My Thomas Built HDX overheats above 55 mph

[CMORGANSKOOL]

Originally Posted by alpine44

Since you already replaced the thermostat, coolant, and cap, the other important items in the cooling system to check are radiator, fan, and water pump.

The point of a radiator is to create an large surface over which heat is exchanged from the coolant to the outside air. That surface needs to be clean on the air side as well as the coolant side to do the job. On the outside look for bugs, dirt, and grime built up between the fins. You can address that with a general purpose degreaser or a special radiator fin cleaner solution and gentle water pressure. (Do not hit the delicate fins with a pressure washer).

To assess the scale buildup on the inside surface of the radiator you need to drain the coolant and take one of the hoses off the radiator. It should not look like a coral reef inside. If it does, you can flush with a scale remover but I would just replace the radiator.

If you have an electric radiator fan, verify that it kicks in. On an engine driven fan you will have a viscous clutch that I would just replace at the mileage of your vehicle.

While you are troubleshooting the cooling system, check the turbo charging system for leaks. Low boost pressure leads to over-fueling and overheating. On the exhaust side, look for tell-tale soot and on the intake side check hoses for cracks and clamps for tightness.

What about the water pump? They typically leak well before the impeller is so eroded that it will affect cooling. I would only take the pump off if everything else mentioned failed to rectify the problem.

I had a similar issue when my water pump went, but a water pump should spew fluid through the weep hole when it dies. If no weep, I would assume coolant system issue too, fan, filter, radiator blockage.

[superFreddie]

Originally Posted by Native

Hey Fred,


I have a similar situation with my CAT so I am very interested in what you find. Please post your tests/results as they are made.

Hello Native, I hope you've had some luck so far but if not maybe my results can help.

(Little backstory first. Being disabled, it is necessary for me to get help doing most work on my bus. We've recently made the acquaintance of a general contractor who is also a professional of many trades, never mind a Jack and an auto mechanic is is one of them along with carpenter and electronics engineer. He's also become our friend.)

So I booked him for a day this week to work through my bus ToDo list with the cooling issue as #1. But out first task was really a Cooks Tour of the rear engine compartment.

We spent nearly an hour tracing lines, hoses and wiring as best we could. His first prescriptions were power wash the filthy engine and replace all the belts (3) which were badly cracked. Note: Need a local Toronto parts source.Then with all the messages and suggestions in this forum, we turned our heads to the cooling system in more detail.

The fan is hydraulic with an engine driven pump feeding a valve and supported by a reservoir. The valve has 2 outlets, one to the cooling fan from the upper half and other to the power steering pump from the lower half. Corresponding to the outlet lines are some electrical inputs. We concluded that the upper level wires matched to the cooling fan line that feeds the viscous fan clutch. So with engine running and fan turning at apparently normal speed, we popped of the plug to see what would happen. Well you've probably already guessed that the fan went screaming into its high speed mode, like a banshee with winds blowing everywhere. So pretty obviously, that is the problem. However, unplugging what is probably a controller or sensor input is only a partial fix. The three wires in the plug included a green probably ground wire, and two white, one reading 13.6v and the other 0v. Reattaching it to the valve box resulted in the fan dropping back to slow speed after a short delay.

My mechanic friend guessed that there's likely an input sensor that either switches off that voltage at the other end or reverses it when the engine temperature reaches a specified level. In reading some docs that I have obtained about this engine, two temperature sensors were referred to as the coolant and intake manifold temp sensors which support the cooling system.

I could put a switch on the wire, either local or remote. This could "force" the cooling fan into high speed when highway travel is on that day's plan or with a remote switch, when the engine temp starts to climb. Alternatively, is there any harm to the fan if left to run at high speed all the time? Ideally, I'd like to obtain a system wiring diagram in order to locate, test then ultimately replace any defective sensor(s).

I haven't yet road tested this resolution because insurance coverage is lapsed at the moment as I transfer the bus from Saskatchewan to Ontario.

But I'm hoping that being able to tackle a hill at full throttle with no fear of overheating may help me maintain speed on the way up.

More to come. Thanks for your interest.

[EastCoastCB]

Have you tried adjusting the fan clutch?
Smartest guy I know made this lil vid-

https://www.youtube.com/watch?v=QVMN7LmKLYA