Won,t come up to operating temperature

98 Mazda MPV v6.have replace all related parts.been wrenching on cars over 35 years.it’s my living.only way to make it overheat was to loosen waterpump belt and it still would not open thermostat.HELP never scene anything like it.none of the radiator shops around here have ever seen such a thing

Not disrespecting your 35 years, but maybe your replacement thermostat was bad. (if you replaced it.)

Running cold means the thermostat is stuck open, Could also be put in backwards.

Air In Cooling System Surrounding The Thermostat ?
I’m sure you’ve Purged the cooling system, eh?

Where Are You Reading Temperature… Gauge, Cabin Heat Output, Radiator/Hose Temp? What Indication Points To “won,t come up to operating temperature”?

Why are you trying to get it to overheat?

How are you measuring the coolant temperature?

Are you getting a P0128, for example?

I agree about the replacement thermostat . . . I’ve ran into a few brand new thermostats which were faulty

Is the water pump impeller actually turning . . . it’s not eroded or fallen off, I hope

Some more info on the complaint might help. Seeing as how it won’t come up to operating temperature I wonder if this means the heater is not working, the gauge reads low, radiator cooling fan never cycles on, etc, etc?

Offhand, sounds like a bad thermostat. It’s also possible to buy a new thermostat over the counter and discover, as I have several times with auto parts, that the “new” thermostat is actually someone’s old reject that they used in pulling a switcharoo on the parts house so as to end up with a new T-stat and their money back.

Some really awful parts stores won’t ever send bad parts back. They’ll just sell them to another customer, won’t mention anything, and hope for the best. And if that guy returns the part, they’ll just repeat. Keep kicking the can down the road . . .

That is about the most despicable thing . . . knowingly selling faulty parts, and then the poor customer, who might be all that knowledgeable, is wondering why things didn’t turn out as planned. Maybe he’ll just give up, and bring the car to a shop to be diagnosed and repaired. And maybe it could have been avoided, if that parts store hadn’t knowingly sold him a bad part

I suppose the best approach is to refuse any parts which look like the box may have been opened and resealed

@VWguru; we still need to know a few things.

How are you establishing that it will not come up to operating temp.

The heater only blows cold air?
You opened the radiator cap and stuck a thermometer in?
You are going by what the temp gauge says?
You got a code stored in the computer that it didn’t reach operating temp?

You could check the thermostat, by placing it in a pot of water and boil it with a thermometer and watch for it to open and record the temp it opened. Simple test, but it works.


It sounds like either the dashboard coolant temp gauge or sender unit is defective or the thermostat is stuck open. Have you tried putting a thermometer in the top of the radiator when the engine is supposed to be at its normal warmed-up temperature? If so, what temperature does it read? If it reads cooler than it should, that would discount the idea that the gauge or sender unit is the problem. Then the first thing I"d do is remove the thermostat and put it in a pan of water and heat it up on the stove to make sure it is opening fully, and at the correct temperature.

Another experiment you could do is start w/a cold engine, remove the radiator cap, and run the engine at idle for 5 minutes or so. If you look inside the radiator at some point you should see a big surge of coolant flowing into the top of the radiator, which indicates the thermostat has changed from closed to open. When this happens, it is like someone turned on the spigot, there’s no mistaking the effect when the thermostat opens. Sometimes this is easier to do by disconnecting the upper radiator hose instead of looking inside the radiator. Just be careful to not let the radiator run dry.

There’s one other common explanation, the radiator fan is turned on when it shouldn’t be. Check to make sure it is working correctly and only turning on when it is supposed to.