New Vapor-Lock-Acting Question
 

On hot days, if I'm at operating temperature and running at significant rpm (for this particular equation, significant rpm is pretty much anything over 2000-2500) and I allow the car's rpm to drop from that point down to idle (as in, roll suddenly up to a stop sign)(as in, throw it in neutral while at driving speed on the road) (as in, accelerate to move in stopped interstate traffic and then back off once i've pulled up in the line again) (as in, pull around a drive through and then stop at the window), it shudders, and frequently dies completely. idle is perfectly normal at all other times.

In a gas engine, my dad says it's vapor lock, fuel getting hot in the hot fuel lines and turning to vapor before it enters the cylinder. However... diesel has a higher flash point and shouldn't be volatile enough for that to be my problem... is it?

It restarts flawlessly and immediately... but having it do that in heavy traffic is quite dangerous.

How do I diagnose this? I've never heard of it in a diesel before. Fuel filters are good, I do know that much.

sounds like your idle is too low.

I've turned the 1-7 dial behind the fuse box up all the way to 7 to test it and had the same problem. Right now it's set to 4 where the service manual recommends - and if the tachometer can be believed, it's holding the factory rpm recommendation.

Thanks...

Is that dial the same one that's in that compartment on the passenger side kickpanel on late model W123? I was trying to figure out how to rotate that, it wouldnt budge.

I have no clue, honestly. No experience with 123s or 617s except for admiring my friend's 300sd

uhm i think that... you are simply calibrating the tachometer... to turn up the idle you have to pysically with a screwdrive raise the idle screw. it should be on the IP be carefull though. it will snap off if your not gentle

On the SDL's, they have an idle adjustment dial that feeds the computer. The idle speed is computer controlled.

Does that mean my computer's going bad and not properly re-idling immediately when it's called for?

Can't say. But, you can disconnect the computer and see if the base idle speed will be maintained. If so, then the computer, or one of it's input signals is the culprit. If not, then the governor in the IP is the most likely culprit.