Advice on Temp Gauge fluctuation on 90 hilux

x 2........

Hi Stu,
"Some people say that not having a thermostat fitted will cause overheating due to the circulation of the coolant being to quick through the system but I have only ever seen no thermostsat cause overcooling"

This is actually true, but the overheating only happens towards the rear of the cylinder head and not the overall cooling system. I have seen it a number of times with heads cracking at the rear due to no thermostat being fitted.
If you ever need to leave a thermostat out it is best to cut the valve part out the centre and put the rest in as this will slow the flow enough to cool the rear of the head properly, but as you say never leave a thermostat out.
Chris

Thanks for the explanation Chris.

Been working on my own vehicles all my life & no-one has ever explained the overheating effect of leaving out the thermostat like that. I always thought the overheating theory wasn't possible but that makes sense.
Cheers
Stu

What people say and believe and what is actually true can be many entirely different things.
Can't see how it can be causing overheating with no thermostat, all thermostats are trying to be "no thermostat fitted" when the real overheating situation is occurring and maximum cooling is required.
Therefore,
I cannot see how the cutting out of the thermostat valve can alter the designed in cooling flows within the cylinder head, as these flows and circulation happen long before the water gets to the thermostat.

Also the same when the thermostat is on the intake side of the engine and not on the exit (hot) side of the engine.

People seem to forget it is the radiator which gets rid of the heat, not the thermostat.

Some of the previous comments don't make sense to me.

They seem to say, if you turn down the water valve on a nursing home wall panel heater it will get rid of more heat from the panel?????? Are we sure in this?

Pretty simple really. The thermostat is an automatic valve.

When the valve is fully open it allows maximum amount of water to flow through the radiator thus getting rid of more heat load from the coolant.
When it's closed or partially closed it restricts the coolant flow so that less coolant goes through the radiator thus allowing the engine to maintain it's optimum operating temperature no matter what the ambient air temp is.

I agree with all the above, I had the same problem with my HiLux and did all the above, took it to the auto sparkie to have it fixed he did the wire repairs and it was still happening, so we did a complete radiator flush and put in fresh coolant supplied from Toyota at great expense to me and that seemed to be the end of the problem sold the vehicle two years later and had no more heating hassles. that was my experience with that problem.
Broodie H3

Just as an after thought, you may have air in the system too this could also lead to this problem that happened to me with my HINO I didn't heat the engine correctly to get the thermostat to open and allow the flow to go through the whole system, in the truck. It may be another option to look at. just a thought.
broodie

If you pull the plug off the sender the gauge should go full scale, if you touch the sender wire to ground the gauge will go to min.
As you say your gauge is jumping towards the low side it sounds like the sender or its wire are shorting to ground somewhere