Delphi Mechanical Fuel Pump stops pumping fuel

[David Downs]

12 minutes ago, Zed Head said:

You might try some experiments, moving the lever by hand, to see when in the stroke the pump actually moves fuel.  There might be too much play at the beginning of the stroke.  Could also be that the leaky valves inside allow too much fuel past on each stroke.  

If the problem is play, you could stack some gaskets to create a thinner spacer, to take up the play.

I removed the valve cover and manually operated the fuel pump arm and was able to pump fuel (full pump motion). Using pump action similar to the eccentric resulted in very little fuel movement. Sometimes after sitting the fuel in the filter would drain back toward the tank. The delay in filling up the fuel filter would cause the engine to starve for fuel (too much air in the system). We could have two issues here. The aftermarket pumps won't hold vacuum keeping the fuel in the filter and the eccentric needs more lift to pull fuel faster to the filter. A new eccentric with more lift may be a solution to compensate for pumps not meeting original manufacture specifications. The fuel check valve will fix the issue with the aftermarket pumps not holding vacuum. Looking forward to an update this weekend after the fuel check valve is installed.

[Zed Head]

I think that the heart of the problem might be the bad valving.  Have you checked pressure, and capacity?

 

[David Downs]

On 4/25/2020 at 1:20 PM, Zed Head said:

This is the bothersome part.  I'm not an SU guy.  But the float bowls should not be siphoning empty with the filter's emptying.

The car should run as long as there is fuel in the fuel bowls. Every time the filter runs out of fuel the car will keep running until the carbs are dry.

[Zed Head]

oops

 

Edited May 5, 2020 by Zed Head

[Zed Head]

14 minutes ago, David Downs said:

The car should run as long as there is fuel in the fuel bowls. Every time the filter runs out of fuel the car will keep running until the carbs are dry.

Does the filter run dry while the engine is running?

[David Downs]

31 minutes ago, Zed Head said:

I think that the heart of the problem might be the bad valving.  Have you checked pressure, and capacity?

 

I have not performed the specific test. However, I think it's clear the Delphi fuel pump is the issue. Maybe the non original pump design is actually a better design.  

[David Downs]

5 minutes ago, Zed Head said:

Does the filter run dry while the engine is running?

Only if the car was parked for a little while. After restart she will run on the fuel that is in the carbs and filter until its dry.

[Zed Head]

I think that this was a good idea, below, and it's telling about the valve quality.  Maybe you can find an old Nikki pump.  Pretty common to have to fix new aftermarket parts.

 

[Zed Head]

Found a video if you decide to dig in to them.  This guy posted here for a while but now has his own web page.  Some pretty good stuff on it.

http://woodworkerb.com/home/datsun-240z-rebuild/datsun-240z-mechanical-fuel-pump-rebuild/

[David Downs]

Fuel check valve is installed. Going to drive Lori (my 240z) this weekend and report back. This is the valve I installed.

https://www.amazon.com/gp/product/B07HNQVF5B/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1

 

[87mj]

When I was a kid, my first car was a '73.  Flat top carb issues aside, periodically my fuel filter would run dry and the engine would stall.  I disconnected the fuel line from the filter and put a long hose into a glass jar of gas which eliminated the tank and lines as being the problem.  Periodically, it wouldn't draw fuel. So I concluded it was my factory pump.  I replaced my pump and had the exact same result. At the time, I was told it was vapor lock which was common with that year but the behavior acted like it was the fuel pump. 

Today, I have a low mile '71.  It runs very well.  But I only drive it once per month during the summer. A year or two ago, if it sat, gas would drain from the fuel filter.  I would need to crank it for about 20 seconds or so to get it started.  My son would witness gas coming back into the filter which resulted in it starting soon after.  That made no sense to me since I knew there was gas in the float bowls regardless of what the filter looked like.  I installed a different pump and made sure the fuel line clamps were tight so no air could get in and allow the fuel to return to the tank.  Now the fuel remains in the filter but I still crank it 20 seconds or so if it has sat for a week or two. So the bottom line is there was no noticeable difference in how quick it started based on how much fuel was in the filter.  I will be curious if that check valve makes a difference.

Edited May 12, 2020 by 87mj

[David Downs]

2 hours ago, 87mj said:

When I was a kid, my first car was a '73.  Flat top carb issues aside, periodically my fuel filter would run dry and the engine would stall.  I disconnected the fuel line from the filter and put a long hose into a glass jar of gas which eliminated the tank and lines as being the problem.  Periodically, it wouldn't draw fuel. So I concluded it was my factory pump.  I replaced my pump and had the exact same result. At the time, I was told it was vapor lock which was common with that year but the behavior acted like it was the fuel pump. 

Today, I have a low mile '71.  It runs very well.  But I only drive it once per month during the summer. A year or two ago, if it sat, gas would drain from the fuel filter.  I would need to crank it for about 20 seconds or so to get it started.  My son would witness gas coming back into the filter which resulted in it starting soon after.  That made no sense to me since I knew there was gas in the float bowels regardless of what the filter looked like.  I installed a different pump and made sure the fuel line clamps were tight so no air could get in and allow the fuel to return to the tank.  Now the fuel remains in the filter but I still crank it 20 seconds or so if it has sat for a week or two. So the bottom line is there was no noticeable difference in how quick it started based on how much fuel was in the filter.  I will be curious if that check valve makes a difference.

Clearly I'm not the only one having these issues. The only difference we are having is my car will start right away if fuel is in the float bowls. The problem for me is when the fuel drains back to the tank and my car burns all the fuel in the float bowls before fuel returns to the filter. I'm assuming by installing the check valve my fuel will always be in the filter and eliminate the air gap between tank and float bowls. Looking forward to a fun day of driving and not being towed.