Fuel Pressure Drops Rapidly after Sitting 7 months

[Schmieder]

When you've got the car up in the air and pull the filler hose from the gas tank (the short rubber hose between filler neck and gas tank) you should be able to get a smaller hose into the tank and siphon the fuel out. It's worked for my 93 Z34, if I remember yours is similar.....
Hope all goes well,,,
Tom.....

I just dropped it with the fuel in it. I used 2 jacks with wood to spread the load. Ends up, the contamination was a deteriorating filter. Pieces of the nylon like material were drawn up into the pump screen and the pump itself. The fuel looked fine and the tank was a little dirty, a few specks but nothing troubling. I still flushed it very well, now it is crystal clear.

Here is a lesson, just because the filter LOOKS clean and IS clean, doesn't mean it's a good idea to reuse it anyways. I made that mistake and it cost me a good pump.

At least I got an ACDelco filter. Pricey but worth it. I always had no problems with ACDelco, great company.

I have fuel pressure now. Going to start it up tomorrow, lost daylight and I don't want to run first start in the dark. Gotta have eyes on the beast while she warms up for the first time after a long slumber.

I wanted to start it up but patience is a priceless value. Besides, I have an HD digital camcorder ready and want a good video of the first start for all of you to view.

[walterdude]

When you've got the car up in the air and pull the filler hose from the gas tank (the short rubber hose between filler neck and gas tank) you should be able to get a smaller hose into the tank and siphon the fuel out. It's worked for my 93 Z34, if I remember yours is similar.....
Hope all goes well,,,
Tom.....

[pocket-rocket]

I imagine the fuel would be a different color other than clear. You would also be able to see water separate from the fuel in the bucket if water contamination were the case. I know we could very easy.

[Schmieder]

Now that I know my pump is dead, after sitting, it seems I may have contamination in the tank. The pump's death seems premature but sitting 7 months with out enough fuel to cover it may have been a major factor.

But now I have to think about flushing the tank. I heard of sloshing around with roofing nails or a chain roughly 5-10 feet long as the abrassive. The tank is only 3-4 years old, I'de hate to think it would need relining. The fuel coming out is crystal clear (i pump only 91-93 octane). So maybe my tank is fine.

If I had contaminant issues, would I be able to see a rusty color tint in the fuel? My fuel is VERY clean looking. Well, well see what is at the bottom of the tank.

Today is a beautiful day, time to get to work on the pump swap soon. I want to pump more fuel out but getting a gallons worth in 15 minutes is crazy. I still have 1/8th a tank left. Not too bad, at least it isn't a full tank....thank God!

[Schmieder]

My previous post was more about how it sounded like you had good flow from the lines, but not out of the valve, meaning obstruction in the rails somewhere. As far as the flow... A good pump will fill a 5 gallon bucket in a matter of a minute or two.

Ha, my pump wasn't anywhere near that amount. I would compare the flow to a Rat taking a piss versus a Gorilla.

Well, I feel better knowing for sure the problem lies in the tank. Better that then an electrical issue........lesser of two evils. It's just a matter of busted knuckles, working on my back under the car. At least I did it once, I can do it again this time with out surprises.

[pocket-rocket]

Well, not the best flow but it is apparent it has a push behind it. Though, from the fuel filter quick disconnect, pumping fuel at a very slow rate. Roughly 5 minutes plus just to get 3/4 a gallon. It is getting worse though, the flow was greater during the fuel rail test yesterday. It is a trickle flow now and I still have 1/4 tank.

My previous post was more about how it sounded like you had good flow from the lines, but not out of the valve, meaning obstruction in the rails somewhere. As far as the flow... A good pump will fill a 5 gallon bucket in a matter of a minute or two.

[Schmieder]

If you are going with a relay in the trunk, just use a 12 volt relay of some sort, use the present wire for fuel pump power to power up the relay and feed 12 volts from the battery through the relay contacts to the fuel pump. Larry

Hey, that's a good idea. I like the way you think.

[AleroB888]

My Walbro pump apparently has a check valve as the electric fuel pressure gauge always reads ~50 psi continuously with the engine off after the pump primes and before start up....

Really, what model is it? I had 3 Walbros including one kit packaged by Racetronix, which is now installed. None of them had check valves. The first 2 failed because of defects the Walbro company admitted they had, and corrected in later production runs.

[trotterlg]

If you are going with a relay in the trunk, just use a 12 volt relay of some sort, use the present wire for fuel pump power to power up the relay and feed 12 volts from the battery through the relay contacts to the fuel pump. Larry

[Schmieder]

This concerns me.

