Oscillating fuel pressure Options

[andreic]

Hello,

in the process of getting a new to me 1972 914 1.7L with D-Jet I replaced all the rubber fuel lines (but not the plastic ones), the fuel pump (three port, with a rebuilt one) and the fuel filter. All the plumbing was done copying the original.

In the beginning I had trouble getting the fuel pump to prime, but after I left it overnight for one night the fuel started flowing well. The pressure was holding perfectly steady at 30psi +/- 1psi. (I hooked up a fuel pressure gauge to the test port on the driver side rail, and hooked up a 12 volt power source to the pump.)

All seemed well, and I started driving the car. I probably drove it for about 100 miles without problems, except that a few times it seemed to stumble on acceleration around 3000 rpm. Some days it would not do it, some days it would do it a couple of times. Once it was a bit worse, and I worried I would not make it home, but in the end it worked.

Then a couple of days ago I was driving it to the remote garage where I keep it, and it started stumbling worse than usual. I made it to the garage and parked it, thinking it will go away again. Yesterday I tried driving it again, and it was much worse, I managed to drive it out of the garage but then got stuck. It would stall, at idle and almost any other rpm; when starting it again it would run at idle for maybe 30 secs, then stall again. Trying to rev it up would not help.

So today I went again with a fuel pressure gauge, and what I saw is the following. When I just start it, the pressure goes to 30psi and stays steady for 15-30 seconds. Then it starts dropping towards 15psi, and the engine starts to die, but suddenly the pressure goes back up to 30psi and the engine runs good again. This cycle repeats itself for a few times, but the pressure drops more and more, until eventually the engine dies.

Any idea what could cause this? The fuel pump was a refurbished one bought from a respected member here (and it is not leaking, the common cause for failure). Could it be some trash in the fuel system? The fuel tank was cleaned and resealed professionally not more than a year ago. Could it be one of the lines under the fuel tank got kinked? Could the pressure regulator be going bad?

I was thinking of starting to debug this by hooking up a 12v power source to the pump directly (I was not yet able to do this at the place where the car is garaged, and I don't dare to try to drive it home), in order to see if the problem repeats itself with the engine not running. Anything else short of starting to pull off the fuel tank and the fuel pump? I'd hate to have to do this, this garage is a public one (think underground garage under big public building) and I don't feel comfortable working on the car there.

Thanks for any suggestions.

Andrei.

[johnhora]

Definately check the fuel tank "sock"
Most likely that's the problem with your pressure...
It runs fine until you drive around and stur up all the trash in the tank.
You can't believe how gummed up one of them can get and how much crap can float around inside the tank. Especially if you have never cleaned and or replaced them.
Be sure to blow out both fuel lines to the tank...