Runs Nice for a Few Min. Then Rough, Then Kills...

[OldSkoolFool]

Gents,

I somehow missed the most recent replies, sorry about that. Thanks again for all the info, help, and time. That's why I love the Z car community!

So, we received and installed a new fuel pump and pressure regulator. At this point we can not get any of the three pumps to make fuel flow to the front of the car. It is like they are not pumping the fuel or not able to for some reason. This most recent stint I was not present for so am going off of my father's description. He used a vacuum to get the fuel flowing from the tank and it flows freely. I am not 100% sure if he stated the fuel pump is actually turning on and running or not. We are getting very close to pulling our hair out now and might be having a professional mechanic look at it in the next couple days.

So, from what I understand from the replies, the fuel pump in a '78 relies on input from the alternator AND oil pressure switch? I am glad I do not have to worry about the ECU. I guess I do not understand what exactly I should be diagnosing right now or where to go from here.

[siteunseen]

I couldn't figure all the switching for mine before I replaced the old pump. Took about 12 ft. of speaker wire and straight wired it to the battery and ran a hose from the hardline before the front fuel filter, in the engine bay, into a gas can. Have a hose pipe turned on or a fire extinguisher nearby and be sure the hose is secure in the gas can or it'll blow out 50psi without going through the regulator. Be safe.

[Zed Head]

So, we received and installed a new fuel pump and pressure regulator. At this point we can not get any of the three pumps to make fuel flow to the front of the car.

He used a vacuum to get the fuel flowing from the tank and it flows freely. I am not 100% sure if he stated the fuel pump is actually turning on and running or not.

So, from what I understand from the replies, the fuel pump in a '78 relies on input from the alternator AND oil pressure switch?

Is the pump mounted in the original location, and is the supply hose a fairly straight shot to the pump? The EFI pumps are typically "push" pumps, they don't pull well at all. In other words if the pump is mounted too high or if there's a loop of hose before the supply inlet to the pump, the pump rotor will just spin air, but won't generate enough suction to prime the pump.

How long did the fuel flow freely? Sometimes there are floaters in the tank that let fuel flow then get sucked down on to the outlet from the tank.

The power to the pump is controlled by the alternator power OR the oil pressure switch. Either. But it should be fairly easy to hear the pump when it runs. You can test the pump, and the pump power wires, by removing the small wire from the starter solenoid, and turning the key to Start. The starter won't engage but the pump will get power. this will let you test pressure and flow, and take the pressure switch and alternator out of the picture since the Start circuit bypasses those safeties.

[FastWoman]

OK, time to start eliminating problems:

Got power to your pump? Disconnect the small wire from the starter. Crawl under the fuel pump, and hold a pair of volt meter probes across the terminals of the fuel pump, while a friend turns the key to "start." (It won't actually crank, because you've pulled the solenoid wire.) You should have 12V at the pump for about 5 sec, and then the power would switch off (without a running engine). You can also hot-wire your fuel pump the way siteunseen suggested.

Does your pump operate? (Yes, it should. It's new.) With power across the leads, you should hear it running.

Is the line clogged from your pump to the engine compartment? Is your filter clogged? Make certain the fuel flows freely. Pull off the fuel hose between the filter and the fuel rail. Put a longer hose on the output of the filter, and put the end into a fuel jug. Run your pump. There should be a voluminous flow of fuel out the end of the line.

Passing all of these tests, you should also have pressure, unless there is a mechanical problem with either the pump or the pressure regulator.

If I had to guess, I'd place bets that you don't have power to the pump. There's a somewhat problematic connection inside the passenger firewall (at least on my car), and the fuel pump relay connections can also get a bit crusty. And also, as I said, check your ground.

[OldSkoolFool]

Thanks for the next round of suggestions/info. Couple things: The pump is mounted in the exact stock location. The line going to it looks to be the original set up from the lower right front corner of the tank kind of behind the passenger rear wheel then to the pump. The diaphragm/tank looking thing that evens out the fuel pulses is present as well. Going from the pump to that has a good loop/180 degree? turn then like another 90 degrees towards the front of the car (I am trying to picture it from memory, car not in front of me) but looks to be the stock set up. Just called my father to clear up what happened last time. Fuel pump is receiving power. It does kind of seem like the fuel pump is spinning air.

When removing the fuel line that goes from the tank to the fuel pump, it does let fuel come out BUT only for about 5 seconds then stops. In regards to a floater being in the tank that would clocg the hole, would this be a floater on the sending unit then or...? Say if it is a floater clogging the outlet, how would/could that cause the car to run well until about warm then run rough? I sure hope it isn't the sending unit as it looks like they are no longer available and used ones on Ebay for $400+. Is it possible there is something else going with a pressure/suction issue in the tank not allowing the fuel to come out? We did try it with the fuel cap open as well.

