fair amount of wear. Should be convex tip.

You may want to grab a new one and try it out.

ROLLGUY · #3 11-22-2011, 04:20 PM

I could be wrong, but it has been my experience that if the idle speed adjuster is not backed out before adjusting the rack damper, the rack damper adjustment will change nothing. When the idle speed screw is backed out, the governor is acting on the rack damper spring, not the idle speed spring (that is the way I understand it anyway).

shadetree77 · #4 11-22-2011, 04:34 PM

thanks for the replies; the rack bolt is new. i just unscrewed the idle screw all the way out and adjusted the rack bolt again, no change

shadetree77 · #5 11-22-2011, 08:10 PM

forgot to mention: has plenty of power throughout rpm range