You could also have crud in the lift pump. As you pump a solvent in it will soak/clean the lift pump as well remember. One way or another until you are pretty certain things are not just gummed up not much sense in doing anything else.

Yes even the return valve can get stuck open with garbage. Basically it cannot be ignored that the car may have run on fuel of questionable overal quality animal fats and all. Anyways it never hurts to clean the fuel system out or at least parts of it now..Especially after running on alternative fuels.

Once again you are lucky that since the car ran on diesel fuel your injector pump elements themselves where not damaged. Your issue I believe and hopefully is pretty minor in nature. Even if it is not crud buildup. Still you have to start somewhere.

Another way to look at this say the injection pump or lift pump or even the relief valve are gummed up? Yes even two out of three are possible even if unlikely.How do you prove it one way or another without cleaning them out? Not technically difficult to do either. If not cleaned out say you do get it running somehow and down the road another clump of fat gets loose and causes issues. This you do not want..

Right now you almost appear to have a problem of an intermittent nature. Some of your tests indicate the rack may be bound up or sticky as well. More likely the lift pump or it's supply is though. I believe the throttle controls the govenor and this in turn controls the rack so there is a possibility of something gummed up or sticky. Also could be crud under the valves in the lift pump. Gummed up relief valves are not unknown either.

I am not stating cleaning the partial system out is going to get the problem solved. To me it is kind of a first step though on the way. Just in case it both is the problem or presents possible issues later clean them. If you do not do things in a systematic fashion it may come back to life with you not knowing exactly what it was. That is a bad senario as it may then come back sometime when you least want it to.

One member I remember posting saw globs of stuff pumped out of his return valve with the solvent soak. Had to clean it a couple of times to get rid of the crud. In his case it did get the engine running.

A car that has used wvo sitting around for a time seems to solidify things more in the fuel system as well. You can never assume that the old owner used high quality wvo or just quickly strained what he got from almost any source and dumped it in the tank. The vegatable oil burner I purchased just used new vegatable oil. Yet it resulted in him having a road breakdown and replacing the car.

I believe what I found that caused the stoppage may not have occured on diesel fuel. Or there would have been far less likelyhood. I am not dead set against people burning alternative fuels. Still many doing so do not attempt to do it properly. It also can complicate troubleshooting and those same people that use really bad wvo seem to lack the knowledge to deal with the resulting issues..