Cat fuel transfer pump: what it does, how it fails, and where it sits on the 3306, 3406 and C7

On a Caterpillar engine the fuel transfer pump is the low-pressure pump that draws fuel from the tank, moves it through the filters and delivers it to the injection pump or the unit injectors. Its place depends on the engine: bolted to the injection pump housing on the 3306 and 3406, and to the high-pressure oil pump on the C7. When fuel pressure reads low, the manuals we read change the filter first and the pump second.

What does a fuel transfer pump do?

The transfer pump runs the low-pressure side of the fuel system. In the US Army’s maintenance manual for the Caterpillar D7G tractor (engine: 3306), it pulls fuel from the tank through the primary filter and the priming pump check valves, then pushes it through the secondary filter to the fuel manifold in the injection pump housing. The pumping spring inside the pump keeps system pressure at 25–42 psi (172–290 kPa).

The pump delivers more than the engine burns. On the 3306 a constant bleed orifice sends the surplus down the return line to the tank, and the manual says why: “This helps keep the fuel cool and free of air.” Unit injector engines work the same way. The Army’s operator’s manual for the C15-powered M977A4 truck says the excess flow cools the injectors, and adds: “The excess fuel flow also purges the air from the fuel system.” The 3208 and 3304 are covered in the article on their fuel lines.

Where does the pump sit, and what drives it?

On the 3306, and on the 3406 with a mechanical injection pump, the transfer pump sits on the injection pump housing. The D7G manual on the 3306:

“The fuel transfer pump is a piston pump that is moved by a cam (eccentric) on the fuel injection pump camshaft. The transfer pump is located on the bottom side of the fuel injection pump housing.”

US Army, TM 5-2410-237-23, D7G tractor maintenance manual (engine: Caterpillar 3306)

All rows come from US Army manuals and reports. The 3406 is the mechanical injection pump version.

Which manual describes your engine is decided by its serial number and arrangement number.

What are the symptoms of a failing transfer pump?

A 2012 US Army Research Laboratory report summarises a failure mode analysis prepared for the Caterpillar C7. Its table names a faulty fuel transfer pump among the potential causes of four complaints: the vehicle cannot reach its speed limit, the engine cranks but does not start, it misfires or runs rough, and it shuts down intermittently or cuts out. The pump is never alone in a row: each list also holds the fuel filter, the fuel lines or the fuel/water separator.

A fifth row is different: fuel in the lubricating oil. The first cause listed there is “Fuel transfer pump seal faulty”, ahead of a faulty unit injector, excessive idling, low compression and the cylinder head.

The 3306 and 3406 manuals put the fuel pressure check under two of those complaints: misfire and lack of power.

Where is fuel pressure measured?

On the D7G the gauge sits on the secondary fuel filter base, and the transfer pump should supply enough pressure for it to read in the green zone. If not, the gauge is replaced and the pressure checked: 25 psi (172 kPa) minimum at full load and 30 psi (207 kPa) minimum at high idle. In its lack-of-power procedure the same manual takes the reading at the “fuel injector housing inlet”, the inlet of the injection pump housing, and wants at least 15 psi (105 kPa).

On the 3406 in the Army’s 621B scraper the reading is taken at the same point and must be at least 30 psi (207 kPa). For the C7 and C15 the Army documents we read give no figure. That comes from the engine’s service manual; forum numbers are left out.

Is it the pump, the filter or a line?

Both manuals work in the same order: below the limit, the primary fuel filter is changed first. Then the pump, in the 3406 manual’s words: “If fuel pressure is STILL below 30 psi (207 kPa), replace fuel transfer pump”. On the D7G a gauge in the red also means new fuel filters.

Before the pressure reading, both manuals send the mechanic to the lines: check them for obstruction, and inspect the connections for leaks and damage. A leaking connection is tightened; cracked or split tubing is replaced.

The pump pulls fuel from the tank through the primary filter, so that stretch is under suction, and a loose joint there usually lets air in instead of fuel out. The manuals do not say this; it follows from the flow they describe. The C7 report lists air in the fuel system, low fuel pressure and damaged or leaking fuel hoses as separate causes of stalling at low rpm. Caterpillar’s own sentence on suction leaks, and ways of finding bubbles on the supply and return sides, are in the 3406E and C15 article.

What sets the pressure differs too. On the 3306 it is the pump’s own spring. On the C15 the pump carries “a relief valve in order to protect the system from extreme pressure”, and a regulating valve in the fuel filter base holds the back pressure that fills the injectors. The C7 report counts a faulty fuel pressure regulating valve as a cause of its own. Checking that valve before the pump when pressure stays low on a new filter is our reading of the layout, not a sentence from the manuals.

In what order should a mechanic check?

1. Identify the engine and its manual. Pump location and pressure limit change with it.
2. Check the fuel and the filters. Water or dirt in a tank sample: drain the tank, change the filters. Gauge in the red: change the filters.
3. Walk the lines. Obstruction, leaks, cracked tubing, especially the connections ahead of the pump.
4. Read the pressure where the manual says. On the 3306 and 3406, the inlet of the injection pump housing.
5. Go to the pump last. New filter, sound lines, pressure still low. On a C7 with fuel in the oil, check the pump seal.
6. Prime the system and look for leaks. Both manuals ask for this once the pump is fitted.