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temperature gauge reading very high

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Hello all,

I have had my 98 LS400 for 3 1/2 yrs and until recently it has been bullet proof. The car is the vvti engine with larger coolant inlet  (same thermostat as LS430). It has 310ks and is due for timing belt and water pump replacement. my problem started last week when on a hot  day returning from the beach (Brisbane) the temperature gauge moved from just below half way to 1 and a half from the red zone. the temp gauge has never moved  before at all its always been rock solid. The air con was on aswell.

I have since replaced the thermostat which had bits of rubber through it and looked pretty sad. so i "think" it was bad but after replacing and burping the coolant system the gauge still sits high.

About 2 yrs ago i bypassed the heater by blocking off the two hoses that connect to the heater control valve (which was leaking hence bypass). had absolutely no problems with that situation for years. however i am now wondering if when i replaced the thermostat and drained the coolant that i may have a air lock through that blocked off circuit. I dont fully understand the flow of the circuit so forgive me if im way off track here.

any suggestions would be appreciated


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as far as i can tell the big fan with the clutch comes on at start up and continues.  the smaller fan at the front of the radiator comes on when air con is on. which to me seems right. i have heard some noise from what i think is the big fan with clutch.  do you know how to check that that one is ok?. cheers

i don't have a radiator cap on the radiator .  the cap is on the res. bottle.


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Thanks 2bling. Haven't flushed the radiator yet as ran out of time over the weekend. My wife drives this car, so we went for a drive on the weekend and I thought it drove well. And when we got home i put my hand on the top of the inlet manifold and it wasnt that hot. This is after 50mins each way driving mixed traffic. This is the problem the car gets overly hot (not red) on the gauge but doesn't feel extra hot. 

I disconnected the bottom radiator sensor tonight to keep the aux fan on high as it hasnt been coming on unless the air con is on. So maybe it is the ECT temperature sensor under the throttle body or both. The temp on the gauge doesn't move from 1 fifth from red but it is definitely worrying me. The rubber trapped in the thermostat spring looked like an old oring. I hosed out the fins of the radiator yesterday and lots of crudd came out but nothing has changed the temperature gauge in cabin. 

Can in flush the radiator in place by dumping the systems coolant and taking off the bottom rad hose and forcing water through the top hose? . I don't have a rad cap on the radiator?

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Just a note in my experience of buying the wrong sized thermostat for a 1998 LS400.

When I went to the the local trusted auto parts store (no names cause I dont think it was their fault). The stat listed for years 90 thru to 2000 was the smaller one. Just thought Id let all know that aint buying OEM the vvti engine in our 98 LS400 has the bigger dia thermostat 64mm dia i think. I got my replacement as the one they have as the LS430 Stat. Bigger inlet coolant thermostat. I know I know not OEM

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