I am having a meltdown here all the usual suspects are rounded up. My 1986 300SDL (that I have spent more time working on than driving no kidding) is a great car has me perplexed. It started and ran fine one day the next started and idled fine but gives me about 100RPM increase when I push the throttle with foot or under the hood so all the linkage is moving the way it should. Then it "falls on it's face" meaning the RPM's go up from about 650 at idle to 750 no matter how much throttle (fuel) I give it it only "boggs down" as I let the throttle down (because it has more fuel than the engine can burn because the RPM's didn't rise) here is what I have done before this happened most has nothing to do with my issue but full disclosure. New glow plugs+reamed prechambers (highly recommended) new glow relay box new alternator new optima new bat. cables all new fuses changed all rubber vac. lines brakes+drilled rotors all new suspension (everything ball joints drag link ect. all new fluids (brake and power steering+filter) thermostat K+N air filter front bearings ABS sensors propeller driveshaft discs carrier bearing engine shocks stereo speakers ect. monark injectors+ balanced cleaned hard lines cleaned gunk in crossover pipe + intake injector return lines+ ALL rubber fuel lines custom interior work I'm sure there is more I did all of this myself and have had it for about 382 days but who's counting so I really know this car but have much to know yet.... Since this we will call it an RPM problem I drained my tank replaced fuel screen (it was OEM but not that bad still had black chunks though) replaced fuel lines out of the tank diesel purged again I did it like a week earlier changed filters (my pre-filter is the aluminum one you sell it had alittle crap in the bottom of the bowl) they where a week old with 50miles on them UGGGH sigh I cleaned out my wastegate and exhaust chamber that leads to it it was pretty plugged with carbon pulled all injectors and put them on the balancer they are fine popping 25psi of each other and spraying fine they had about 300miles since new but want to cover all bases? Today I disconnected the exhaust thinking maybe the cat is plugged or collapsed no differance my main vac. pump (front of motor left of power steering pump) puts out 22hg on mityvac It's getting air, fuel, and the exhaust has good pressure out the tail pipe I did a compression test (injectors were out again so what the hell?) all at 400 to 410psi long block has 50K on it cam is good intake valves are good still has crosshatching in cylinder walls (I had oil pan off) The little electric plug that goes to the injection pump (two prong) I put the ohm meter in one socket and against bat. ground and got 615 and the other socket got like 1400 maybe a problem?? If car is running and you unplug this it shuts it off so it is important (injection time control I don't know) I even plugged it in backwards and it started up run fine but no help with RMP's (at this point I'll try anything) OHHH let's see here I disconnected the battery maybe to "reset" a freaked out computer that didn't work!! UMMM I made sure my fuel cap was on tight told you I was grasping at straws here! My town is 12,000 the two diesel mechanics in town didn't have any real ideas except the clogged cat but I was going to do that anyway after bouncing ideas off them (I wanted to pay them to fix it!! I told them take it fix it I'm gettin' pissed) they both said "with all you have done to this car you know more than I do" Benz dealer says the same thing you'd think I'd feel good about that but I'm still not able to figure this one out and makes me feel like crap!!! I'm all out of straws anyone got any for me to grasp?? Is there a throttle position sensor (I just thought of this) I know there is a crank position sensor It's gotta be something simple I don't see