5.7 LX EFI random idling stall - electric fuel pump?

[hnt]

Hello folks,

Engine: 1996 5.7LX EFI 250 hp (non-vortec) S/N OF761024

I installed a replacement long block last year and have about 55 hours on it. My engine was from a transition year because it had the later 880 block but had the older (non-vortec) heads. The engine uses a throttle body with a VST tank and a mechanical fuel pump to feed the VST tank (not cool-fuel). The Mercruiser specs for the mechanical fuel pump are 3-7 psi at 1,800 RPM. I was unable to locate a replacement 880 long block with provisions for the mechanical fuel pump so I converted to an electric fuel pump. I used a Carter CRT-4389 fuel pump, 4-8psi, 50 gph as recommended by the engine supplier.

The electric pump is mounted in almost the exact location as the mechanical pump was, below the VST tank. After installing the pump and changing the fuel filters I noticed that the electric pump would not draw fuel trough the filters and to the VST without priming the pump and disconnecting the feed to the VST to bleed the air out. The old mechanical pump would draw fuel and fill the filters without priming or bleeding. Once it is getting fuel to the VST it runs and starts fine and runs strong up to 4,700 rpm WOT @ 24gph without problems.

My only operational issue is that after idling for a long period of time it will occasionally stall and be hard to restart. It never stumbles or stalls underway or at higher power settings. It happened yesterday at my mooring after returning from fishing. I let the engine idle for 20 minutes and it died and would not restart. I removed the fuel line from the electric fuel pump to the VST and was not getting fuel while the pump ran with the ignition switch on. In a few seconds fuel came out and I reinstalled the line and the engine fired and ran perfectly. Just like when I installed new fuel filters the pump was not drawing fuel (had lost its prime). It is definitely a fuel starvation issue before the VST.

I never had this with my mechanical pump. I am wondering if I have a vapor lock situation. Is the electric fuel pump more prone to vapor lock? Do I have a weak electric pump? It seems to deliver plenty of fuel while running and I get a full 24gph from my flo-scan at 4,700 rpm WOT. I have a copy of SB-99 on vapor lock and I might try relocating the fuel pump and water separating filter further away from the engine but it makes me wonder if that was the problem, why didn?t it happen with the mechanical pump?

Things I have done so far:

• Confirmed 30 psi fuel pressure (except when starved for fuel).
• Complete cleaning / service of VST and injectors.
• Rebuilt pressure regulator.
• Temporarily bypassed the fuel pump oil pressure safety switch to allow pump to run continuously.
• Replaced fuel filter elements and checked for water and contamination.
• Replaced IAC valve on throttle body
• Replaced all fuel lines and re-checked all fittings for air leaks
• Replaced Anti-syphon valve at tank
• Checked tank vent system.
• Ran on remote fuel tank and did not experience stalling (but problem is very intermittent anyway)

I appreciate your thoughts and suggestions on this issue.

Thanks,

HNT

 

[alldodge]

Your having the issue mostly caused by the electric fuel pump. The mechanical pump actually makes some suction when it runs, and the electric has less suction. Things to look at would be the location of the fuel filter. The filter should be level with the top of the fuel tank or a bit below it. Same goes for the fuel pump.

While Carter is an OK pump its not the best at this type of work.

 

[hnt]

No Title

Thanks All Dodge,

Here is a pic of my set up. Fuel pump is low. I will check the height of the water seperator filter but I think it might be a bit higher than the top of the tank. Path for fuel goes up pick up tube,through anti-syphon, through Racor 110 filter, through floscan sensor, through water seperator to electric fuel pump and up to VST.
Maybe this is too much suction for this electric pump. Can you recommend another electric pump that might have better suction?

H

 

[alldodge]

Racor 110 filter, through floscan sensor, through water seperator to electric fuel pump

You have two filters and a flow scan, your making it kind of hard for that little pump. That is a lot of area to keep full. I would think about removing one of those filters and my first choice would be the racor

 

[hnt]

Yeah I thought about removing the Racor. Floscan recommends a filter before the sensor but I have never had any contamination in it. I think I might remove it and see if the problem goes away.

