Fuel rail pressure problems

[ed653]

I'm trying to help a friend with what seems to be fueling problems on his '79 280ZX. He lives in a different city, which limits what I can do. Here's a recent note from him:

"I connected a vacuum meter between the intake manifold and the fuel pressure regulator and hooked a pressure gauge to the fuel rail and took the Z out for a drive.

For about the first five minutes the vacuum gauge read normal and the fuel pressure would vary from about 30 to 36 lbs and the car ran quite well. About a mile down the road the vacuum gauge started acting erratic and the fuel pressure also varied around 28-30lbs seeming to respond to the changes in vacuum. A few minutes later the vacuum gauge read much lower than normal and the engine ran rough. I pulled the vacuum line off the regulator and plugged it and the vacuum reading went up to normal and the car ran fine. I started back home to place an order for a regulator (which I am quite sure is faulty) when suddenly the fuel rail pressure gauge dropped to about ten lbs and the car nearly stalled. I nursed it home and when I get a little more ambition will check the fuel filter, suction hose to the fuel pump and possibly the pump itself. "

Is he on the right track?

I happen to have a '76 Jaguar XJ-S V12 which I believes has a very similar Bosch EFI system. What I find strange is he says there is a vacuum port on the FPR and hooked DIRECTLY to manifold vacuum. Naturally, when he disconnects it the rail pressure goes up. I know later Jag V12s had vacuum ports on the FPRs, but in this case there is a temperature controlled valve that closes when rail temperature gets high, thus venting the FRP vacuum port to atmospheric pressure, and increasing the rail pressure by about 15 PSI. Maybe someone here can tell me how it's supposed to be on the 280 ZX.

Please excuse the cross-listing. I'm not sure where this belongs.

TIA

Ed