Hard Starting, but runs fine after finally starting

[dzman1331]

Hi, 1976 280z. Was starting good and running fine. Lots of vacum hoses replaced on

refresh as well as AFM and air regulator. Recently problem started,, would not start on several

attempts. Had to do it 3-4 times then would start and run fine. After starting cold engne

goes up to about 1100 rpms and as it warms comes down to 800 where set.

One thing I do not hear is the relay click when I turn the key one before starting and

I don't hear the pump come on. Haven't done fuel presure bleed down test yet, but believe the

pump not coming on to bring up presure is the problem.

Looked in FSM and didn't see how the system works. Am I correct that the pump should spool up first then shut off with presure? What activates this. Any suggestions would be appreciated...

[Zed Head]

It's described in the Engine Fuel section. The pump should only have power at Start and when the engine is running.

Two possibilities are that your fuel system might not be holding pressure or your cold start valve is not working. Sounds more like a fuel pressure drop. Will it start when warm, after a very short shut down (before pressure can bleed off)?

[dzman1331]

Hi Zed Head, Yes will start fine after that initial start. Then let it sit an hour or so and hard to start again. Is this the presure regulator or the pump letting presure bleed off?

[dzman1331]

I should haave asked, What is the 5 seconds of pump run when you first turn the key on?

[superlen]

FYI.

The fuel pump turns on when the air vane in the AFM moves off it's at rest position. (There's a switch in it specifically for the fuel pump circuit) So you won't hear the fuel pump when turning the ignition to on, only when you try to start and the pistons take their first few breaths and cause the air vane to move will the pump turn on.

The above isn't your problem I don't think, but just wanted to add some more information about how the fuel pump circuit operates.

Lenny

[superlen]

Also, there is no pressure switch to electrically monitor the fuel rail pressure. The pump runs when the air vane is being sucked open by air. The pump shuts off when the air vane closes. From your description, I think your fuel pump circuit is working ok. I agree with ZedNoggin and think that your losing pressure after it sits. (leaky cold start injector, leaky primary injectors, regulator, ect).

Search for pressure gage in the EFI forum and you'll come up with the quick/easy fuel pressure gage to connect to monitor. Once you have a gage on there, you can prove/disprove the leak down easily.

Lenny

[dzman1331]

thanks for the replies.. What holds the presure in the fuel system. The pump or the FPR?

[superlen]

The FPR does. Pump has the ability to generate about 60psi if I remember correctly. The FPR holds this to 36psi.

[Zed Head]

There is also a check valve at the outlet of the fuel pump. FPR at one, check valve at the other, to hold the pressure. The check valves do fail occasionally, search the term and you'll find a few threads about it.

superlen is right about the AFM vane and the fuel pump contact switch. But the Start circuit (key at Start position) does supply power to the fuel pump directly. When the engine starts and you let the key go to On, the AFM vane switch takes over. That gives one way to check the pump and fuel pressure without starting the engine - remove the wire to the starter, then turn the key to Start. The fuel pump will run, but the engine won't turn over.

[dzman1331]

if I have to replace the FPR, looking at it, can it be removed w/o having to remove the injectors or does everything have to come out?

[Zed Head]

Download the FSM. Open the the Engine Fuel section. There is a sub-section for Removal and Installation - http://www.xenons30.com/reference.html

[superlen]

Zed,

I was just checking back on this thread to add that the START signal will also turn on the pump regardless of air vane. I wasn't quick enough, and you busted me already. To clarify once again for posterity: Either of the these two conditions will turn on your fuel pump.

1) The air flow vane is moved off of it's at rest position.

2) The key is in the "start" position.

Sorry for the confusion.

Lenny

Lenny