Fuel Pressure Regulator Basics

Caterpillar C7 fuel pressure diagnosis.

This post concerns how fuel pressure is developed and regulated. I’m writing it after getting a fuel pressure problem in the shop the other day. It was a Caterpillar C7, EPA 04 model. The 3116/3126/C7 Cat engines have the same type HEUI fuel system as the Navistar DT/EGR/Maxxforce engines. In fact, Cat and Navistar developed the system together. The fuel system is (relatively) low pressure, and the injectors are actuated with high pressure engine oil.

The symptoms on this Caterpillar C7 were a long crank to start, under all circumstances.

Hot or cold, initial startup or after road time, it cranked 15 to 20 seconds or even more, before starting. A warm engine should start almost instantly.

Even before scanning it for codes I had a feeling I was dealing with a fuel problem. However, I’ve learned the hard way that it’s best not to make assumptions until I see evidence. There was a code for low fuel pressure and none for injection control pressure, so I was comfortable starting with the fuel system.

Checking fuel pressure on this engine was easy – there is a Compuchek port on the filter base, so an adapter just snaps onto the port. When I cranked the engine the fuel pressure built slowly, taking 15 seconds or so to build to spec, and the engine started. After it was shut off, the pressure dropped to 0 within a few seconds.

  

Fuel pressure is regulated on pump output.

To address a fuel pressure problem, you first need to know how pressure is developed and regulated. These same principles apply to any hydraulic system, whether it’s power steering, auto trans, or engine oil supply. If you don’t understand how it works, you are destined to just replace parts one by one, hoping you replace the right one before the customer’s money runs out.

The first principle is that PUMPS DON’T PUMP PRESSURE! Pressure is developed by a restriction in the pump output. Think of a garden hose. With the end of the hose open and water flowing, there is almost no pressure in the hose. If you put your thumb over the end you can feel the pressure build and the water stream comes out harder. The same principle applies when making fuel pressure. Restrict the flow, and the pressure goes up.

Caterpillar C7 Fuel schematic

One of the most common misconceptions is that fuel can be regulated on the supply side, in other words on the pump input side. If you restrict on the pump suction side such as the fuel pickup it will starve the pump, like ice in the pickup line or a restricted separator. This may also cause low fuel pressure, but the point is that pressure is regulated on the pressure side, and fuel must FLOW THROUGH the regulator to control and limit pressure.

Fuel can be returned back to the tank, or to the suction side.

Depending on the system, fuel after the regulator can be routed back to the tank or to the suction side of the system. This Cat C7 and the late, great Detroit Series 60 route it back to the tank, through the regulator on the back of the head. Older Mercedes 900, Volvo, and some Navistar engines return it back to suction, or pickup side. The advantage to returning fuel to the tank is that any air getting into the system, like during filter changes, is quickly bled off to the tank. Returning to suction means any air needs to go through the injectors, and it sometimes takes lots of time and effort to get an engine running right after a filter change or especially after running out of fuel. Air in the fuel can lower indicated fuel pressure, because the air is being compressed, instead of properly pressurizing the fuel.
 

Modern high-pressure common rail systems differ in that there are actually 2 fuel systems, a low-pressure system and a high-pressure system that supplies the injectors. Here, the two systems need to be diagnosed separately to see which is causing a problem. The low-pressure system is controlled like the traditional unit injector systems like on the C7.

The C7 application had a Racor separator with the same size line in and out, so it was a simple matter to bypass the unit by taking the lines off and connecting them together. This showed no improvement. Looking in the fuel tank with the engine running, no bubbles were emitted from the return, so no air was being sucked into the system. There was no fuel smell or overfilling of the engine oil, so it wasn’t a leaking injector. The only other realistic possibility was a leaking or stuck open regulator. This was supported by the fact that fuel pressure dropped off so quickly after the engine was shut off.

  

The regulator on the C7 is on the rear of the cylinder head. 

Removing it for inspection was easy, just had to crawl under the truck and wedge myself up by the transmission. (No doghouse in a Freightliner M2)

The problem was obvious. The hole for the check valve was worn oversize and the end of the valve was sticking out of the valve body. Replacing it cleared up the long crank and the engine started repeatedly after a few revolutions, hot or cold.

C7 fuel regulator. Failed (left) and replacement.

C7 fuel pressure regulator design has two parts.

The design of the regulator is interesting, with two parts. The upper part is a check valve with a very light spring that holds some pressure in the head when the engine is shut off. The lower part regulates fuel pressure during operation. The regulator plunger has a tiny pinhole in it. When the check valve failed to seal with the engine off, the fuel leaked through the pinhole, into the return. Thus, zero fuel pressure seconds after shutdown. I can only guess at the purpose of the pinhole, maybe to prevent hunting at idle or to dampen regulator oscillation.

Caterpillar C7 fuel pressure regulator

Detroit Series 60 fuel pressure regulator has two parts.

The old Series 60 regulator is worth mention here. Its fuel system is much like the C7. On the back of the head is an elbow on the fuel return port. The regulator is screwed into this elbow. These regulators are known for problems, not necessarily with the regulator itself. The elbow in the head looks just like a regular brass pipe elbow, but it’s not. It has a small (I think .02 in.) orifice in it. This hole actually regulates the fuel pressure at higher engine loads and is exactly the right size to trap pieces of injector seals after an injector change. When the hole is plugged the engine may run, but not well and won't have any power.