I guess i am still not understanding how the electric pump can provide enough suction and flow at higher power settings but struggles sometimes aftter a long idle. As I understand it the electric pumps runs constantly but when the VST is full the needle valve in the VST shuts off the fuel coming in. I guess while there is no flow the electric pump must cavitate or develop a small vapor lock condition. When the fuel level drops in the VST after idling for a while and the needle valve opens to let fuel flow into the VST the electric fuel pump loses its suction and cannot draw fuel out of the filter. I guess by removing the Racor filter it would make it that much easier for the electric pump to draw fuel.

Do I have this right?

 

[alldodge]

The VST pump should be running so long as the motor is running. Any pressure on the fuel line to the injectors which exceeds the regulator setting is released back to the tank. The tank is just like a carb bowl, and the low pressure pump keeps the pressure there until the float drops and allows fuel to fill.

I think your new pump just cannot supply enough fuel, may need to think about another model, (regular) Merc, Holley Red, (high end) Weldon, Aeromotive

 

[hnt]

Thanks for explanation and your help!!!

So I did a bunch of reading last night....

I found that Floscan recommends only a course filter before the Floscan sender of 30 micron or larger and the unit they acutually recommend for my sensor is 238 micron. The Racor 110 that I have installed before my sensor is a 10 micron filter with a max flow of only 15 gph! (My WOT fuel burn is 24gph.). This unit should have never been installled for this application.

I found the following in the troubleshooting section of the Flocscan manual which I think explains how I can be getting bubbles in my system too.

"VIII. FUEL FILTERS A dirty fuel filter, or one that is too fine (1-25 micron) will draw vapor bubbles out of the fuel, causing fluctuations and high readings. Replace it with a new 30-micron or coarser filter. "

I have owned the boat for six years and it performed well with the mechanical fuel pump even with the too restrictive Racor 110 installed. I never checked to see if the fuel filter was up to spec becasue I never had any issues. But I guess the switch to the electric pump with less suction power has exposed the problem. That poor little pump has been trying hard and doing pretty well given that the boat runs so well. I will remove the Racor this weekend and try it. I will hold off on declaring the problem solved until confirmed but it seems we are on the right track.

Thanks again for your help, I will report back the results.

H

 

[hnt]

I double checked and found that the Racor 110 is rated for 35gph for gas and 15 gph for diesel so the total flow was not a problem. It is 10 micron and the Floscan recommends 30 micron or larger so today I removed the Racor filter and connected the fuel line directly to the Floscan sensor. Fuel now goes up the pickup tube, through the anti-syphon valve to the Floscan sensor.then to the Mercruiser water seperator filter, then to the electric fuel pump and then to the VST.

The boat performed flawlessly at all power settings and I let it idle for over 1 hour with NO STALLING!!!!. It seems that this has solved my problem.

Thank you for your help.

Cheers,

HNT

 

[alldodge]

Cool, great to hear. Hope you have an event free season

 

[hnt]

Well I guess I was a bit premature. After running the boat another 10 hours without incident tonight it stalled again at idle and would not restart. I had to remove the fuel line going from the electric fuel pump to the VST and run the pump for a few seconds before fuel came out. The flow was eratic at first but eventually came out at a good flow. I hooked the line back up and it started and ran fine again. I am clearly getting cavitation, loss of prime during extended idle sometimes causing a loss of fuel going to the VST and fuel starvation. It only happens during extened idle. I just ordered a Holley Marine Red fuel pump from Summit Racing. Thanks for the recommendation All Dodge! I hope to have it by the weekend. Hopefully a more robust electric fuel pump will solve the problem.

Cheers

 

[hnt]

Okay installed the new Holley Maring Red pump. It runs fine, just like before. I guesss i will need to do some extended idling to see if that solved the problem.

 

[alldodge]

Hope you have better luck, and the Holley Red should have no issue with keeping up with supplying fuel

 

[hnt]

Thanks, I will put some hours on it and will report back.

 

[hnt]

No Title

I had a chance to run the boat this weekend with the new fuel pump. As you can tell from the picture, I decided to mount the fuel pump on the bulkhead instead of mounting it on the engine. I wanted to get it a bit farther from the heat of the engine just in case that was a factor. The engine ran great and no stalling with extendeded idle. I did notice a more stable fuel flow on my floscan at idle with the new pump so I hope this solved it.


Thanks

 

[alldodge]

Sounds good, now more fun boating times