Coolant and temperature

There is some rather strange behavior with this coolant system. It’s a '91 Subaru Justy. 1.2L 3 cylinder. Here are the facts:

  1. Engine temperature gauge gets maxed out after a few minutes.

  2. Sometimes the coolant reservoir is empty

  3. sometimes the coolant reservoir is full

  4. sometimes coolant is seeing leaking in various parts under the hood. no visible leaks though

Does anyone know what the problem could be? I was thinking maybe the thermostat is seized shut and not letting anything in? But if that was the case, wouldn’t the reservoir be full all the time? I’m lost and looking for help here.

I can think of several things to check right away. None of your listed symptoms exclude either of these.

  1. Thermostat
  2. Water pump (or the belt running it)

There is no water pump (that I can see). I assume the water pump would be belt driven but there are only 2 pulleys: crankshaft and alternator–that’s it! And if it is the thermostat, how do i check if it’s bad? Or do i just have to replace it to find out?

Should have added this to the first message -

Is the radiator getting quite hot when the temp gauge goes up? That should be pretty easy to check. If it isn’t (and it probably isn’t or the engine temp would be ok) it is probably completely dry inside or if wet, the coolant isn’t flowing through it properly.

When things are working correctly, it is pretty easy to detect the thermostat opening. Start the engine while cool and keep your hand on the top hose between the engine and radiator. As the thermostat opens, you will feel that hose heat up quickly. If it doesn’t, there are about three possibilities. The coolant is very low, the pump isn’t pumping, or the thermostat isn’t opening.

On some cars, if the coolant gets real low, it is tricky to get it to fill back up properly. If there are large air bubbles in the block, you might have to open a bleeder valve somewhere to get it to refill properly. Until that is done, you will have effectively no coolant flow.

Radiator stays cool all the time. If i rmember correctly (this is a friend’s car and i don’t have it here to look at) there is only 1 hose that goes to the thermostat which is under the air intake and carbeurator thingy. And that hose is always hot. When I get a chance tomorrow I’ll look for another hose b/c I don’t see how only 1 hose would be very effective.

But on a lighter note, it runs fine when the temp is maxed out.

There most certainly is a water pump. It is “possible” that it is electrically driven, but I doubt it.

Thermostats are petty easy to test, but are so cheap that generally you just replace them if you go to the trouble to take it out.

To test a thermostat, remove it from the engine and drop it into a pot of water on the stove. Heat the water. Watch the thermostat. It is pretty obvious when they are opening. Record the temp where it opened (you will need a thermometer) and compare to the temp that it is supposed to open at. You will get that info from Subaru or a service manual of some type.

One thing to look at, is there an “empty” pulley in front of or behind the alternator pulley?

There will be two hoses going to and from the radiator. If you found the one from the thermostat housing, it should go to a top corner of the radiator. Check the opposite bottom corner for the return line. It will almost certainly be there.

If it is really getting that hot, it will not run fine indefinitely. That does bring up another possibility. It could be the temp sensor or gauge is faulty. But, if it starts out in the cold position and gradually works it way to pegged, it is “probably” working correctly.

And one last thing. If the car has been significantly overheated, replace the thermostat, period. Overheating can/will damage the spring that opens them and cause them to open at higher than the rated temp in the future.

It could be he overheated it bad once and that has set the thermostat to open at or very near the end of temp gauge scale.


No offense, but if you don’t think your car has a water pump, I doubt you are ready to tackle repair/troubleshooting yourself. I recomend taking it to a shop.

If you insist though, follow the top radiator hose back from the radiator to the “engine”. Where it joins your thermostat will almost certainly be in a little housing there. Replace it with a new one or just take it out. If that doesn’t cure the problem it is more likely than not your waterpump. Is there another coupl of “pulleys” that looks like a belt should be running on?

If your car is overheating and you keep running it, you can and well cause severe damage to your engine. This includes everything from a warped head to outright faliure.


It could very well be that there’s a pinhole in one of the hoses that’s spraying coolant and it only happens when the engine temp gets high enough to cause it. If the hoses on the car are over five years old, its a good idea to replace them, since you don’t know when they could go out. Also, if the temp gauge gets pegged, either shut the engine off, or turn the car heater on. You’ll blow a head gasket sooner or later and those things ain’t cheap or easy to replace.

Ahem. Former Justy owner here.

The water pump on the Justy is gear-driven directly, and that is why you see no belt. Unless you have the A/C option, there should only be one belt driving the alternator, and that’s all.

