I need a mechanics advise on cooling system

[Steven94]

So since I have bought my 94 4runner 3VZE with automatic trans, I have problems with my cooling system. My engine takes a long time to get to operating temperature. If you are familiar with the auto trans 3VZE, you know it will not shift into overdrive unless it is at operating temp. My runner also sometimes gets cooler while driving along. So I will be driving at operating temp, then the gauge will all of a sudden show cooler (about 1/4 full gauge or 1/2 normal operating temp). So this kicks it out of overdrive.

It is working better than when I first bought it. I have replaced the following as far as the engine is concerned: Coolant temp sensor (the old one was so corroded it was easily broken in half), radiator cap, thermostat and vacuum hoses replaced with silicone. The engine runs real smooth and shows no other signs of a problem.

I have spent a lot of time trying to figure this out. I have come across a thread not from this forum saying the casting on the back of the engine can be full of crystals and causing a coolant blockage and air bubbles to be trapped in there. Have you ever heard of this? Any other reason why my engine may do this?

 

[mikebontoft]

trapped air bubbles would cause it to overheat.

have you checked the fan clutch to see if it's operating correctly? There are not too many conditions that can cause overcooling. It also wouldn't be the first time I've heard of a faulty thermostat out of the box.

 

[Steven94]

My engine never seams to over heat. At least not in any way I have heard of. It has not stalled or smoked or anything like that. It is always hot when I get home, and takes a while to cool down to touchable temp (at least an hour). It was increasing in engine temp when I would blast the heater (it would go from 1/4 to normal engine temp in a few seconds). Though it hasn't done that recently. When my mechanic replaced it he said the old one wasn't even bad. I have been close friends with this guy for many years so I trust him, but he is also confused with what is causing this problem. I really want to figure this out so it drives better. The 4runner has been one of my better vehicles, but I know it can run better than it does.

 

[mikebontoft]

I know it's not overheating. You need a better method of reading the temperature though. Don't forget that gauges lie. You may have replaced the sensor, but it could be trash or the gauge itself could be faulty. It should actually cool off if you have the heater on, not the other way around.

The mechanical fan clutch could be stuck but you'll probably notice that the fan won't quit.

Or you have a faulty thermostat. Again. Unfortunately not uncommon.

 

[Steven94]

Yes I have heard that it should cool off if the heater is on before, but this is not the case with mine. Will it cause it to shift out of overdrive if just the gauge is faulty? Like I said, once it is at operating temp it will drive and shift perfectly. When my friend pulled the old thermostat he inspected it and said it was fine, but replaced it anyway with the one I bought. The problem existed before I bought the truck.

The fan on mine is connected to the engine. It is not electric. So if the engine is on, the fan is running.

 

[fourwd1]

First, do you know that there are 2 coolant temp sensors?
One for the dash gauge and one for the EFI ECM.
It is the EFI temp sensor that controls the trans O/D.

 

[Steven94]

The one I had replaced was on the coolant bypass casting on the back of the engine.