When your dirt bike or ATV overheats it makes perfect sense to blame the weather or anticipate a clogged or damaged radiator upon inspection.

Riding in extreme heat or mud sometimes results in overheating and probably comprise the top reasons for an overly hot engine along with using old coolant. But what if you have fresh coolant, balmy weather and the radiator has no damage or clogs?

Most likely the water pump has given out.

A bad water pump prevents enough coolant from circulating, resulting in overheating and the plume of steam discharging from the radiator and/or a blazing hot engine case. Follow the evidence to determine whether you have a bad water pump. The giveaways include:

  • Coolant dripping from bottom of clutch over
  • Milky oil and steam in sight glass
  • Water pump impeller doesn't spin

Milky oil - You'll never look at coffee the same again

The inner clutch cover houses the water pump so you will need to remove the inner cover exposing the clutch to inspect the water pump. (Have your bike's owner's manual handy since this is not a step-by-step guide but rather an overview to help get you started.) Coolant dripping from the clutch cover most likely comes from a leaky weep hole and a failed oil side water pump seal usually causes the milky oil. The weep hole lies between two seals - one that keeps motor oil from the water pump (the oil side water pump seal) and the other to keep coolant in the pump. When the coolant side seal fails fluid drips from the weep hole and eventually leaks out the cover.

Impeller shafts wear out or the blades break preventing adequate flow of coolant. Unfortunately, you can't really tell if the problem lies with the impeller since you obviously cannot visibly check to see if it spins while the motor runs. Once inside, if it works properly it won't turn by hand but it also won't turn with a seized bearing. Conversely, a broken shaft gear allows the impeller to turn by hand but prevents it from spinning while the engine runs.

Additionally, the water pump consists of many small parts and you cannot replace it as a whole piece like the fuel pump. One of those greater than the sum of its parts type of things. Anyway, the following components of the water pump need inspecting:

  • Pump Drive Gear and Drive Pin
  • Pump Impeller Shaft
  • Pump Oil Seal
  • Pump Shaft Bearing
  • Pump Coolant Seal
  • Impeller
  • Pump Cover Gasket
  • Pump Cover

Find all of these individual water pump parts on the inner clutch cover (or right side crank case cover). You can't replace just one of the parts instead replacing the water pump requires a kit that includes all of the above. If a gasket leaks or the impeller stops working then other parts will soon follow so you always want to replace all of the water pump components similar to replacing the chain and sprockets regardless of the condition of one over the other.

Lots of parts go into a dirt bike water pump

If you plan to tackle this project yourself follow the steps below:

  1. Drain engine oil
  2. Drain coolant
  3. Remove radiator hoses
  4. Remove pump cover
  5. Remove the inner clutch cover (right crank case cover)
  6. Remove impeller (or drive gear depending on the model) from shaft
  7. Remove impeller shaft
  8. Remove coolant and oil seals
  9. Remove bearing
  10. Install new parts in reverse order

Most bike models require removal of the kickstarter and others need the rear brake pedal moved out of the way to allow full access to the clutch over. Have the following tools handy:

Well, guess what? A water pump replacement or inspection obviously requires an oil and coolant change (don't reuse what you drained) and since you have eyes on the clutch now offers a good time for some additional inspection and replacement parts, if needed, including:

And don't forget to check the condition of:

Depending on the dirt bike model you might have to use OEM water pump parts (click for KTM/Husky) but if aftermarket options exist Boyesen and Pro Circuit offer upgraded pumps that improve coolant flow through the radiator.