I think the relief valve is part of the output fitting itself on the injection pump. I guess about the last test is to examine the fuel flow from the relief valve when you have little power.

Just submerge your clear fuel line from the return valve into a container and watch for air and fuel flow. If you have no air and fuel flow with no power in your case it very well can be the injection pump.

To be really valid because of your description the test has to have the clear hose fed back into the passenger compartment to verify the same senario is occuring at running loads. If It passes the test your injection pump is almost certainly going to be the issue. I imagine this kind of driving test requires two people to preform.

This type of issue is not that uncommon on cars running used vegatable oils from what I have heard before. You could check this out on one of the sites that is more slanted to running wvo. You would not be the first to have a twenty thousand mile approxamate lifespan on an injection pump running wvo. At least there are still good used pumps around if it comes to it.

If you have the time available and indications during the driving test are good. For the time and effort involved I would introduce laquer thinner into that pump and let it soak for a day.

We have always assumed the bores and pistons get abnormal wear. This may still be true but I do not know if it has ever been proven. What if accumulated fats or substance clumps get warmed up a little and reform somehow into element intake port restrictions?

Do the test and if indicated soak the pump. This is important enough to the majority of us for you to post back. The opportunity of purchasing non running used wvo cars is there sometimes.

We really are not aware if the injection pump will respond to an internal cleaning. Odds are certainly not good but at least one person had the solvent break up stuff and spew it out of the pump. It actually fixed his compaint.

Also since the viscosity of the wvo is heavier. Clamping off the return line when you have no power is also indicated. We know these engines will run diesel fuel at very low base injection pump pressures.Yet have no knowledge of the minumin feed pressures required with wvo. If for example the relief valve where totally defective. The addition of the electric pump could have been totally meaningless.You would lose any pressure instantly through the open relief valve back to the tank.

Now for example if your return valve has failed somehow and there is really next to no operational pressure in there. It would be subject to fuel starvation easier than a car on diesel fuel.

The clamping off of the return line will do not harm and will let the pressure rise. The placement of your 4-7 pound booster pump is not the best even for testing. Usually it is at the back to attempt to get the thicker viscosity fuel up to the the lift pump.

I also think there is a definative test but allow that I do not follow the wvo thing very much. If the car will run on wvo and will not on diesel fuel it is pretty clear there is really too much wear in the element bores. This last statement is considered to be the most current belief. Again proven or not I am not sure. To us non wvo users it sounds okay.

There has been so much issue around the use of alternative fuels something could have fallen through the cracks. I personally doubt that though. Once again let us know how this problem works out and what you do.