Foretravel Owners' Forum For owners and Fans of Foretravel Motorcoaches September 13, 2025 ...

Question about Cummins Fuel System

June 09, 2014, 09:22:40 pm

My sketchy understanding of my '93 C8.3 mechanical fuel system: the transfer pump sucks fuel (under vacuum) from the tank through the first filter.  After the transfer pump fuel goes to the 2nd filter under (low) pressure, then on to the injection pump.  From the injection pump it goes (under high pressure) to the injectors.  Excess fuel not injected into the engine returns to the tank.  That's the extent of my knowledge.

Question: Does my engine have a "overflow valve", or some other type of device that restricts/controls the flow of excess high pressure fuel back to the tank?  If so, where is it physically located?  Is it something that needs to be replaced periodically?  Does it "wear out" or "get weak" leading to reduced engine performance?  The reason I ask is I came across this pitch for a "adjustable overflow valve".  (See below).  I'm just wondering if this is something that would be applicable to my engine.  Any thoughts, pro or con?

TORK TEK CUMMINS ADJUSTABLE OVERFLOW VALVE OFV010 - Tork Teknology

Quote Selected

Re: Question about Cummins Fuel System

Reply #1 – June 09, 2014, 10:07:48 pm

Excellent question. You have the basics down.

On Bosch inline injection pumps, there is a fitting that screws into the side of the injection pump.
It has a spring loaded ball that keeps a certain amount of fuel pressure in the pump for delivery to the high pressure part of the pump. Over a period of time, the steel ball may get wear channels or deposits on it and may result in lower pressures than specs call for and result in less power, uneven running and harder starting.  It is a cheap and easy to replace part if the engine installation allows access. May take a microscope to see the damage.

With Bosch inline injection pumps, the lift pump is frequently mounted on the side of the injection pump and is cam actuated.

Pierce

Quote Selected

Re: Question about Cummins Fuel System

Reply #2 – June 09, 2014, 10:45:49 pm

Should have added that the primary filter (the first filter coming from the fuel tank) can be a small, clear, see through filter as found on generators and some cars to a large, approx. 30 micro filter like many of our diesels have. The lift pump is usually between the primary filter and the secondary filter. The secondary filter may be a 10 micron filter on the mechanical engines to a 2 micron filter on the electronic engines, especially with common rail injection. The engine operator's manual will have the micron specs for the particular engine.

Excess fuel returns to the engine not only from the relief side of the injection pump mounted valve but also from the high pressure fuel injectors. The engine only uses a small portion of the fuel supplied and the returning fuel carries off heat from the injectors. Thats why the fuel tank is quite hot to the touch after driving. Normally, big rigs have the fuel tanks out in the open so a lot of the heat is taken away by the passing air. Our Foretravels trap a lot of the heat in the compartment. Another reason for keeping the tank fairly full on a long trip as there is more fuel area to dissipate the heat. In our Detroit 6V-92TA, the fuel goes through the DDEC II engine computer to cool it. Our engines have a sensor to transmit the fuel temperature to a tool like the Pro-Link 9000.

Pierce

Quote Selected

Re: Question about Cummins Fuel System

Reply #3 – June 10, 2014, 10:16:32 am

Pierce,

Thanks for the good info on the fuel system.  As a old shade-tree hot rodder used to dealing with Holley carbs, I find the diesel engine fuel system very new and interesting.  I am trying to gradually educate myself about how it works, and how to maintain it.  While researching the "overflow valve" subject, I found a interesting document (link below) that discusses "Low Pressure Fuel System Diagnostic Procedures".  Although issued by Dodge, and directed to truck owners with the 6.9L Cummins engine, it does apply to the Bosch P7100 injection pump which is also used on the mechanical C-8.3L.  I would think this test procedure would be helpful to anyone (with this engine) who suspects a possible problem with the transfer pump (lift pump) or the overflow valve.

TSB 14-07-96 Rev. A

Quote Selected

Re: Question about Cummins Fuel System

Reply #4 – June 10, 2014, 11:42:11 am

Chuck,

Good link to the illustrations. The fuel return relief valve is illustrated well in figure 4 (called the overflow valve). It's on the back side of the pump (between the pump and the engine block) and you usually need small hands to R&R it without getting a few scratches and a lot of bad words.

Bosch inline pumps are the best and found on most high quality engines from John-Deere tractors to our Foretravels. They are not much different from 100 years ago. I remember the older pumps had an oil reservoir that needed checking ever few thousand. Then they connected a small oil line from the engine to the pump and finally in the '70s it was an automatic oil connection when the pump was installed. They typically have a timer with splines that runs off the timing chain/gear that the pump inserts into. It advances the injection timing as the RPM goes up (like centrifugal advance on a distributor).

The other pumps found on diesels are the rotary type. Some Cummins, Rabbit diesels, Case tractors, Perkins, Ford/Navistar, etc. have these. They are much cheaper to make but frequently fail before TBO on the engine. They are lubricated with diesel instead of engine oil so seals and everything else have a shorter lifespan. The inline Bosch pumps frequently outlast the engine TBO several times over.

Another interesting tidbit in later diesels is the role the injection plays in starting the engine. When you see a puff of black smoke on startup, it may be because during cranking, extra fuel is delivered to the injectors plus the injection timing is retarded so the piston is further up toward top dead center where the air temperature is higher.  As soon as the engine starts, the injection timing is advanced to it's normal setting.

The latest engines are all electronic with common rail injection pressures up from 3000 psi on the mechanical engines to at least 25,000 and soon to be 45,000 psi at the injector. The computer/Bosch Piezoelectric injectors are capable of precise metering of injection up to FIVE different times in each firing stroke of the engine. This is why 5.9 Cummins engines have evolved from the reason fast food drive-thru lines have "turn off diesel engines" signs to almost as quiet as gasoline engines. Check your UPS truck the next time. It has a 4 cylinder version of the Cummins "B" engine and is very ear friendly now. My only neighbor is 500 feet away but his old 5.9 at startup made our Detroit 6V-92TA seem very quiet by comparison.

Pierce

Quote Selected