Well, tells you that I know absolutely nothing. Apologies are in order. I’ve never worken on a gear driven waterpump other than on my bike.

bernse = dumbass

Couldn’t it be a bad radiator cap? If the cap goes out, you can’t maintain pressure in the radiator, so the coolant boils off.

If I’m right, the repair costs $10 and takes only a minute.

Not likely.

Even a completely missing cap would not cause the temp to climb to the top of the scale quickly. A cap only comes into play (for the most part) once the coolant has reached its boiling point.

Thanks to Anthracite for the gear driven water pump info. That mean pretty much one thing, if the engine is turning, it is extremely likely the pump is turning as well.

Normally, as the system heats up and expansion occurs, coolant goes from the radiator to the reservoir, raising its level some. As the system cools down, its pressure drops (it can produce a vacuum) and coolant goes back into the radiator, lowering the reservoir level.

OP points 2&3: With some system leaks, the reservoir level will drop significantly as its fluid replaces what has been lost. Sometimes, though, instead of sucking fluid from the reservoir, the system will suck in air through the leak. (Thus to check the level in the system, it’s necessary to look in the radiator–COLD ENGINE ONLY–normally the fluid will be up to where the bottom of the radiator cap goes.) It’s possible that your system is acting this way, sometimes drawing from the reservoir and sometimes not.

OP point 4: A cooling system pressure test will find most external leaks. In tricky cases, flourescent dye and ultraviolet light inspection can help.

OP point 1: To be certain the gauge reading is right, measure the temperature of the engine, preferably where the gauge’s sending unit is located. This may not be feasible without special tools (infrared thermometer or contact pyrometer probe). Assuming the gauge is right, I’d like to know precisely what a “few minutes” is. If it’s 2 or 3 minutes, I would suspect a blown head gasket to get to max on the gauge that quickly. If it’s 8 or 10 minutes, I’d suspect a flow problem–faulty thermostat or significant air pockets.

A few thoughts on the thermostat: all modern cars I know of have a thermostat rating of 195 degrees F. A somewhat imprecise but fairly effective way to check one is to drop it into boiling water–it should open right away. Do this for your own curiosity–as previously mentioned, it should be a pretty inexpensive part and I would not trust one that has been heat-stressed to work consistently.

Overall thoughts: if you’re lucky, the heart of the problem is leakage. Find and fix the leak(s), replace the thermostat, refill the system making sure there are no air pockets, and it may work fine. Of course, it’s possible there are further problems, but the above is at least a good start.

I’m just going from memory, but I’m almost positive there is only 1 big fat hose going from the thermostat to the radiator. The other hose goes from the other corner into the reservoir. Tomorrow I will get on my hands and knees and look for a hose that goes underneath the block somewhere.

And Bernse, apology accepted. I know fairly well how to navigate my way around an automobile. FWIW I replaced the starter and unseized the motor in this undestructible POS already. But I never heard of a gear driven water pump either. No blood, dude, peace.

Unless the car is of some very unique design, the radiator must have two large hoses----the hot water pours in from the engine, through the top hose. Then, it flows downward through the radiator, which is cooled by the rushing air. The cooled water exits the radiator at the bottom hose, where it goes back into the engine, and the process starts again. (It also makes other detours along the way, but you get the basic idea.)

Gary has you on the right track here.

If you suspect the thermostat at all, just replace it, as they are generally dirt cheap. (The gasket is often sold separately; don’t forget to buy one.)

How about the car’s heater; does it work? The heater is just a miniature car radiator–called a heater core-- mounted inside the car’s dash, with a fan blowing across it.

Generally speaking, if the heater is blowing cold, the car is low on coolant, or the water pump isn’t pumping. If it’s hot, you probably do have coolant and flow.

But first things first: check the coolant level.

No, aside from its gear-driven waterpump and its balance shaft (needed for the 3-cylinders), the Justy engine is a rather conventional design. I replaced a radiator on it, and it was just like any other radiator I’ve ever seen. I’ve replaced a thermostat, heater line hoses, and all of the cooling hoses, in fact, and it’s just like any other car. Aside from its freaking stupid computer controlled carb versions, it’s normally a rather easy car to work on.

Replace the thermostat, filled reservoir , everything runs A-ok now. Gas mileage is back to like 50 miles per gallon and acceleration is neck and neck with a Jeep Cherokee (I raced it today in the streets, hehe). Thanks for the info everyone.