Mechanical Fuel Pump issue

[Bhart]

My wife and I went camping last week and for the first 30 miles of a 120-mile trip the motor home ran well. When attempting to go up a steep hill the motor started chugging like it was running out of gas. Once over the hill it ran great for the next 30 miles of level travel. Once again when confronted with a hill it would start chugging. Replaced 10-micron fuel filter just after the selector valve and replaced the fuel filter at the carb. We made it to the camp site without any issues hills and all. On the way home it would start chugging on the hills and speed would reduce to 35 mph. I replaced the 10-micron filter again and the problem still persisted.
I have a Cater electric fuel pump that I use to prime the engine before starting. The momentary prime switch is a rocker like the battery boost switch. Just for fun on the next hill I depressed the prime button, and the coach ran up the next series of hills without fail.
I stuck a toothpick in the rocker switch to keep power on the electric fuel pump until we got home. It was mentioned in anther post that a mechanical fuel pump either works, or it doesn't. My pump seemed to work on level roads but would fail when under a load. I have ordered parts to install the dual fuel pump system.

 

[RF_Burns]

You may just be experiencing a drawback to our large shallow fuel tanks. When you get down to below 1/2 a tank, the fuel pickup can be left high and dry when going up or down a hill. The issue is more pronounced on fuel injected system because they do not have a reservoir like the carb in the fuel bowl.

I bet if you switched to your front (Aux) tank you will find the gauge reads higher. Best fix is to refuel once you hit the 1/2 full mark.

See this resource with diagrams to better explain the problem.

Resource icon

Great Gas Tank Mystery

Nov 26, 2019

Great information as to why the fuel system does what it does.

• Christo
• Fuel System & EFI

 

[Bhart]

You may just be experiencing a drawback to our large shallow fuel tanks. When you get down to below 1/2 a tank, the fuel pickup can be left high and dry when going up or down a hill. The issue is more pronounced on fuel injected system because they do not have a reservoir like the carb in the fuel bowl.

I bet if you switched to your front (Aux) tank you will find the gauge reads higher. Best fix is to refuel once you hit the 1/2 full mark.

See this resource with diagrams to better explain the problem.

Resource icon

Great Gas Tank Mystery

Nov 26, 2019

Great information as to why the fuel system does what it does.

• Christo
• Fuel System & EFI

Tanks were full.

Resource icon

Great Gas Tank Mystery

Nov 26, 2019

Great information as to why the fuel system does what it does.

• Christo
• Fuel System & EFI

 

[werewilfs]

Tanks were full.

Resource icon

Great Gas Tank Mystery

Nov 26, 2019

Great information as to why the fuel system does what it does.

• Christo
• Fuel System & EFI

It's also possible the electric pump being inline with the manual pump restricts the manual pumps suction a bit. It doesn't take much. I found the same issue after wiring in an electric pump with my manual pump. I needed to do it "quickly" (campground fix) so I just put it after the selector switch as I had a break in the fuel line there where a filter was installed. I'm going to go back now and put the electric pump before the selector so it's only on when I want it to be on.

 

[pvfjr]

Since your tanks were full, it sounds like the fuel demand on hills was exceeding your fuel pump's current flow rate.

• You could be sucking air
  • There could be cracks forming in the rubber lines on top of the tank
  • Sometimes, leaks in the suction line, above the liquid level, can suck air instead of leaking fuel
• You could have restrictions
  • Pinched rubber lines on top of tank due to worn body pads
  • Gummed up fuel pickup sock
  • Other kinked/pinched/clogged lines
• You could have a poorly performing pump
  • Check valves in mechanical pumps can indeed get gummed up and leaky, reducing max flow rate and pressure
  • Mechanical wear can cause a pump to receive inadequate actuation (very uncommon, but it happens)
  • Check oil for gas content, just to be safe and ensure gas hasn't been pumping into the crankcase due to a leaky diaphragm

Remember that things that reduce flow are additive. So swapping a slightly clogged filter can seem to help a tiny bit, but not quite enough. An electric pump could make it seem like "it's not the pickup sock", but you have two pumps working together--that certainly helps overcome restrictions.

 

[Bhart]

Since your tanks were full, it sounds like the fuel demand on hills was exceeding your fuel pump's current flow rate.

• You could be sucking air
  • There could be cracks forming in the rubber lines on top of the tank
  • Sometimes, leaks in the suction line, above the liquid level, can suck air instead of leaking fuel
• You could have restrictions
  • Pinched rubber lines on top of tank due to worn body pads
  • Gummed up fuel pickup sock
  • Other kinked/pinched/clogged lines
• You could have a poorly performing pump
  • Check valves in mechanical pumps can indeed get gummed up and leaky, reducing max flow rate and pressure
  • Mechanical wear can cause a pump to receive inadequate actuation (very uncommon, but it happens)
  • Check oil for gas content, just to be safe and ensure gas hasn't been pumping into the crankcase due to a leaky diaphragm

Remember that things that reduce flow are additive. So swapping a slightly clogged filter can seem to help a tiny bit, but not quite enough. An electric pump could make it seem like "it's not the pickup sock", but you have two pumps working together--that certainly helps overcome restrictions.

Paul, I have replaced the rubber lines on top of the tanks with metal lines. From past experience of socks being clogged no fuel would flow out of disconnected fuel hose, mine was free flowing. No gas in oil.
Same setup a year ago with no issues in 400+ miles of trips. I just believe that the mechanical pump is faulty

 

[Carl Stouffer]

Paul, I have replaced the rubber lines on top of the tanks with metal lines. From past experience of socks being clogged no fuel would flow out of disconnected fuel hose, mine was free flowing. No gas in oil.
Same setup a year ago with no issues in 400+ miles of trips. I just believe that the mechanical pump is faulty

I've been fighting this sort of "vapor lock" symptoms ever since I got my coach. I have finally (a few years ago) solved 98% of the problem after doing nearly EVERYTHING that can be done as a preventative measure. I still occasionally have that 2% rear it's ugly head once in a while. I think it is sometimes attributable to bad gas. Sometimes there is no rhyme or reason for it, other than where I got gas the last time.

I will hope for the best in about a month as we are going from Tucson, through Phoenix, where the temps are likely to be over 110 degrees, up to the Mogollon Rim in AZ. Where I live in Tucson is at about 2600', Phoenix is at about 1300', Aspen Campground at Woods Canyon Lake, where we are heading, is at about 8000'. This will be our third trip up there in July, and I'm hoping for the best. Past trips have gone fairly smoothly, so I'm expecting this one to go that way as well.

The addition of a Carter 4070 electric fuel pump, and moving my fuel lines to the outside of the frame, have made the biggest difference. Having that pump, back by the tanks, PUSHING the fuel up front works way better than the mechanical pump PULLING fuel all the way up front. I don't this is the fault of your mechanical pump, although it is not that hard to replace, so you might as well go for it.

 

[mech0001]

Classic symptoms of a clogged fuel filter.
Check that first.