water temperature gauge goes to red; using a temperature gun reading of top of radiator was 150F, top water hose at 140F coolant not hot, gauge red

First item that I would check is the temperature on the outlet side of the engine oil cooler. Another place is on the head behind the thermostat. If those read the same as you have been seeing I would suspect the wiring harness. It's a B model machine manufactured in Japan when there were issues with wiring harness insulation. I've found it less expensive to purchase an automotive temperature gauge with sensor. Put the sensor in and most automotive sensors are twelve volt so run the gauge from a twelve volt battery. Maybe even find a manual gauge with a capillary tube and check it that way. I've looked at plenty of machines from the analog era where the monitor panel noise maker wires were clipped and a set of manual gauges were installed.

Nige said: IMHO you're crediting a 30-year-old machine with far too much in the way of electronic intelligence....... Those E-Series excavators were only one step past being steam-powered. Click to expand... Wow! The machine is that old? Well that may be a different kettle-of-fish. But I'll still advise, to have the tubes flushed properly. The best way is to remove the bottom the core.

Watch it I remember when they were new

Nige said: IMHO you're crediting a 30-year-old machine with far too much in the way of electronic intelligence....... Those E-Series excavators were only one step past being steam-powered. Click to expand... Lol yes I remember the E's, a friend of mine had one, he used to call me every minute to put back on the tracks. They kept running off. Luckily it was a long stick version, so I was able to just lift them back on using the same machine.

I will aim the temp reader at the sender and report what I find. Thank you. Nige said: Is low coolant flow also a possibility.? Stuck temperature regulator maybe.? Your comment "after a couple of minutes the gauge went to RED" appears to support that theory. When the gauge is in the RED have you tried aiming the temp gun directly at the coolant temperature sender.? Click to expand... Nige; today Sunday I took reading before I started the engine at 630 PM and one hour later 730 PM. The before starting temperatures were to of radiator 88F; top of hose 85 F; temp sender 75; bottom radiator hose 75F. After one hour temperatures were top of radiator 147F; top hose 126F; temp sender 135F; bottom hose 108F. After one hour the temp gauge in the cab was just over the middle/center of the gauge with the red zone to the right the dial was not close say 1/4 inch from red zone.

lake side bob said: today Sunday I took reading before I started the engine at 630 PM and one hour later 730 PM. Click to expand... Were you working the machine during that hour or was it just sitting idling.? lake side bob said: After one hour temperatures were top of radiator 147F; top hose 126F; temp sender 135F; bottom hose 108F Click to expand... I dunno about anyone else but that sounds like low coolant flow to me. I would not expect the temperature at the sender to be significantly higher than both the temperature of the top hose or the top of the radiator. I would suggest to look at the temperature regulator next.

Nige said: Were you working the machine during that hour or was it just sitting idling.? I dunno about anyone else but that sounds like low coolant flow to me. I would not expect the temperature at the sender to be significantly higher than both the temperature of the top hose or the top of the radiator. I would suggest to look at the temperature regulator next. Click to expand... the engine was only idling. after one hour of idle the temp was top of radiator 147F top hose 126F temp sender 135F It does not make any sense to me, why the temp sender is higher then the hose going to it via the metal elbow, but lower then the top of the radiator. Do you think it its time for a new radiator thermostat? I take it the temp regulator Nige wrote is the radiator thermostat? I can flush the radiator out with it still attached to the machine, but maybe switching out the thermostat first and see what that outcome is, might be the next item to try. thank you for replying, much appreciate every ones suggestions. This site is sooooo useful. The access to so many minds and experienced people.

lake side bob said: It does not make any sense to me, why the temp sender is higher then the hose going to it via the metal elbow, but lower then the top of the radiator. Click to expand... A lack of flow through the system would explain that. Question is what's causing it. lake side bob said: I take it the temp regulator Nige wrote is the radiator thermostat? I can flush the radiator out with it still attached to the machine, but maybe switching out the thermostat first and see what that outcome is, might be the next item to try. Click to expand... Cat call it a temperature regulator, which is exactly what it is supposed to do; regulate the coolant temperature within a set range once the engine has initially warmed up. See the parts illustration below. I would not suggest to replace the regulator just yet. Remove it and dump it in a pan of boiling water. Does it open.? If it doesn't then replace it and re-test the cooling system performance. If it does then you are in for a radiator removal and internal cleaning. This will involve de-soldering the top & bottom tanks because the radiator is a single unit.

Might want to check the belt on the water pump and how the pump turns

If the belt, fan, thermostat or cowlings are defective, the temperature change would affect the whole radiator system. If the radiator is blocked at the bottom, that area would have a lower temperature since the coolant wouldn't be circulating in that section. If the coolant have not been changed for a long time it would leave some sticky residue mostly at the bottom of the radiator that would result in blocking the inner pores. If you use water rather than coolant. That makes it worst. Water causes corrosion much quicker wether hard or soft. I would have the radiator removed and cleaned properly, you may even have to remove the bottom tank to expose the tubes and use a flat brazing rod and have them cleared. Or maybe replacing the radiator. Been through this same problem a few times. Based on your explanation of your problem i think this your last resort. Greetings to all from Antigua and Barbuda.

