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.

[andreic]

I think I got this sorted out. I towed the car to my house, hooked up the 12v source, and the pressure did exactly as before (started at 30psi, and dropped while the pump was running to about 10-15psi after 20-30 seconds, oscillating between 10 and 30psi). The pump was making weird noises, as if it was struggling to pump.

So I decided to pull out the tank. With the tank out of the car (but still hooked up), the pump runs fine, makes no unusual noises, and the pressure holds steady at 30psi. I let the pump run for 2-3 minutes and all was well. So I am now reasonably convinced that the supply line was kinked under the tank. (When I put the tank in I had lots of trouble not kinking it, so probably it was almost kinked, and then it must have moved and completely kinked after a week or so.)

I will now try to replace the lines from the tank with more rigid lines (high pressure fuel injection ones, I read they have less of a tendency to kink), and make them much shorter, 22in seemed to be the recommendation.

However, as I posted in a new thread, I don't seem to have a fuel sock in the tank. I am now debating whether to take the tank apart completely and install a sock, or go as is. I will probably end up ordering and installing a sock.

Now regarding the pressure drop with no power source to the pump. With the pump running I get a steady 30psi in the system. When I disconnect the power, the pressure drops almost immediately to about 20psi, and then it takes 7-8 seconds to drop to 10psi, from where it drops more slowly. This sounds to me like something is leaking in the fuel system. Which one should I suspect, injectors, cold start valve, or the regulator? How worried should I be?

I assume that if the injectors or the CSV leak, this would make the engine run very rich with bad consequences for the life of the engine and for the fuel economy. How can I test if the injectors leak? Do I take them out and put them in plastic cups, and run the pump? The CSV is harder to take out...