Have a 2009 Subaru Forester XT 5 speed manual.

The cars temps are running high. 98c while driving at about 100km/h.

Temps will drop to 80c very quickly if the car gets into higher rpms. Ie 4000rpm.

Not sure what could be the issue.

New GMB water pump Thermostat verified working Cheap eBay alloy radiator

Car has been bled.

Additional Info: TO provide some further details about the car;

It had a recent shortblock replacement due to a overheat (Air lock)

The current up to date list of modifications/changes are as follows;

Currently 2.5 fuel tanks old brand new Subaru EJ257 shortblock ARP Head studs Reconditioned heads - Pressure tested and shaved flat. Genuine head gaskets VF 52 Turbo Upgrade (Stock was a TD04) Water pump replaced at time of build eBay spec Radiator (Currently swapped out to a genuine rad, more on this later) PCV Delete - venting to atmosphere for the moment. TGV and Air pump Delete Throttle body coolant ports blocked off

12.07.2018: Today I pulled out the eBay spec radiator and replaced it with a genuine used radiator (I have another project car that is the same except auto). I installed this radiator and my temps have dropped a bit, but I feel something is still not quite right. During my test drive, I noticed while going down hill with acceleration my temps would be approx 90c then drop down to 82c.

Once I began driving along, either straight road or up hill and the temps would climb back up.

Naturally the car will cool down coasting down a hill, especially in a manual with engine breaking spinning the pump quicker, but I put the car into neutral to see if the temps would go down still, they did. This never happened when I had the eBay spec radiator in the car. (and yes, naturally the car will heat up when driving up hill with more load on the motor)

The standard radiator is much smaller and holds quite a bit less then the eBay one.

There were no huge temp spikes during acceleration with this radiator on the test drive, another positive.

I did remember testing my thermostat, and maybe its a bit "lazy"? It didn't fully open till about 85-90c.

I have been considering putting in a lower temperature thermostat into the car for added piece of mind, but I just want to make sure that its not a radiator or flow issue before changing it out.

13.06.2018 Update: Drove the car again, noticed it was low in the coolant overflow bottle. Pressurized the system while cold before heading out. Was loosing a small ammount of pressure, nipped up a hose clamp and have started driving it again. I have now noticed there is only heat on the drivers side in the car, everything tells me its not likely to be a heatercore issue - how can it get only half hot...?

  • Google indicates that the thermostat on that year doesn't open until 196*F, or around 92 C. I'd only be worried if you were hitting 110-120C on a regular basis, and maybe not even then. Coolant doesn't boil at the same temp as water, far from it.
    – 3Dave
    Jul 11, 2018 at 21:56
  • My thermostat starts opening at 78c and when I tested it if was fully open around 85c. I just don't think it's right for the car to see those kind of temps at Highway speeds
    – Booki
    Jul 12, 2018 at 1:06
  • You’re well under 220*F. Those temps are well within spec.
    – 3Dave
    Jul 12, 2018 at 3:47
  • Temps may be in spec, but I have never owned or even heard of a car having thermo fans running when driving at high way speeds. Thats ridiculous in my opinion
    – Booki
    Jul 12, 2018 at 9:25
  • Rockauto indicates that your car's OE thermostat is 170°. I do not think you could find a thermostat which has a lower temp than that. A 170° thermostat begins to open at 170°. I do not know at what temp it will be fully open. Are you using an OEM Subaru thermostat, or an aftermarket brand? (Eg, Carquest, Motorad, Gates, etc)
    – sam
    Jul 12, 2018 at 14:27

3 Answers 3


This Forester owner forum suggests that these are normal temperatures.

Keep in mind that when the car is fully warmed up, the coolant (in the engine) will at least reach the boiling point of water. This is why cars use a 50/50 antifreeze/water mixture; to prevent the coolant from boiling. The engine oil will get even hotter than the boiling point of water. This causes water in the engine oil to evaporate.

You will see temps drop when you hit higher RPMs because the water pump is connected directly to the crankshaft. So for example, if you are idling at 750 RPM, your thermostat will probably be fully open. Then, if you start driving down the road at 3000 RPM, in theory the water pump will be moving coolant 4 times faster than when you were idling. It may take up to 30 seconds for the thermostat to react and partially close, restricting the flow of coolant from the radiator into the engine.

The bottom radiator hose should be cold, or maybe warm on very hot days. This indicates that your radiator is operating correctly: it is successfully cooling the hot coolant which is leaving the engine via the upper radiator hose. The thermostat controls how much of this cold coolant is brought into the engine from the radiator, via the lower radiator hose.

If for some reason your upper radiator hose is cold, this would indicate you may have a problem.

  • 2
    Note to add, the cooling system is under pressure as well to prevent boiling. Both this and the antifreeze / coolant allow temps greater than 100c
    – RemarkLima
    Jul 11, 2018 at 22:29
  • At the kind of temps I am getting I would at least think my radiator would have a warm bottom hose as well. The thermostat would be well open at that time and with the air going through everything the temps should drop. Just seems to strange to me. Other users report temps of mid 80c during highway use
    – Booki
    Jul 12, 2018 at 1:08
  • 1
    Yep, mid 80's is fine... Check the top hose (carefully!) to ensure it's all working properly. Remember radiators are specced for extended periods of max load, so must be able to cope with the engine running full throttle. Highway use is very light loads.
    – RemarkLima
    Jul 12, 2018 at 6:42
  • Yes, mid 80's is what I want. My car is running mid 90's at times with high way driving, unless I start going down hill, the temp swill fall to a good level low to mid 80c, because of the down hill driving I suspect maybe it is a flow issue somewhere. I updated my original post with additional info, I have swapped the radiator to a genuine one (although from a automatic car, but basically the same to me).
    – Booki
    Jul 12, 2018 at 9:43
  • This amateur racing site takes a very scientific look at coolant. Worth a read if you want a better idea of how the water-coolant mixture and pressure of the system raises the boiling point.
    – sam
    Jul 12, 2018 at 14:04

You likely have a partial water blockage in the engine passages. At high RPM, the water pumps spins forcefully enough to push water past the blockage. How often was the coolant changed throughout the life of the car? If not often enough, there is likely scale buildup throughout the system that eventually breaks off in chunks which cause blockages.

