Fuel pump dying briefly, getting closer to solving my issues (automotive issues :) )

[Dave M]

wait a minute... car is supposed to have 14.4V when running... 13.5 minimum to charge battery. Check voltage at relay socket... should be same as you measure on your battery emergency post and ground.

If for some reason at your pump location you only get 13.1... then I would say your fuel pump relay is bad.

Good point, but like I said, the relay is brand new. For fun, I'll swap in the old relay and check the voltage. Then I'll use the switch I made to bypass the relay and see what voltage I get at the pump power connector.

Dave

[Tiger]

Oh... I didn't know you got new fuel pump relay... Check the voltage at the battery too.

[Dave M]

Oh... I didn't know you got new fuel pump relay... Check the voltage at the battery too.

Volyage at batt. is 14+. I think the fact that I have the DMM attached to the power supply at the pump with a safety pin (again, I'm cheap) might have something to do with the 13.XX volts I'm geting.

I'll get some better info and post back later.

Dave

[gmannino]

Coming from a extreme electrical background, I would rule out voltage problems at your fuel pump. Don't bother trying to troubleshoot the fuel pumps electrical system because you mentioned that the voltage drops from a low 13v to a high 12v.

This small amount of voltage is not enough to effect the pressure output of your fuel pump. The fuel pump is rated at 12 volts anyways so .5 volts difference would not be noticeable even with a fuel pressure gauge.

Mechanically, your fuel pump has a much higher probability to fail than electrically. I would but a fuel pump, swap it, and try it for a few days. If your problems continue, return it.

Also, things to look at would be fuel filter, and fuel injectors. Try a highly concentrate fuel injection cleaner to see if you get better results.

[fujioko]

Hmmm,

Your first post indicated that you electrically by-pass the BMW control circuit and the fuel pressure goes up.

Very odd,

When you say ”Actuated the fuel pump with a switch at relay”, I assume you pulled the relay and jumper the socket ? Is this true?

It would be perfectly safe to temporary drive the car with the relay jumped, have you tried this yet.

Perhaps the trouble is not the main power to /through the relay, but rather the signal from the ECM. A crappy connection might allow the relay to shutter, thus effecting the fuel pressure.

From the info provided, I see an electrical fault, not a mechanical fault.

Best of luck!

[calibra]

Hi did you get to the bottom if this

[bsell]

"...when I actuate the pump with a switch at the relay, I get a solid 51-52 psi (proper opressure according to bentley)."

This is an improvement over the normal state (with relay in the system) and is worth noting... What is the voltage at the pump with this jumper set up? If it is also higher, then you are in the right ballpark, faulting the relay.

I am concerned that you have 13.1V at the relay power supply. Corroded connection somewhere between the relay connection and the alternator/battery? Or is your alternator only pushing 13.1 volts due to a tire battery or worn alternator?

"I noticed that the voltage does NOT drop significantly prior to the pump quiting. The voltage only drops slightly from 13.1 V to high 12.XX V just after the sputter,..."

This rules out a loose connection between the relay and the power supply (alternator/battery). This drop can be caused by high current draw to get the pump spinning back to pressure after electrical power supply droop caused by a failing fuel pump relay. My guess is dirty contacts in the relay. I'm not sure what kind of relay you have but you can usually pull the cover off and see what the power contacts look like. My guess is they are corroded and barely making contact after all of the years of use. A homeroy fix is to sandpaper the contacts to get them clean but that only makes the contacts shorter, creating more of a gap/less contact pressure in a tired relay. This is only a temp fix at best.

A failing fuel pump can draw too much power, causing excessive current draw across the fuel pump relay contacts, making the relay die quickly. Did your relay jumper wire get hot during the test? As in too much current running through it? If so, it may be time for a new pump and relay.

Just some things to think about,

Brian