Machine has about 3400 hrs and original head gasket as far as I can tell. I know these c3.4 engines are known for head gasket issues but want to make sure Im not overlooking anything else before I go that deep. Coolant temp will slowy climb to 240-250 degrees where it sets off the overheat warning then slowly returns back to normal temp range a few seconds later. I can't imagine the coolant temp is actually fluctuating that fast and wondering if the temp sensor could be faulty? My idea is water pump failing or and air bubble developed in the system possibly from a failing head gasket. So far I have checked the fan operation (working), checked for air bubbles in coolant reservoir (nothing obvious), cleaned the radiator and replaced radiator cap, performed boil test on the old thermostat (worked fine) but replaced it anyways since I was there. No changes, still overheating. Thinking next logical step is a new water pump, if that doesn't fix it guessing it will have to be time to do the head gasket. Disclaimer: I noticed tiny air bubbles forming in back edge of the head gasket where it appears to be leaking compression but no signs of oil/coolant contamination as far as I can tell. I bought used and put roughly 500 hrs on it and has been seeping just a little coolant out of the reservoir when ran hard for an extended period of time. I'm sure it's due but hoping to get through the summer at least before addressing head gasket.
Have you tried to verify temp Guage is accurate? Laser temp gun can be tricky to use accurately but should get close. With no bubbles in coolant reservoir I'd do more diagnostics before pulling a head. While your at it with the temp gun check rad in and out Temps. Could give an indication as to water pump or rad health
Yes I forgot to mention but I did use a laser temp gun and monitored temps from cold start to max operating temp and where consistently showing about a 10 degree (cooler) difference compared to digital readout. I also measured rad hose where coolant flows from the thermostat housing to the radiator and the return hose which showed about a 30 degree cooler temp difference. I called my local Cat service center who instructed me to use a coolant system pressure tester to see if pressure immediately started building which it did not. It seems to only build normal pressure when system gets up to temp. I guess the next step will be to buy a combustion leak dye test kit and see what the results are.
Fan operation- working. How about a laser tach and rpms to spec. This has been an issue as I recall. (Probably from on here. . )
If you start the engine from cold then unplug the connector to the fan solenoid, does the fan go to high speed.?
Nige, yes when I unplug the fan solenoid it goes to high speed. Is that what it is supposed to do?
Yes, that is a test that the fan solenoid and the hydraulic motor are working correctly.
Nige, what do you make of this temperature fluctuation? It can go from normal temp to 240 and back to normal in a matter of 60 seconds or less. Surely coolant can't rise and fall in temperature that quickly?? Makes me wonder if air is somehow entering the system causing a hotspot by water pump cavitation or head gasket failure.
Temperature rising and falling within a short space of time would certainly suggest the possibility of air in the cooling system somewhere, or something impeding the coolant flow. A pressure test of the cooling system when the engine is warm might reveal something.