'78 280z cuts out under load

[marcuspoccia]

After 20+ years of sitting, got the z running last summer. After driving a few times, the car began cutting/stalling when giving it a heavy throttle press while driving. The cutting has no relation to speed or rpm, but more the load/stress the engine is under (acceleration/hills). Last fall, we found the car would drop about 10 psi of fuel pressure before cutting out. 

After this past winter, we have started by replacing all of the rubber fuel lines from the tank to the engine, along with replacing the fuel filter. We cut open the filter and did not find any noticeable contaminants.  We had one more successful small drive where we could really get on it, only for a small skip really high in the rpm's, but it would take a full throttle pull or two. 

After that ride, we took the car out the next weekend for a much longer, more strenuous ride, and the cutting slowly came back. We drove a couple back roads normally where it ran decent, then took it one exit up the highway which is where it really began to struggle. We had the car running at about 3500 rpm in 4th for 5 or so minutes there. We struggled to get the car back home after that highway ride. 

Tonight we took the car once again, and it ran better than after the highway ride, but not well. 

I am looking for some advice on what you guys would do. I think it has something to do with a blockage or something in the fuel system. Thoughts?

[Zed Head]

Sounds like the fuel pump is dying.  They've been known to die by overheating, the longer they run the worse it gets until they can't build pressure.  You should never lose pressure under load, that's when pressure goes to its maximum.

The cutting/stalling is typical of running lean, which is what will happen when you lose fuel pressure.  All signs point to the pump, since you've cleared the path to the fuel rail.

OR - there's junk in the tank clogging the pump inlet.  20 years is a lot of time for rust and old gas gunk to build up.

Edited April 29, 2020 by Zed Head

[siteunseen]

Shade tree work around, clamp on the return fuel line to the tank builds higher pressure.  This for testing only.  Not a fix.  Those pumps are $250 if you can find one.  Bosch no longer makes them but Beck Arnley does, same pump to.

[marcuspoccia]

6 hours ago, Zed Head said:

Sounds like the fuel pump is dying.  They've been known to die by overheating, the longer they run the worse it gets until they can't build pressure.  You should never lose pressure under load, that's when pressure goes to its maximum.

The cutting/stalling is typical of running lean, which is what will happen when you lose fuel pressure.  All signs point to the pump, since you've cleared the path to the fuel rail.

OR - there's junk in the tank clogging the pump inlet.  20 years is a lot of time for rust and old gas gunk to build up.

What if i said the fuel pump was replaced last summer, because the old one was completely dead?

[Zed Head]

7 hours ago, Zed Head said:

OR - there's junk in the tank clogging the pump inlet.  20 years is a lot of time for rust and old gas gunk to build up.

That's what the last part was about.  But, there's no guarantee that "new" is good.

In short though, you're losing fuel pressure somewhere.  You need to get it back to where it should be.  The EFI system depends on proper fuel pressure to work correctly.  It's critical.  There's no pressure feedback signal to the EFI control unit.  The FPR and fuel pump are what control fuel pressure, and the ECU"assumes" that it is controlled.