I once watched a service video by Gates explaining the best practices for installing their water pumps. It recommended a very thorough flush of the engine block with a pressurized machine.

In my own experience, by the time the water pump needs to be replaced on a car there is stubborn buildup throughout the cooling system (even on a well maintained car). It is most evident in the heater core. I've had new water pump seals fail after just a few months due to tiny particles floating around in the coolant. The solution has been to use coolant flush and dishwasher powder aggressively and drain the radiator five times in a row, then change the coolant again after three months and six months.

  • I wish it was this simple, the car actually has a brand new short block. The coolant always appeared to been have maintained prior replacing the shortblock.
    – Booki
    Jul 12, 2018 at 1:04

I cannot imagine any condition where I would be concerned if the thermostat was operating correctly and the temperatures were within the margins of the operation parameters of the engine.

However, your concern appears to be that there is something weird going on so that you are getting uneven cooling through the radiator. I also cannot imagine a case where the temperature of the water hoses will be much different upper to lower. Both should be hot if circulation is occurring and the water pump should be circulating the water unless the engine is not running, but possibly not through the radiator or does not reach operating temperature. It is not likely that it could go elsewhere without you seeing signs of it so the entire radiator should be nearly the same temperature as the engine - just a little less (assuming not severely cold weather).

According to later comments, a new short block was installed with a new water pump. I would assume that a new thermostat was also installed - the difference in the operation of the new thermostat could explain any difference in the performance that you are seeing. However, as long as the engine temperature is not out of the range spec'd for it, there is really nothing to be concerned about unless you are aware of other things that the responding commenters are not aware of that gives you reason for concern.

In any event, the entire cooling system should achieve nearly the same temperature as the engine within a small difference if the operating temperature is achieved and it should do it within a short length of time. If it is severely cold ambient temperatures you might experience some differential top to bottom, but otherwise not. The radiator system is designed to cool the engine and to make sure that it gets hot enough at low ambient temperatures - like -40 degrees F. And likewise that it remains no higher than the upper limit under pressure so that coolant is not lost to boiling overflow.

If you are really concerned, you might try impeding the flow of air over a portion of the radiator to determine whether or not the engine will begin to overheat by running the engine at 1500 rpm to when the thermostat opens - I would be more concerned that the temperature drops to 80 degrees C after having been at 92 - it suggests that the thermostat is sticking open once it opens and not responding to the temperature changes. If the 80 reading is within specs, you have no recourse. You might try another new thermostat and see if it responds the same way.

  • Thanks for the comment JDubya, I have updated my original post with some more information about the car. I am indeed worried about uneven cooling throughout the car. In my climate (Australia), It is our winter at the moment, average max temps are about 12c. Its rare we get any temperatures lower then 0c. I was quite concerned that with some acceleration the car will spike from 90~c to 80c~ quickly. You may be right that there is a issue with a lazy thermostat.
    – Booki
    Jul 12, 2018 at 9:36
  • From other forums and members I have spoken too, most people seem to get mid to low 80c temps while driving at high way speeds. Stop start normally low 90's. Mine is hitting the stop start tempretures at the high way, its not ideal I would say. While the temps are nothing to really worry about in terms of how the engine was designed to run, I do want to preserve the longevity of the engine and run it ideally around 80c. Perhaps I will look into a thermostat that opens sooner.
    – Booki
    Jul 12, 2018 at 9:40
  • Really, Booki, you should not see much change in the temperature (on a standard gauge anyway) speeding up or slowing down, etc., the engine is large enough that it should be able to maintain at least the normal winter (low) temperature. The speed of the water pump's affect on the temperature is almost minuscule compared to the shear mass of the water. When you speed up the engine, the water just bypasses depending on where the thermostat is located (open/close).
    – JDubya
    Jul 12, 2018 at 20:59
  • The only temperature variation that you should see is from the overload or underload caused by speeding up and slowing down and it should be quite small. If others are seeing 80c temps and the thermostat is designed for 92c open and the weather is cold near 0c then the engine is being operated too cold - so much so that the thermostat is having inconsequential effects - it is closed totally and the engine cannot heat the coolant sufficiently to satisfy heater load and air blowing by.due to motion.
    – JDubya
    Jul 12, 2018 at 21:00
  • It suggests that you have too much coolant for the size of the engine. The spiking that you mention is the wrong way. The water pump has little effect when the engine speeds up if there is a thermostat because the engine is not getting cooled water, it is circulating already heated water. If the thermostat is slow to react, the temp should rise, but it might be slow to close (held open somehow) causing the temp to drop, but it should rise up again unless the thermostat is faulty (stays open). Maybe your engines are just so much smaller than I am used to.
    – JDubya
    Jul 12, 2018 at 21:09

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