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3.0L Mechanical fuel pump fails when hot

T

tgmmartin

New member
I have recently been having some fuel pump problems causing the motor to cut out when warm.

Background and symptoms:
-I have a pressure gauge between the mechanical fuel pump and carb. It reads 0psi when the boat dies.. So obviously fuel is the cause of the issue.
-The fuel pump is new with less than 15 hours on it and has been giving these issues since new.
-The motor starts and runs fine when cool. But seems to have problems when warm, and more so when it is left to heat soak. Whenever the boat cuts out it will restart after a few minutes rest. The fuel pump is hot to the touch, but I can leave my hand on comfortably enough for 10+ seconds.
-I have played around with hoses, filters, electric pumps, and primer bulbs between the fuel tank and the pump (problems occur in all configurations). But I I am very certain that any of inlet blockages, siphon, venting are not the cause.
-Currently the setup is: fuel tank, hose, primer bulb, hose, mechanical fuel pump, hard line, fuel pressure gauge, hard line, carb.
-Whenever the boat cuts out (and the fuel pressure is 0psi), the primer bulb is soft. So clearly the pump is failing to draw fuel from the tank. But I can immediately and easily reprime the pump (and even get pressure on at the gauge and fuel up to the carb) using just the priming bulb. So there cannot be a venting, siphon, pickup or blockage.

It seems very likely it is a vapor lock in the pump. Let me know if you think it is something else.

Questions:
Will replacing the fuel pump (again) be likely to fix this problem?
Has anyone done a clean, safe, and reliable replacement of the pump with an electrical setup? Is so, what did they use?

For what its worth, the new fuel pump is a Carter M60337 which is as far as I am aware the same as the Mercruiser item. I chose the Carter as it was significantly cheaper than the Merc

Thanks!!
 
racerone said:
Test run with a portable tank and hose.------Check thermostat and cooling system.
Click to expand...
Ayuh,.... Agreed,.....

To run an electric fuel pump, you need to run it's power through an oil pressure switch to kill the power when the motor stops,....
 
I’m wondering the OP said this is a mechanical pump and why was there a primer bulb on an inboard engine?
Was the fuel pick up tube removed & inspected for blockages and the anti-siphon valve checked? Fuel vent in the tank open?
 
I rigged this up to test the fuel pick up tube and anti siphon valve for air leaks also found that a good anti siphon valve opens between 1-->2" of mercury vacuum. Some pick up tubes have a screen on them that can clog, this one didn't.
You might have to pull the sending unit for the gas gauge on your tank and look inside to see if there's crud floating around in the bottom of the tank.
This is very odd because mechanical pumps either work or they don't usually. Electric pumps can have issues due to bad connections, failed oil pressure switches etc but mechanical pumps are usually 100% reliable.
Fuel pick up tube and antisiphon valve test for vacuum leaks.jpg
 
I'd bet on crud in the tank accumulating on the screen and restricting the flow....engine cuts off, vacuum in the suction line drops and the crud falls off the screen...you can plumb a vacuum gauge into the suction line, at the inlet to the pump to test for this...

FWIW, there should NOT be a primer bulb in the fuel line on an I/O's engine...temporary use for troubleshooting is not the same thing as having it installed all the time.
 
Thanks all for you replies and suggestions.

I have tried the following setups:
-pickup, inline fuel filter, mech pump
-pickup, inline fuel filter, electric pump, mech pump
-pickup, primer bulb, mech pump

I removed and cleaned the tank last year. But there was a little bit of dirt in the inline fuel filter, so I suppose there is a small chance that the pickup filter is clogging. But I am reluctant to assume it is this as the primer bulb happily pulls fuel through when the mechanical fuel can't.

The primer bulb is temporary only. I put it on yesterday just to help troubleshoot.
There is no anti-siphon.
The engine temperatures are good.

Sounds like the brains are saying not to rush into thinking the mechanical fuel pump is the problem.

I will follow racerone's suggesting and run it off a clean portable tank. This should allow me to conclude whether it is fuel pickup or mechanical pump.

Thanks.
 
The engine temperatures are good.

Whats Good? The actual temp would be good to report!

Just you saying A, B, C is good does not say much. Actual numbers/details are better
Also why did you replace the original mech fuel pump to begin with? Was it the same issue and trouble shooting the original gave you the same result you have now?

There is no anti-siphon.??? Why is there no anti-syphon valve? Did you remove it? Was it already missing? If already missing and replaced with a barbed fitting before you acquired this boat and or issue then that would be a sign of a fuel issue already. There is NO reason to remove permanently.

