Coolant Temperature Sensor 1.8 Pre2000

[RobbyV]

Hello

Can someone let me know the location of the coolant temperature sensor for 1.8L pre 2000 (engine 7A-FE) please.

Problem I have is that the engine temp guage struggles to get to 'norm' (half way). And when it does, it goes back down to about 1/3 position if I'm doing 40+ mph. Plenty of coolant in reservoir. Plenty of heat in hoses. Plenty of hot air inside the car when heater fans are on.

BTW, I did have water pump replaced recently....

Cheers.

[gazza1286]

Your problem is the thermostat.

It is not closing correctly and is probably open permanently. The engine is therfore overcooling. To check this run the engine from cold. You will see that the upper hose warms up and the lower hose will also seem warm quite early on.

What should happen is the upper gets very hot. The lower remains cool. Then several minutes after the needle reaches halfway the lower hose gets hot very quickly.

The fact that the needle falls when you increase engine speed is also a symptom of a failed thermostat. What is happening is that as you accelerate 'relatively cool' water is being pushed at a faster rate through the unrestricted system as the stat is open too wide. The heat is being lost through the radiator and the temp at the sender is dipping.

[RobbyV]

Thanks very much Gazza1286. I'll check that out tomorrow.

Cheers.

[RobbyV]

OK. Starting engine from cold...Temp guage after ignition not moved from pre-ignitionig state (ie at cold stop).

Idling at 1.5K rpm (high?). Hose to radiator top (#1) warms up after a few mins. Hose from rad bottom to stat mounting (#2) remains cool. Temp guage rises a little -just getting into marked zone.

A few more mins later, revs about 1.2K, 1.3K. Hose #1 getting hot. Hose #2 lukewarm at bottom. Temp guage rises a bit more. Hoses to/from heater matrix are hot.

A few more mins later, revs about 1.0K. Hose #1 hot. Hose #2 fairly hot (cooler that #1) at bottom of hose but much cooler at top of hose where it connects to stat housing. Temp guage rises a bit more but not yet at mid point.

Takes another 5 mins idling (15 mins total) before temp guage approaches mid-point. Idle revs about 850/900.

So, if the thermostat was stuck open, should hose #2 not be the same degree of hottness (!) from rad bottom to stat housing?

Thoughts welcome.

Cheers.

[gazza1286]

Yes it's the stat.

What I failed to explain accurately in the 'should' paragraph was that the bottom hose will remain stone cold until several minutes after the needle has hit halfway. This is because the stat is locked closed and stops circulation until operating temperature has been reached. In your case the stat is partially open and is allowing some circulation.

There is usually a temperature differential between the top hose and bottom hose anyway. Some of the heat is lost to the atmosphere when the coolant passes through the radiator matrix. This occurs even when the car is stationary.

The temperature gauge and the operation of the stat are totally independant of eachother.

The first reports upon the temperature of the coolant in the system. The second reacts to the temperature of the coolant.

[RobbyV]

Cheers gazza1286.

I'll get the stat replaced and post the outcome.

Many thanks for your advice.

RV

[gazza1286]

The presure test your mechanic did for the coolant leak may have hastened the demise of your stat which may have been on its last legs.

[RobbyV]

OK. Did partial drain of coolant. Replaced thermostat. Refilled.

From cold start, initial idle speed is 1.5K rpm (was 1.8-2K pre stat change). Within a few mins, temp guage start to rise and idle revs decrease. Top hose warming, bottom hose cold.

Few mins later, idle revs below 1K rpm, temp guage approaching nomal. Top hose hot, bottom hose still cold. Few more mins later, temp guage at 'normal' (which for my car is a hairs-width below mid point). Idle revs at 800 rpm - normal.

Let it idle for 5 mins, then swithched off. A minute later, coolant in expansion tank was sucked into radiator. So topped up coolant to max.

Rested for an hour. Checked coolant - at 1/4 up from min, so topped up to half way. Started up again, temp guage rising normally, idle revs falling normally. BUT, no hot air inside car. Possibly airlock. So, squeezed radiator hoses gently for a few mins and also heater matrix hoses.

After idling for a few mins and revving to 3-4K rpm, hot air inside. Rad top hose v hot, bottom hose cold, heater matrix hoses hot.

Took for a 30 min spin. Some urban driving with traffic, some A-road at 60mph. Idle normal at 700 rpm (no electrics), 800 rpm lights on. Temp guage remained 'normal'. Checked hoses - all hot.

So, I think all is OK.

Many thanks to gazza1286 for advice. TOP MAN!!!

[gazza1286]

Very pleased.

Keep an eye on the coolant levels over the next few days. It takes a few heat/cool cycles to fully purge the air out of the system.