To recap:

-Car originally started and ran very nice for a few minutes then would run more and more rough to the point of so lean it would kill. Fuel pressure was never held at the 30 PSI mark, usually was around 15 PSI.

-Installed three different pumps and now we cannot even get the car to start. No pressure read on the gauge.

FastWoman, looks like our Z's are identical twins. Yours a five speed too?

Zed Head, the wife and I took the 911 from WI to the West Coast from Washington through Oregon. Beautiful country! Loved it. Here's a pic from Crater Lake:

[240260280z]

Forget the tank and run hoses to a jerry can next to the rear wheel. The hoses will connect to the hard lines running to and from the engine (the pump will be in series on the send side of course).

[FastWoman]

Yes, mine's a 5 speed!

So you're saying fuel will flow out the line via gravity for about 5 sec and then stop? Is your tank full? What happens with the gas cap off? Does it keep flowing then?

Loose rust can float around in the churning fuel until it progressively clogs the intake screen. Then the engine starves for fuel and dies. When the pump is shut back off, the rust falls away from the screen long enough to let more fuel in, until the pump runs long enough to clog the screen again. Repeat, repeat, repeat.

Blue's suggestion would rule out rusty tank issues.

If you want to look inside your tank, there's a round inspection lid in the hatch area under the deck lid. It's about 6" dia, with two screws, as I recall. When you remove that, you'll be looking at the fuel level sender at the top of the tank. There's a ring you can turn by knocking it with a screwdriver. Then the sender comes out (delicate -- be careful). Pay attention to the position of the alignment tab. Once that is out, you can look inside with an LED flashlight. Be careful not to make sparks.

You can also pull the drain plug on the bottom of the tank, passenger side (10mm wrench, as I recall) and see if rust comes out.

Edited October 16, 2013 by FastWoman

[Zed Head]

Yeah, when it's not raining you can see some great scenery in the Pacific Northwest.

I've never let fuel run from the supply line for 5 seconds or more, but I'm pretty sure that fuel should flow freely until the tank is empty. There is a spring loaded one-way (in) vent in the gas cap that can get stuck though, but you tested with the cap off and still see the problem.

People that have sealed their own tanks sometimes clog the screen to the outlet pipe with sealant. I think that I've read of fixing that problem by running some thin wire up the outlet hose to poke the material off of the screen. If you've tried three pumps and none will generate pressure, I would focus on the fuel supply line from the tank. Blue's suggestion will confirm that your pump works if you want to be sure you're not wasting your time.

[OldSkoolFool]

Current update:

We tried Blue's suggestion and same symptoms were happening... it was not pumping out fuel (and yes we did have the gas can high enough). But here is the good news... my father hooked the fuel pump directly up to a battery, while it was hooked up to the 280Z's fuel tank, it shot out fuel like crazy. So it seems like a fuel tank issue is ruled out... phew! Sounds like we have a voltage/current issue. We plan to spend a little more time this weekend to dig in and figure it out.

[240260280z]

That is good news! most 280z efi problems are wiring: http://www.atlanticz.ca/zclub/techtips/electricalconnections/index.html

[Captain Obvious]

Cool. Sounds like good progress.

78 used alternator output AND oil pressure switch to control the fuel pump. Unless the key is in START, the alternator needs to be spinning AND there needs to be enough oil pressure to close the switch in the oil pressure sender unit. If those two conditions (spinning alternator and oil pressure exists), then the fuel pump relay should close and send power to the fuel pump. The fuel pump relay is located on the relay bracket up in the engine compartment near the battery. It has four wires going to it... Black, white/black, and two green/blue wires.

If you separate the connector and the relay and then short the two green/blue wires together at the connector, the fuel pump should run. If shorting the two G/L together doesn't cause the fuel pump to run, then the problem is between that relay and the pump. If the pump runs, then your issue is upstream of that point and could be a bunch of things.

Doesn't matter what position the key is in. You can do this test with the key in your pocket. Give that a try and see what happens?

[Captain Obvious]

The power to the pump is controlled by the alternator power OR the oil pressure switch. Either.

Zed Head, I used to think (up until a few days ago) that the fuel pump on the 78 would only run if there were both oil pressure AND a spinning alternator. However, I've now taken a closer look at the design and I no longer believe that to be the case.

I now believe, just as you suggested earlier, that the fuel pump on the 78 will run if there is oil pressure OR a spinning alternator. It's really a convoluted screwy system and if I get the chance, I'll put together a description of system operation. But in the meantime, thanks for the help.