As suggested, When intermittent fuel pressure loss occurs, Many times it is caused by the pick up tube getting clogged in the fuel tank typically by a floating filter screen that fell off the end of the pick up tube. The use of a primer bulb from an outboard that works when used may only be that way as due to filter in tank moved away from pick up tube (as an example)

Ditto on RacerOne suggestion. A known good stand alone fuel source with line directly to mech pump may help in troubleshooting!
 
kghost said:
The engine temperatures are good.

Whats Good? The actual temp would be good to report!

Just you saying A, B, C is good does not say much. Actual numbers/details are better
Also why did you replace the original mech fuel pump to begin with? Was it the same issue and trouble shooting the original gave you the same result you have now?

There is no anti-siphon.??? Why is there no anti-syphon valve? Did you remove it? Was it already missing? If already missing and replaced with a barbed fitting before you acquired this boat and or issue then that would be a sign of a fuel issue already. There is NO reason to remove permanently.

As suggested, When intermittent fuel pressure loss occurs, Many times it is caused by the pick up tube getting clogged in the fuel tank typically by a floating filter screen that fell off the end of the pick up tube. The use of a primer bulb from an outboard that works when used may only be that way as due to filter in tank moved away from pick up tube (as an example)

Ditto on RacerOne suggestion. A known good stand alone fuel source with line directly to mech pump may help in troubleshooting!
Click to expand...
Temperature happily sits below 175F. Open circuit cooling with a 140F thermostat.

Good question on the symptoms of the old pump. I was having the fuel starvation issues and there was fuel making its way into the oil (oil was thinning and smelt like fuel).

I had thought that replacing the fuel pump, cleaning the tank, new pickup, new hoses, new filters would solve my issue.

The fuel starvation issues do seem more prominent in rougher conditions with more bouncing off waves. So I am leaning back towards a (still) dirty tank and pickup issues. Testing on a tote will be telling.

The only thing this doesn't mentally resolve for me is the idea that it only happens when the engine is warm / has been sitting hot. I suppose this could just be correlated to usage stirring up the tank plus some degree (excuse the pun) of heat meaning vapor locking is more likely.

When I bought the boat there was no anti-siphon. I will happily re-fit one when these fueling issues are resolved.
 
tgmmartin said:
Temperature happily sits below 175F. Open circuit cooling with a 140F thermostat.

Good question on the symptoms of the old pump. I was having the fuel starvation issues and there was fuel making its way into the oil (oil was thinning and smelt like fuel).

I had thought that replacing the fuel pump, cleaning the tank, new pickup, new hoses, new filters would solve my issue.

The fuel starvation issues do seem more prominent in rougher conditions with more bouncing off waves. So I am leaning back towards a (still) dirty tank and pickup issues. Testing on a tote will be telling.

The only thing this doesn't mentally resolve for me is the idea that it only happens when the engine is warm / has been sitting hot. I suppose this could just be correlated to usage stirring up the tank plus some degree (excuse the pun) of heat meaning vapor locking is more likely.

When I bought the boat there was no anti-siphon. I will happily re-fit one when these fueling issues are resolved.
Click to expand...
Fuel in oil...............Typically that is due to blow by (Bad rings/ cylinder bore issues ) or carb flooding.

I am guessing you felt the fuel pump was leaking into the pocket it fit into? Or the diaphragm leak hose going from Fuel pump to carb was full of fuel?

"""The fuel starvation issues do seem more prominent in rougher conditions with more bouncing off waves. So I am leaning back towards a (still) dirty tank and pickup issues. Testing on a tote will be telling."""

this is good info, Loss of pump pressure with a "thought to be intact system with new pump)" may suggest the pick up tube may be cracked.

If you have pressure some times but after time or in rough water, what could cause a loss of pressure?
1. Vacuum, Is vent line free of debris and have you confirmed there is no blockage?
2. Floating but submerged pick up tube filter.
3. Loss of fuel line integrity (Air leak) within tank...........

Vapor lock in my opinion on a 3.0 liter should not be an issue.

Also note: When running a 140* thermostat engine temp on gauge, not under load or high rpm, should be ~150-160*F NO HIGHER.
If you are seeing 175* F you have a water flow issue.
If on a trailer and using a garden hose you definitely have a water flow issue caused by a restriction, as the garden hose is pressurized and will overcome a bad outdrive impeller short comings to some degree.
Lots of things going on so when I say, INFORMATION its best if you tell all.....................WHY? because we will all get tired of tid bits being dropped here and there causing extensive posts to draw the info needed to give you accurate advice,

We are on post 15 now and still new info is being presented and yet we dont have any basic info on the,
1. Year of boat
2. Original engine or replaced? If so what year is engine
3. Engine serial number
4. Recent history of issues/repairs to anything so we have some idea if a can of worms or an isolated problem
5. How many hours of use if known?

