How Does Excavator Thermostat Work?

Ive got a recent issue with temps rising to warning levels on my Bobcat Excavator. Its got the VT kubota engine (4cyl, turbo 58hp)
After about an hour of moderate to heavy use at rpm, the temp creeps up to the yellow zone on temp guage at which time i idle back and take a break. The machine will go back to normal temps within 5 minutes and i can continue with heavy use for another 20 minutes or so then will creep back up to warning zone.

This has happened regularly when i use the machine over an hour. Never overheated or went into the red zone.

I checked the fan belt first. It was tight but i could turn the fan by hand and make it slip the belt. Book says deflection on belt should be 1/8" with 5 pounds of pressure so i tightened it up so where i can turn the engine over by turning the fan blade. still had the heat up problem but seems like the belt was looser after an hour of use.

I heat gunned the upper and lower rad hoses about 5 minutes after one of these episodes (185 upper, 170 lower)

I blew out the rad from the engine side to out and got some dust but not much more. Its hard to see and get to in an ex.

I checked coolant level its down an inch inside of rad just covering the tubes.

Overflow was full almost to top so its been pushing stuff out but not sucking it back in.

I started the cold engine with rad cap off and saw no movement of coolant but this was cold engine and i figure the t-stat would be closed and shutting off flow anyway.

After all these checks it still does it no change.

Do i need a new belt or a new excavator? Coolant has likely never been changed, hrs on machine. I can see the tops of tubes and they are open no blockages showing.

Have not yet replaced or checked out thermostat.

Bobcat recommends running machine at WOT doing what im doing which is rpm, ive been running at rpm trying to save some fuel. Running at seems to accelerate the heat issue.

Its interesting that it cools down to the normal level pretty quickly and it stays there for a little while.

Could i just have a slipping belt not turning the pump or fan fast enough at higher rpm?


Also my hydraulic oil cooler is attached to the radiator and those temps are 180/150 so not out of line.

Outside air temp today was 85 degrees high humidity

I thought that difference should be much more also.
Whats a quick way to test the water pump? I looked into rad with cold engine and started it up saw no fluid movement.

If the tstat was stuck wouldnt it overheat pretty quick? It takes a while of hard work.

Not necessarily. They can stick at any position so it is possible it is only partially open. Since testing a thermostat requires removal it is wise just to replace it, especially given your conditions.
I would replace the thermostat, flush thoroughly and refill, and thoroughly clean the radiator with coil cleaner and water. The newer style radiators are quite hard to clean well with air.
It would not be common for the fan speed to be off unless the belt is visually worn. Especially since you tightened the belt. Actually sounds like you over tightened since the belt felt looser after running. some random thoughts

where is the radiator located? Does the fan push air OUT of the radiator (toward the outside of the machine) or pull it from outside in? Either way, the air has to go somewhere or it has to come from somewhere--meaning, if it's in a compartment look down inside that compartment and see if there's any place for air to get into/out of. If it's got a bunch of mud/dirt/poo/etc in there and enough air can't go in or out, you need to find out why and correct that problem. This was somewhat common on some of the skid loaders for a little while as the belly pan would get plugged up and people never clean their stuff, and they'd run warmer than normal because of reduced air flow.

also, could be an issue with the water pump. Not likely as Kubota water pumps are pretty near bulletproof, but worth mentioning since I've run into an impeller being loose on the shaft in the past.

how about air filter and engine oil level? They ok? Diesels require air to cool them internally, reduced air flow through an air filter can contribute to heating. Engine oil level being lower than specified, same result.

Does it sound like the engine is lugging all the time while running? I'm maybe thinking that there could be possibly a valve sticking, causing a constant load on the engine?

Valve adjustment--it been done? Can also contribute to running warmer than it should. I think they're supposed to be done about hours, right? Couple of things noticed this morning before running the machine. I marked the pump pulley and belt with a sharpie, idled it for about 5 minutes and shut it off. The marks werent even close which tells me the belt is slipping and that was with no load, not hot and idling.

The belt looks decent from the outside but hard to see inside of v but it is intact. Its a napa belt and it looks like it may be sitting slightly higher in pulley than my alt belt so maybe not exactly the right belt.
Also the belt seems slightly loose again like its stretching or the slipping is causeing it to get thinner.

Also the cap gasket surface is deteriorating. It looks intact but the rubber turns hands black just rubbing it.

I ran it for about an hour and got same results as yesterday, very slow warmup to yellow caution area, 5 minutes idling brought it back to normal.

Once i got it back to the yellow caution again i stopped and got 190f at top rad hose 180f at bottom hose. 200 on top radiator tank right next to the cap.

Think im going to order a new belt, thermostat and cap and change the fluid see what happens.

You might be having a problem with your temperature gauge. Compare its reading to an accurate temperature reading from the head with an IR gun.

The gauge is electronic and part of the computer but gets a signal from a sender. It doesnt display actual readings just green, yellow and red. This issue is new as of about a month ago and always stayed midway in the green.

The small difference in temps between upper and lower hose is a clue thats somethings not working right i think.

The entire engine, cooling system and hydraulics are burried inside the machine with vents to let air in and out. The back half of the machine is hot to the touch when the temp goes up.