Highway & Heavy Parts Answers Your Diesel Engine Questions: Fuel Systems

Recently, at Highway & Heavy Parts, we've expanded our product offering to our customers. We now offer more fuel system components for all major OEMs.

We receive a lot of questions about fuel system failures or customers asking us for certain parts that will fix these issues. To that end, today we're going to be answering some of your questions on fuel systems!

If you want more info on fuel systems, check out our video:



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Answers to Your Fuel System Questions


fuel system components customer questions hhp | Highway & Heavy Parts


We've rounded up some of the questions that we see a lot on diesel engine fuel systems and are bringing you the answers you want:


What are some common reasons I would find fuel in my oil?

Fuel in the oil can come from an injector over-fueling to the point that it can't burn all the fuel, and it's getting by the rings down into the fuel.

Another reason you might find fuel in the oil is that the injector o-rings have failed. This allows fuel to come up into the cylinder head and into the oil. Or maybe fuel is coming through the body of the injector, contaminating the oil.

Another thing to think about when regarding fuel in the oil is the fuel pump. You might have fuel pump seals leaking into crankcases. So, there's a number of different reasons you might find fuel in your oil.


Do we sell any electrical components for outside of the motor, like electrical fuel valves or modules?

We do have a full line of Bosch after-treatment valves. Whether it's a DEF pump, a dosing valve, a metering unit, a supply module, sensors, or things of that nature, we do offer those components as well.


What is the difference between a lift pump, fuel pump, fuel transfer pump, HEUI pump, and fuel injection pump?

So, the first three, the lift pump, fuel pump, and fuel transfer pump, can all be kind of gathered into the same group. Those are auxiliary pumps that supply fuel to the main pump, whether it be an injection pump or common rail pump, or anything like that. 

The HEUI pump is a hydraulic electric unit injector pump, and it's actually an oil pump. Those hydraulic electric unit injectors are oil driven, so that is a high pressure oil pump that supplies oil to the injector to supply fuel. 

A fuel injection pump is typically an older style pump. It has barrels and plungers, and lines coming off it going to the injector nozzle. That creates high pressure in the injection pump.

Having problems with your oil  pump? Read our oil pump failure analysis.

How is a flow matched set of injectors better or different?

Flow matched injectors are a much  more precision set of injectors that are matched across the set. On a typical standard injector, say we're talking about a Caterpillar injector, there is a certain specification window that that injector has to fall within. There's a high limit and a low limit. The injector falls within that range and it gets applied a trim code. If the injector is just outside the range, maybe that trim code gets it in the range. So, you can have injectors from the low to the high and all still be within specification.

But with a flow matched set, you're looking at an injector  limit, high and low, that is actually cut in half. It is a much more precision injector. Everyone one of those injectors is built mechanically to fit in that window. So, a flow matched set of injectors are all going to use the same trim code, all the way across the board. We're not changing the flow electronically on one or the other to make it match. They're mechanically matched, and that trim code being the same is going to give you a much better idle, a smoother idle, and better performance out of your flow matched injectors.

Want to know more about trim codes? Check out our post explaining fuel injector trim codes


If you have any other questions about fuel systems from any major OEM, let us know! We'd be happy to help you out.

If you're in the market for diesel engine replacement parts, our ASE Certified Technicians can help! Give us a call at 844-304-7688, or request a quote online