Premature Fuel Cut, Maybe?

[cajafiesta]

Hey Folks, been piddling around on the 82 NA diesel rabbit I bought.  Just sort of going through it and cleaning up odd stuff I don't like.

I added an aftermarket tach today.  It's a W-signal tach.  Admittedly, I have no way of verifying that the tach is accurate until I get my hands on a handheld, optical tach to read crank RPM and set the W wire tach accurately. 

But I digress.  Before I installed the tach, I noticed that there seems to be a fuel cut around what my ear says is 3-3500rpms. The car seems to drive fine below 3000rpms.  It doesn't seem to be down on power. It seems to start fine after about 10 seconds of glow plugs. No oddities from my perception.  The car makes a small amount-whisps out the exhaust- of black smoke at full throttle.    Installed the W wire tach today and set it to around 800rpms reading at idle.  This is pretty close, ballpark.  Revved the car up after it was warm and, sure enough, the engine hits something that is limiting rpms.


It seems to be happening around an unverified 3000-3500rpms.  It it, seemingly, for sure below the factory cut that I've read is something like 4500rpms.    It is not a "soft" cut.  It sounds like fuel is being stopped and then added back, then stopped, rapidly.  Sounds pretty much like a spark ignition hitting a factory rev limiter.  But it's happening at 3000ish rpms.  The motor puffs a small amount of light colored smoke when it taps the max rpm that can be achieved.


I assume, logically, that it has to be something fueling related.  I'm ordering a shop manual for injection pump setup info as I type this.


As I'm new to these cars, I don't have a solid foundation of "common issues and how the symptoms present themselves."  I figure there's a chance this is something obvious and y'all can pinpoint it immediately. 


Anyone have any smart ideas? 


Thanks for your time an consideration.  Much appreciated!

[ORCoaster]

Got me stumped.   

[fatmobile]

A vacuum gauge on the front of the pump will tell how hard the pump is working to pull fuel.
 I have a dash-mounted vacuum gauge on most of my VW diesels.
 Maybe there is a fuel line restriction that is withholding fuel at higher demand.

[ORCoaster]

If you can run it off a bottle or jar with the inlet and outlet hoses going in and out you can eliminate the rest of the fuel system and possibly get a handle on where the problem lies.   

Once you run it unrestricted you can add the filter into the system and if you get the symptoms at 3K then perhaps a new filter is in order. 

Running a vac gauge is the best way to determine if a filter is overdue for change.  You never can tell what gets sucked up off the bottom of the tank when you forget to keep a bit in there just because of that. 

I pulled my tank down a few months ago and found someone's attempt to siphon fuel from the tank had gone bad.  The hose they used had a hose barb on it joining two small sections to make the length and one of those parts ended up staying in the tank and fouling up the sending unit for me.  I could never be really sure whether the amount of fuel I was showing on the gauge was real or not.  It fluctuated depending on the position of the floating hose.  Very annoying.

[cajafiesta]

Thanks for the suggestions, folks.  I'm going to see if I can get into it a little more thoroughly this weekend.  I'll report back what I find.

Thanks again to everyone for the responses.