I think you may get the point.

Good thing its cold as F^CK other wise you might be on your own with the lack of complete info needed from the get go..........many would just not respond due to to many details being revealed to late.

Everyone has problems here. it of the upmost importance to put ALL the info you can in the original post about the boat BEFORE you describe your issue/problem. Many things are NOT generic and based on model year and other factors many here will have experience in the problem reported but without the specifics (3.0 liter with fuel problem) What does that tell us. There have been 3.0 liters used for over 40 years.............Lots of changes in 40 years........
 
tgmmartin said:
Temperature happily sits below 175F. Open circuit cooling with a 140F thermostat.

Good question on the symptoms of the old pump. I was having the fuel starvation issues and there was fuel making its way into the oil (oil was thinning and smelt like fuel).

I had thought that replacing the fuel pump, cleaning the tank, new pickup, new hoses, new filters would solve my issue.

The fuel starvation issues do seem more prominent in rougher conditions with more bouncing off waves. So I am leaning back towards a (still) dirty tank and pickup issues. Testing on a tote will be telling.

The only thing this doesn't mentally resolve for me is the idea that it only happens when the engine is warm / has been sitting hot. I suppose this could just be correlated to usage stirring up the tank plus some degree (excuse the pun) of heat meaning vapor locking is more likely.

When I bought the boat there was no anti-siphon. I will happily re-fit one when these fueling issues are resolved.
Click to expand...
If running a 140 stat should be running more like 150-155. That pro has nothing to do with your issue just keep in mind.

Mechanical fuel pump quality sucked these days. If your into hot rods it is a very common issue these days. Over last 3 years I have put two Carter mechanic, fuel,pumps on my 65 pontiac engines that out out way too much pressure. I have fuel pressure regulator installed , wanted to mention that the cheap liquid filled gauges I’ve used on them change pressure when the engine y gets hot. As temp rises indicated pressure drops.

When it dies if you take off gas cap does it show the tank is under vacuum? Have you used a vacuum gauge on suction side of pump?
 
Just to clear on the temp question. A can't recall exactly how hot it is running, but it definitely isn't past the 175 on the gauge (vertical). I have previously completely checked through the cooling performance and been happy that it doesn't run hot.

I have been reading through:
Mercruiser installation instructions (wrt fuel) and fuel delivery bulletins.
iBoats' https://forums.iboats.com/threads/how-to-testing-your-boats-fuel-system.278642/
PPT's https://www.perfprotech.com/blog/ar...w6VYi8XKOOM5HuWua4lX2JFGJwc3ixwAUdvmoLeIsyR8O

When pulling apart and inspecting the fuel system (again), one thing that jumped out at me was how tight I had installed the hose tail into the fuel pump. I had been concerned about splitting the housing or stripping the threads on the aluminium fuel pump when I tightened the tapered NPT pipe fitting into the parallel (NPSF) fuel pump. I had barely done it up finger tight. So I have reinstalled all fittings following the instructions to hand tight + 1&3/4 to 2&1/4 turns. And used the correct Loctite 567 rather than teflon tape.

If there was a chance of air leaking in, then hopefully this has solved it.

I also have a vacuum gauge ordered which should allow me to diagnose a lack of pump suction vs blockage or venting issue.

I have also upgraded the hoses and fittings from 5/16 to 3/8 following the Mercruiser bulletins and instructions. I don't expect this will help much. but it didn't cost a lot.

I have a tote tank all set up to be substituted when/if the fuel problem appears again.

I also have a fuel safe 3/8 transparent fuel hose that I may temporarily install to be able to see what is occurring.

There definitely isn't air rushing in when I open the fuel cap. But again, I think the vacuum gauge will really help my diagnosis on this.


Thanks all for your input. I think I have a path forward to getting to the bottom the issues and will post back when I have new info or an outcome. It's summer here in New Zealand, albiet a bit windy this weekend to get out.
 
tgmmartin said:
Just to clear on the temp question. A can't recall exactly how hot it is running, but it definitely isn't past the 175 on the gauge (vertical). I have previously completely checked through the cooling performance and been happy that it doesn't run hot.

