C9, Poor Fuel Economy and Power


 Ask Dave / Posted 3 years ago / 1351 views

Question: – How do you feel about the fuel system modules on the market that claim more power & fuel mileage ? I have a 2005 C9 in a 40′ Beaver Monterey with 23000 miles on it, we get an average of 6 mpg with a tow car on a trailer.

Submitted by Rodney Luker – Hemet, CA

Dave’s Answer: Looks like you do have a problem and it is fuel related, but not directly related the fuel you buy daily. The CAT C-9 should get 7.5 to 8 mpg. The Cat C-9 (400 hp) needs a constant flow of diesel fuel at 58 psi low idle to 81 psi at high idle to run properly.  Many Cat C-9 engines cannot maintain constant fuel flow with the fuel tank located 38 feet away from the engine. According to CAT, the tank should be no more the 4 to 6 feet from the engine. Air entering the fuel system is a common problem on coaches built after 2005, some manufacturers used plastic fittings in the lines and they do not last, (a problem that, over time, needs attention.). Air can enter the fuel line at the connections going in and out of the primary fuel filter. The water filter in most cases is aftermarket and needs to be maintained on a regular basis. Parts can dry out and allow air leaks. The fuel transfer pump cannot overcome the air in the lines and cannot maintain constant fuel flow. Remember low fuel pressure results in low power, and air in fuel system can also cause injector failure. Caterpillar has recommended adding an electric fuel pump into the fuel system before the engine (after the filters) that will provide and maintain constant fuel flow and fuel pressure. I would recommend replacing the primary water/fuel filter with a CAT filter at the same time. This system has been installed on several BAC member coaches with great results. You can expect more power and mileage back up to 8 mpg. BAC members are reporting a 1 mpg or more change after this fix.

I can provide a parts list for the second fuel pump and filter replacement upon request.

View the parts list.

We had the following followup questions on this subject.

1.      It seems to me that the kit includes a toggle switch so as to use the pump only intermittently.  I was under the impression that this pump would run continuously to supply better fuel flow. Am I not understanding this correctly?


2.      I originally thought that a separate Electric Fuel Pump was required in addition to the Priming Pump Kit with the Fuel/Separator 206-5756. What I see now is that ONLY the 206-5756 is required, is this correct? Any long term info on the longevity of the Priming Pump? I would assume that it was not originally designed as a 100% duty cycle pump.

Dave’s Answers

The Caterpillar Electric Fuel Pump needs to be wired into an on/off key circuit so every time engine starts and runs the electric pump runs, it runs all the time engine runs. Note: this Caterpillar electric fuel pump was made for continues running on off road equipment, and on road trucks. The constant fuel flow and pressure it provides will meet and exceed requirements for Caterpillar C-9 engine and down to Caterpillar 3126. It will  run 100 % of the time with little or no problems. Note many questions I received from  members indicated that they think the Electric Fuel Pump is a priming fuel pump with an electric motor, the same as the Racor water/fuel filter, it is not.  The Cat Electric Fuel Pump runs constantly to provide constant fuel flow and pressure for heui injection systems.



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