I hot wired the fuel pump relay terminals 3 & 5 and also measured 12v.

Well, not the best flow but it is apparent it has a push behind it. Though, from the fuel filter quick disconnect, pumping fuel at a very slow rate. Roughly 5 minutes plus just to get 3/4 a gallon. It is getting worse though, the flow was greater during the fuel rail test yesterday. It is a trickle flow now and I still have 1/4 tank.

The Fuel Rail is clean and obstruction free, that I remember checking while swapping injectors. I'm going to blow out the fuel lines when the tank is removed with mild air pressure.

It very well could be bad wiring issue but I strongly feel the pump is dead/dying. And at this point I should just get the pump swap over with before I continue onward. It would make it easier to start diagnosing an electrical issue ruling that out. I may even route my own (heavier gauge) wiring replacing the original wires to the fuel pump. As for the pumps ground wire, can I ground it to the frame using far less wire or is the ground routed to the PCM as a switch. I heard they are ground controlled.

I come to realize how simple the circuit is for the fuel pump and checked all angles. I pierced the GRY wire in the trunk leading to the pump and the voltage was good there too, 12v. The grounds, both, were strong. One was for the fuel level sender so I tested both to be sure I got the right one.

I have a good question for you guys. This is a techy thing I want to try if it is okay...........I want to place the fuel pump relay in the trunk and wire the fuel pump w/ heavier gauge and MUCH shorter use of wire. The battery is in the trunk as well so the circuit to the fuel pump would be roughly 2-3 feet versus 9+ feet. I would get a spare fuse box and rig it up like a factory would and bolt it down somewhere safe and out of the way.

What do you guys think? If I had an electrical supply issue, that could solve it.

I will update the thread after I swap the pump.

[pocket-rocket]

When I push the schrader valve, I get a weak spit of fuel and a trickle. But I get good flow from the fuel lines.....

This concerns me.

[Schmieder]

update, I got a wiring diagram. After digesting it I found several ways to test the pump.

One, I'm going to 'jump' the relay terminals. ORG to GRY, This will be conclusive regarding the relay.

During that jump I can insert a pin into the pump wires in the trunk to verify the voltage is good.

Then test the ground.

Should all that be ok, then it really is the pump. I have already ruled out the filter, FPR, leaky injectors, leaking lines, etc.

[Schmieder]

I don't have a pressure gauge handy atm, but i can tell for sure there is not enough pressure. I tapped the Schrader Valve many time in the past to check for pressure hours later and immediately after prime. There is just not enough pressure and what little pressure there is drops off to zero in seconds. As it 'wants' to run immediately after prime but refuses any combustion after sitting a few seconds after prime.

I know it is a ghetto way to diagnose fuel pressure, I should get a gauge. But it is too obvious. Oh wait a minute, I have an electric pressure reader with a schrader valve adapter It is a dash gauge from Glowshift but it should work just fine.

I'm going to hook that up so I can get some real numbers. Just for curiosity to see if I even come close to 44psi before it drops off like a rock.

When I push the schrader valve, I get a weak spit of fuel and a trickle. But I get good flow from the fuel lines.....


Thanks for the help guys, at this point I really think the pump is dying/dead. But I will try the wiring fuel pump to batt idea, after I digest the pinout.

[trotterlg]

Guess you could measure the prime pressure, but if it tries to run on prime then it sounds like it is getting some fuel, so I would either direct wire the pump or measure the pressure you are getting on prime. Seems like an easy test to avoid pulling the pump. Larry

[pocket-rocket]

I have a good question, the relay is for prime and then when the oil pressure sensor registers pressure for the PCM to keep the pump on (run mode) what relay? Or is it shared?

The OPSU switch and fuel pump relay from the PCM are wired in parallel. This way if for some odd reason the output on the PCM or the relay itself fails, the OPSU continues to send current to the fuel pump so you can make it home.

One misconception people have is that once the engine is running the OPSU switch feeds the fuel pump to keep the engine running. I've run 60v6s without the OPSU in place because I needed to run a pressure gauge on a vehicle that had a dummy light and the vehicle ran just fine after the initial prime on the fuel pump. The OPSU either wasn't on the car at all, or I plugged it in for some reason, but left it hang (which would still register 0 pressure to it anyways). Once the PCM registers RPM and that the engine is running, it turns the fuel pump back on by way of the fuel pump relay.