Thanks Klaus,

I guess there are at least 3 explanations possible.

1) the regulator is ok but it should regulate at about 40-45 pounds. The pump is getting weak and is not putting out enough pressure (especially when fuel is being used by the engine) to fully activate the regulator. I had a fluctuating pressure on an old 164E and replaced the pump. The problem was really dirt - with the new pump the pressure still fluctuated but it was a hell of a lot higher.

2) one way you suggest. Pump may be good enough to activate the regulator but the regulator doesn't know what it's doing and bleeds fuel back to the tank when it shouldn't.

3) the other way you suggest. Pump and regulator are ok but manifold vacuum is uncertain. I think that if you rev up the vacuum should be less, so pressure should go up, and mine went down.


I didn't try the vac line test - but will next time I am near the car. If the regulator is good closing the vac line or revving up should move the pressure up. I also saw this on a different car. If pressure doesn't go up it could be that the pump is really weak, or regulator is sluggish.

ksg