Fuel regulating pressure low (but fine when jumping pump)

[sileighty]

Hi there. I have a 1986 325e. I tee'd a fuel pressure gauge to the cold start valve connection. When I jump the fuel pump using a fused wire, I get 36 psi, which is right on spec. However when I replace the fuel pump relay and start the car, my idle pressure is 24 psi. When I step on the throttle the fuel pressure rises, so at WOT I get 35 psi. But for my car the pressure is supposed to be 33.4-39 psi, so I am significantly lower than that range.

What I don't understand is that the fuel pressure is correct when jumping the fuel pump, but incorrect when the car is running.

I have already changed:
- main fuel pump
- fuel filter
- fuel pressure regulator
- fuel tank

The 30 and 87 pins for both my fuel pump relay and oxygen sensor heater relay are burned, but the oxygen sensor heater relay looks far worse. I tested the resistance of the fuel pump relay pins 30 and 87 while they were connected (I put a 9v battery on the other 2 prongs) and the resistance was 0.4 ohm, which is the same resistance as my leads when they are touched together. I also noticed the same pins (30/87) on the oxygen sensor heater relay were 2.2 ohms, so I'm guessing that relay is bad, but I don't think that would affect fuel pressure right? Also the voltage coming from the socket that pin 30 on the fuel pump relay plugs into is 12.18 (my battery voltage) so there doesn't appear to be an electrical problem up to that point.

With the engine off and the fuel pump jumped, I clamped the fuel line before the fuel rail (but after the fuel gauge) and the fuel pressure shot up to 90-100 psi very quickly before I turned off the fuel pump, so the fuel pump appears to be plenty strong.

I did a fuel delivery test and got about 32 oz in 30 seconds (supposed to be at least 30 oz in 30 seconds) so my fuel filter should still be fairly clean.

I removed the fuel rail and jumped the fuel pump. Neither the injectors nor the cold start valve leak a single drop of fuel. I also traced the fuel line from the main pump to the fuel rail while the pump was jumpered and didn't find any leaks. I also changed the gas tank because the car sat for 10 years.

I started investigating fuel pressure because when my car gets to about half tank, I experience misfires and a significant loss of power. I can pretty much barely get home when this happens. I've read that fuel pressure drops as the fuel tank empties (the DIY guides say to run the tank down so that there is less fuel pressure when you need to do work with the fuel lines). The car has been almost completely topped off since installing the fuel pressure gauge so I haven't seen what low pressures it reaches when I experience severe stuttering, but what I suspect is that as the tank empties and fuel pressure drops, it drops so low (from its already low starting point) that it can't fuel the engine. But I can't figure out what the problem is.

What I'm going to try next is a set of rebuilt injectors (not reusing mine, they have no leaks but have 251,000+ miles on them) and new relays, but I don't know if these will help, so I was wondering if anyone had any ideas?

[Madhatter]

The fuel pressure regulator is working. That's why the pressure changes. It requires vacuum of the engine running to function as it's basically just a valve operated by a diaphragm, thats why you get different pressure without the engine running when you jump the fuel pump.

If it starts playing up ONLY when you get to a half tank or less, you most likely have fuel pump issues. Most always happens in cars with a lift pump that is no longer functioning. Places more demand on the pressure pump which worsens as the fuel level drops.

Clamping off the fuel line and reading the pressure isn't an indication of anything other than the pump trying it's hardest to push fuel through. It's meaningless because pressure is directly related to flow, so if the pump isn't actually flowing fuel through it, you don't really know what it's operating condition is like.

[dnguyen1963]

I'm guessing that the in-tank transfer pump is not functioning correctly.

[TobyB]

Yep, the regulator is working, but I agree the low- pressure pump probably isn't.

The fuel pressure regulator is referenced to the manifold, so at idle, when you have
-10 psi of air pressure in there, the apparent pressure at the rail drops by that -10.
To test this, pull the little vacuum line off the back of the regulator at idle, and watch
pressure jump back up.

To test the low- pressure in- tank pump, you have to unhook the fuel line where it comes
out of the tank under the back seat, rig up a catch can (in case it IS working)
and then fire the fuel pump circuit. Don't do this for long, as the main pump doesn't like
running dry for very long.

hth

t

[sileighty]

Sorry to bring up an old thread, but I wanted to come back to tie up loose ends. Everyone was right, the problem was my transfer pump was bad. I didn't realize this before because when I jumped the transfer pump with a battery, it ran and flowed fuel, so I thought it was good, but it wasn't powerful enough to move a fuel pressure gauge needle at all. I then went and replaced the transfer pump with a Chevy Vega fuel pump.

This was done over a year ago and my car has ran well ever since. However 3 days after changing the fuel pump I got rear ended, so my car hasn't been very nice to look at. Here's pics I took when I was changing the fuel pump:

See the pictures of the new transfer pump here.

Long overdue, but thanks for the help everyone.