Cat 3126 Fuel System Diagram (HEUI Pump Diagram)

When you are finding evidence of leaks, those leaking spots can be difficult to find. All you have is a puddle of fuel and a few drops to go by to start your search. Finding the actual leak will take some time unless you have a good diagram to follow.

We will get to the diagrams in a minute but if you have a leak, then look for and examine the check valve. This little valve regulates the pressure and if it is not put in place properly, it could be one source of your leak. It does degrade and wear out.

To see the diagrams and learn more about this fuel system, just continue to read our article. It provides the best diagrams possible and the best information you need to know about this fuel system. Take a few minutes to see how both can help you solve a problem with your engine.

Cat 3126 Fuel System Diagram


When you do a search for this diagram, there will be a lot of images that come up. While many are labeled for the specific engine systems you are looking for, not all represent a solid diagram of those systems. Many are labeled badly.

We are not going to use one of those images that pop up in that category. Ours is coming from the Caterpillar Engines Troubleshooting which you can explore further by clicking on that name.

While it may not be the best diagram, you can use it as a guide to match up with a better one in the image category of your search engine results page.


#1. is the plug and #2 is the fuel filter. The instructions underneath the diagram are: “If the engine is not equipped with a fuel priming pump, it may be necessary to fill the fuel filter with CLEAN fuel. The following procedure will allow only filtered fuel to enter the fuel system.”

We will have other links later on in the article that will take you to manuals. These manuals take some searching but there are a lot of diagrams inside of them to help you work on your 3126 diesel engine.

Cat 3126 HEUI Pump Diagram

The pump diagram is a lot better than the previous one. You may have to enlarge it a little to get a better view of all the components. If you can’t do that then just click on this link to see it at full size.


What Are The Fuel System Components?


The components for this fuel system are all numbered. Unfortunately, the corresponding list was not part of the diagram we pasted into this article. Those components are listed below:

Diagram of components for the HEUI fuel system above

  1. Unit injector hydraulic pump

  2. Oil flow to engine

  3. Oil filter

  4. Engine oil pump

  5. Oil cooler

  6. IAP sensor

  7. Injectors

  8. Fuel supply rail

  9. Fuel pressure regulator

  10. IAP control valve

  11. Fuel filter

  12. Fuel tank

  13. Back of cam gear

  14. Speed/timing sensors

  15. ECM

  16. Boost pressure sensor

  17. Accelerator pedal

  18. Accelerator pedal position sensor

  19. Batteries

  20. Coolant temperature sensor

  21. Data link

  22. Exhaust brake relay

  23. Inlet air temperature sensor

  24. Transmission relay

  25. Vehicle speed sensor

  26. Inlet air heater relay

  27. Inlet air heater lamp

  28. Fast idle lamp

  29. Check engine lamp

  30. Speedometer and tachometer

  31. Cruise ON/OFF switch and Cruise SET/RESUME switch

  32. PTO ON/OFF switch and SET/RESUME switch

  33. Service brake switches

  34. Neutral and clutch switches

  35. Fuel transfer pump

Cat 3126 Parts


One of the best places to go to get spare parts will be direct to Cat. They have a parts website or set of web pages where you can find the spare parts you need. This link will take you to the web page that has the fuel system kits.

Another place to look will be Diesel parts Direct and you can find their merchandise at this link. This online company has spare parts for a variety of diesel engines in case you do not own a Cat 3126.

There are a lot of outlets selling diesel spare parts across the internet. If you do not need one of those nationally advertised ones, do a local search to find the nearest parts store to your location.

3126 Common Problems


There seem to be only 2 major common problems you will run into with this engine model. The first is the ECM and it may be underperforming requiring a re-flash or update.

Both of those actions require you to go to the dealer or authorized repair shop to get it done. If you go anywhere else, you may lose data or have the ECM corrupted.

The second common problem is with the fuel system. There have been a lot of complaints about the performance of this system because lower-grade fluids contain particulates that build up and cause problems.

Valve drop is another issue that comes with this motor especially when you do not do great maintenance methods or use the lower grade fluids.

Cat 3126 Manuals


You should contact Cat first to see if they have any manuals you can download. You can try this link and the link to the manuals is the middle one after you scroll down a short way.

Another website to go to is this link. It has the service manual online and is easy to view. The website has a download and a print option ready for your use. Finally, you can get several manuals at this link.

Some Final Words

Working on your fuel system is a lot easier when you have the right diagrams to help you. These diagrams should get you on the right track but double-check with the manuals that we linked to.

Having the proper diagrams will make your work a lot easier and hopefully it will make it a lot faster. If you still cannot find your problem then the best thing to do is to go to an approved mechanic or the dealer directly. They will have the expertise, the knowledge, and the tools to get the job done right.

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