I have been reading through:
Mercruiser installation instructions (wrt fuel) and fuel delivery bulletins.
iBoats' https://forums.iboats.com/threads/how-to-testing-your-boats-fuel-system.278642/
PPT's https://www.perfprotech.com/blog/ar...w6VYi8XKOOM5HuWua4lX2JFGJwc3ixwAUdvmoLeIsyR8O

When pulling apart and inspecting the fuel system (again), one thing that jumped out at me was how tight I had installed the hose tail into the fuel pump. I had been concerned about splitting the housing or stripping the threads on the aluminium fuel pump when I tightened the tapered NPT pipe fitting into the parallel (NPSF) fuel pump. I had barely done it up finger tight. So I have reinstalled all fittings following the instructions to hand tight + 1&3/4 to 2&1/4 turns. And used the correct Loctite 567 rather than teflon tape.

If there was a chance of air leaking in, then hopefully this has solved it.

I also have a vacuum gauge ordered which should allow me to diagnose a lack of pump suction vs blockage or venting issue.

I have also upgraded the hoses and fittings from 5/16 to 3/8 following the Mercruiser bulletins and instructions. I don't expect this will help much. but it didn't cost a lot.

I have a tote tank all set up to be substituted when/if the fuel problem appears again.

I also have a fuel safe 3/8 transparent fuel hose that I may temporarily install to be able to see what is occurring.

There definitely isn't air rushing in when I open the fuel cap. But again, I think the vacuum gauge will really help my diagnosis on this.


Thanks all for your input. I think I have a path forward to getting to the bottom the issues and will post back when I have new info or an outcome. It's summer here in New Zealand, albiet a bit windy this weekend to get out.
Click to expand...
Any chance you are loosing spark when it dies
 
tgmmartin said:
I have recently been having some fuel pump problems causing the motor to cut out when warm.

Background and symptoms:
-I have a pressure gauge between the mechanical fuel pump and carb. It reads 0psi when the boat dies.. So obviously fuel is the cause of the issue.
-The fuel pump is new with less than 15 hours on it and has been giving these issues since new.
-The motor starts and runs fine when cool. But seems to have problems when warm, and more so when it is left to heat soak. Whenever the boat cuts out it will restart after a few minutes rest. The fuel pump is hot to the touch, but I can leave my hand on comfortably enough for 10+ seconds.
-I have played around with hoses, filters, electric pumps, and primer bulbs between the fuel tank and the pump (problems occur in all configurations). But I I am very certain that any of inlet blockages, siphon, venting are not the cause.
-Currently the setup is: fuel tank, hose, primer bulb, hose, mechanical fuel pump, hard line, fuel pressure gauge, hard line, carb.
-Whenever the boat cuts out (and the fuel pressure is 0psi), the primer bulb is soft. So clearly the pump is failing to draw fuel from the tank. But I can immediately and easily reprime the pump (and even get pressure on at the gauge and fuel up to the carb) using just the priming bulb. So there cannot be a venting, siphon, pickup or blockage.

It seems very likely it is a vapor lock in the pump. Let me know if you think it is something else.

Questions:
Will replacing the fuel pump (again) be likely to fix this problem?
Has anyone done a clean, safe, and reliable replacement of the pump with an electrical setup? Is so, what did they use?

For what its worth, the new fuel pump is a Carter M60337 which is as far as I am aware the same as the Mercruiser item. I chose the Carter as it was significantly cheaper than the Merc

Thanks!!
Click to expand...
I had a similar problem on a diesel engine in a tub grinder. The inlet was picking up debris in the tank or in the suction line and plugging the line. When the engine stops, the suction is relieved and the debris loosen and clear the line. You can test this by running a temporary line into the tank at another location, and feed it through a filter screen to the pump. If the engine keeps running the line is bad, if it quits, cut open the filter screen and see what is inside. You can also run the engine from a temporary 5 gallon bucket and see what happens.. Hope this helps
 
When I was having my fuel system problems this past summer what fooled me was this, I never had a load of bad gas before. When I rebuilt the carb, then went through the ignition system, and still no improvement, I felt it must be related to the boat's fuel system. Hooked up the outboard tank and it ran perfect, so I thought, oh, it must be the pick up tube screen or the anti siphon. Took them apart and guess what, they were fine, the last thing left is bad fuel in the tank, which I will pump out this spring! The pick up tube used originally didn't have a screen to clog!
Lesson learned
never assume anything
when I tested the pick up tube and anti siphon valve with a vacuum tester, (see post #5) they tested perfect. Anti siphon worked the same as a brand new one.
so I get to pump out 30 gallons of bad gas!
 
A belated update.