Was that one of the engines 'thinking here" that the impeller would turn loose of the shaft or itself where it was epoxied or something like that. Wasnt that a Mitsubishi Id start with a manual gauge, just to rule that out. Then seeing what the top and bottom readings were the flow thru radiator is too slow, allowing it to cool the smaller amount of water that is flowing thru it to have a greater difference in temps. 126f- 108f

Start the engine with the radiator cover off. Allow temp to build to for the thermostat to open if theres circulation visible then the pump is good. If the pump is bad, the coolant or water will just boil and flow from the cover the minute the thermostat opens. I may sound a bit old school. But my methods work on both old and new.

Nige said: A lack of flow through the system would explain that. Question is what's causing it. Cat call it a temperature regulator, which is exactly what it is supposed to do; regulate the coolant temperature within a set range once the engine has initially warmed up. See the parts illustration below. I would not suggest to replace the regulator just yet. Remove it and dump it in a pan of boiling water. Does it open.? If it doesn't then replace it and re-test the cooling system performance. If it does then you are in for a radiator removal and internal cleaning. This will involve de-soldering the top & bottom tanks because the radiator is a single unit. View attachment 364521 View attachment 364522 Click to expand... Today August 18 2026, I took out the temp regulator and placed it in 200F water, it did open; did this twice so that's not the problem. Moving on we did a continuity test of the wire from the temperature sender. used a long brown wire from the temp sender wire that is yellow with green strip, that plugs onto the sender, it was not plugged to the temp sender, brown wire back to cab, put one lead of electric tester on the brown wire other electric tester lead was connected to the yellow/green strip wire going into the control box the elect tester zeroed out. This was with the auxiliary power on (not engine running no temp regulator reinstalled yet) and also with the auxiliary power off same result. So the wire from the temp sender the the plug that attaches to the plug end with wires coming out of the dash this is fine. The temp regular is fine. Any further suggestions would be greatly appreciated everyone.

Use a hot plate and put the temp sender in a pan of water hooked up to the system and see what it reads. Or you can use a manual gauge. The monitor panel itself can be the problem. As I recall you can remove the display and dismantle it to check the circuit board.

lake side bob said: Any further suggestions would be greatly appreciated everyone. Click to expand... Have you tried looking for flow in the cooling system as suggested in Post #32 above.?

After trying and still unsuccessful with all other methods, have you yet thought of having the radiator properly cleaned internally?

Kool J said: After trying and still unsuccessful with all other methods, have you yet thought of having the radiator properly cleaned internally? Click to expand... I’d still prefer your idea of trying to verify coolant flow first. If there were any doubts after that test I would then suggest a sticky beak at the water pump impeller as suggested above by @HATCHEQUIP before going to all the effort of removing and disassembling the radiator.

Nige said: I’d still prefer your idea of trying to verify coolant flow first. If there were any doubts after that test I would then suggest a sticky beak at the water pump impeller as suggested above by @HATCHEQUIP before going to all the effort of removing and disassembling the radiator. Click to expand... I picked up the new temperature regulator today and compared the old with the new. See photo below. the new one has a rubber o-ring sealing off the regulator, the old one does not have one and it does not seal one can see through it (12 to 3 on a watch face) in the photo, maybe it wore out over the decades. Both have the pin hole to let air out/escape. That has to part of the problem.

lake side bob said: I picked up the new temperature regulator today Click to expand... Genuine OEM or aftermarket.? Did the one you removed have a Cat part Number on it.?

It is from the Caterpillar dealer, part number from the parts book for the machine is 9Y33655 the new number is 1265869. The old regulator part number is 9Y3088 with 180F stamped into it, I am looking at it. Note different part numbers. I put the new one in the machine. On the new regulator it has 82C which is 180F. ran the engine at idle for 45 mins the temp did get up to 155F. Increased engine RPM an after 15 mins it only got to 174F. Operated the hydraulics moving arm stick, bucket in the air this did not get coolant 50?50 mix above 174F. The coolant did not get that hot to open the regulator. I ran the radiator the entire time with out the cab. I filed it the brim, at the start the coolant flowded a little bit out of the radiator it was full to the brim. At the end it was about 1 inch down from the brim and I would say it was boubbling AT 174f. At the end the water temp sender unit read 148F The metal elbow above the sender was 171F The top radiator hose was 154F I used a meat thermometer to test the water 50/50 mix in the radiator it read 174. I take it the new regulator never got hot enough to open? I might dig a hole with the machine to see if that gets the temp up. Any suggestions would be greatly appreciated.