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Open Roads Forum  >  Class A Motorhomes  >  General Topics

 > Holiday Rambler in tank fuel pump quit. Cant find relay?

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dougrainer

Carrolton, Texas

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Posted: 01/12/21 03:01am Link  |  Quote  |  Print  |  Notify Moderator

Broncorambler wrote:

The 300 cummins starts and idles fine. Goes into high idle fine and even revs if not under load. Once in gear with brake set or driving it will stumble, rebound and then stumble again in a cycling way like running out of fuel.

My diesel mechanics scan tool shows no fuel pressure from any aux fuel pump, but it does show vac. pressure from hi pressure pump drawing fuel to the engine. Dont know for certain there is an aux pump, but assuming there is because tank is over 20 feet from the engine and seems should be one. Manual makes no mention of one tho?
On top of tank there are number of wires going thru a portal into the tank and seems like more than needed for sending unit.
Two good diesel mechanics believe Cumminns ISB300 engines have an aux pump, but could they be wrong?
Manual does mention that clogged tank vent can give issues and guess id better check it out, but didnt ghink could be culprit.

Hoping might be some good techs on the Monaco group provuded me, but busy next couple days. Thanks much for replies and advice.


On the Manual I sent you the link to. Go to the Calibrating part of the Fuel tank sending unit. You will see WHY there are more wires than you think is necessary. I was waiting for others to reply. I NEVER saw any In tank fuel pump on a Roadmaster Chassis. The fuel pick up and return is just 4 hoses. The 3rd/4th hose is for the optional Aqua Hot and Genset feeds. The good news is, since the Fuel pump system is part of Cummins, you should be able to get it fixed by a Cummins Service Center. Doug

oldave

Tx

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Posted: 01/12/21 09:06am Link  |  Quote  |  Print  |  Notify Moderator

Doug is right, those wires are for the electronic sender.

Have you changed the fuel filters? There should be 2.
Most problems like yours are filters. Diesel fuel can develop/grow algae,
scum or whatever in the tank and can quickly clog filters.
If you have that you will probably see it inside the primary filter, on top.
It is crazy but the fuel is sucked all the way from the tank, there are after
market pumps that can be used.

Broncorambler

Bishop, CA

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Posted: 01/12/21 02:07pm Link  |  Quote  |  Print  |  Notify Moderator

Wow, thanks guys. Already did both filters so next is to check tank vent for clogging. Amazing that vacuum pulls fuel all that way!
High pressure pump likely ok or it wouldnt idle and run?
Only stumbles under load. Thanks

oldave

Tx

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Posted: 01/12/21 02:46pm Link  |  Quote  |  Print  |  Notify Moderator

Have you noticed the tank sucking air when you remove the cap after driving?
If it is the vent you could probably loosen the fill cap and solve the problem
Don't drive like that if the tank is full as it may slosh fuel out.

Broncorambler

Bishop, CA

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Posted: 01/12/21 03:48pm Link  |  Quote  |  Print  |  Notify Moderator

Thanks can certainly test it with cap loose and if runs fine, its the vent.
Simple, thanks.

filadelfia

Apple Valley Ca.

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Posted: 01/12/21 05:18pm Link  |  Quote  |  Print  |  Notify Moderator

Sounds like a bad lift pump. As Doug said any Cummins shop can replace not too expensive.
Don 2005 HR Neptune
2006 Jeep Liberty toad

Broncorambler

Bishop, CA

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Posted: 01/12/21 09:44pm Link  |  Quote  |  Print  |  Notify Moderator

Great to hear wont be dropping tank. Haha. Would it run at all if bad lift pump? Hoping clogged vent tube. Wish me luck tomorrow.

Ivylog

Blairsville, GA and WPB, FL.

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Posted: 01/13/21 09:46am Link  |  Quote  |  Print  |  Notify Moderator

It’s NOT a bad lift pump which is only used for starting the engine... learned that the expensive $$ way. I had the same problem and by putting a short length of clear plastic tubing ($5 in hose, pipe nipple, & 2 clamps) after changing the lift pump, I could see small bubbles in the fuel with lift pump running and the engine running. A big change in temperature made a fitting (which was tight) at the tank start leaking. Removing the hose and putting back on (not as tight) solved the air getting into the fuel which gave me full power again.

Spend $5 putting a clear hose at a fuel filter to check for air in the fuel system.


This post is my opinion (free advice). It is not intended to influence anyone's judgment nor do I advocate anyone do what I propose.
08 HR Navigator 45'
Sold 04 Dynasty after 14 great years.


Broncorambler

Bishop, CA

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Posted: 01/13/21 11:46am Link  |  Quote  |  Print  |  Notify Moderator

Thanks Ivylog. Can you better explain short hose to check for air? Is leaking hose coupling on top of tank or at filters? Sounds like simple fix.
Love to talk by phone if could get number privately?

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