I have successfully:
-installed new 3/8 fuel line and fittings
-removed the gauze on the fuel tank pickup
-installed a serviceable transparent inline filter (reasoning is to give larger surfer area than the in tank pickup gauze, and to be able able to see debris building up)
-checked the anti-siphon valve (there isn't one installed - I will fit one once I have everything resolved)
-installed a high-quality vacuum gauge at the inlet to the mechanical fuel pump

I had the boat out on the water last week. Boat ran great at high rpm for 20 minutes out to a beach, sat for an hour and a half, then 20 minutes back home. No issues whatsoever.

But.... at the start of the day, the vacuum gauge read ~0inHg at idle, then when I was flushing the boat at home it read 3inHg (mind you this is at idle/low load, and low fuel flow). So I think there is a bit of a restriction forming and I can see the inline fuel filter building up some yellowish varnish.
I did test physically restricting the fuel line by crimping it, and the fuel pump happily pulled 8inHg. So the pump is pulling ok.

Next:
- I will clean the inline fuel filter and see if vacuum reduces to near 0inHg - this would confirm that it is clogging causing the issues.
- I will remove the fuel from the tank and give it another clean with the aim of removing any varnish or debris.

For what it's worth, temp was at 150F (60C) all day long.

I've also bought and carry a tote tank - for when things go wrong and to aid in diagnostics.
 
Last edited:
tgmmartin said:
A belated update.

I have successfully:
-installed new 3/8 fuel line and fittings
-removed the gauze on the fuel tank pickup
-installed a serviceable transparent inline filter (reasoning is to give larger surfer area than the in tank pickup gauze, and to be able able to see debris building up)
-checked the anti-siphon valve (there isn't one installed - I will fit one once I have everything resolved)
-installed a high-quality vacuum gauge at the inlet to the mechanical fuel pump

I had the boat out on the water last week. Boat ran great at high rpm for 20 minutes out to a beach, sat for an hour and a half, then 20 minutes back home. No issues whatsoever.

But.... at the start of the day, the vacuum gauge read ~0inHg at idle, then when I was flushing the boat at home it read 3inHg (mind you this is at idle/low load, and low fuel flow). So I think there is a bit of a restriction forming and I can see the inline fuel filter building up some yellowish varnish.
I did test physically restricting the fuel line by crimping it, and the fuel pump happily pulled 8inHg. So the pump is pulling ok.

Next:
- I will clean the inline fuel filter and see if vacuum reduces to near 0inHg - this would confirm that it is clogging causing the issues.
- I will remove the fuel from the tank and give it another clean with the aim of removing any varnish or debris.

For what it's worth, temp was at 150F (60C) all day long.

I've also bought and carry a tote tank - for when things go wrong and to aid in diagnostics.
Click to expand...
In increased vacuum could be from filter restriction assuming it’s on the suction side, but also you fuel level dropped as you ran for 40 min so the pump also has to lift the fuel more as it gets lower.

If you are getting crud from the tank you’ll need to clean it out
 
Where you are located I don't know the regs but those clear filters aren't Coast Guard approved in the USA for inboard gas engines, and it sounds like you have to remove the fuel gauge plate and look down in the bottom of the tank, and see what it is, that is fouling the fuel system.
I would:
install a proper marine water separating fuel filter is none is currently installed
try running it on the outboard gas tank with fresh fuel (make sure the tank vent is open) check the fuel pump vacuum also if it's good then (should be about 10" Hg vacuum) that suggests that you have crud in your main fuel tank that needs to be cleaned out
If your fuel system had crud in it, that might explain the overheating of the original fuel pump, and zero fuel pump pressure, eh?
Also if you still have it get rid of the primer bulb, not safe in an inboard gas boat, and is not needed if the fuel system is in good shape.
 
One (hopefully) final update.

After the last outing (March 2026), the crud visible in the fuel filter indicated to me that the fuel tank needed a really good clean out. So I removed the tank and had a radiator specialist clean it out. It's now squeaky clean.

I ran the boat both Saturday and Sunday over the weekend - for about an hour each day. Running on plane for 20+ minutes runs.

No issues with the fuel system whatsoever and the inline fuel filter is very clean. The vacuum gauge (on the suction side) topped out at 3inHg. I am ok with this 3inHg reading and suspect it is along the lines of what Dieter has said that lower fuel levels required more suction.

For completeness, and to alleviate everyone's concern about the safety of my fuel system: there is no primer bulb, and the transparent fuel filter has been replaced with a high quality aluminium fuel filter. I have tried to fit a water separating fuel filter, but there isn't space in my engine bay.

Happy days. Thanks everyone for your contributions.
 
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