Intermittent coolant fan operation on 2010 Forester

Did you "burp" the cooling system? Purge all the air out, maybe all you have is an air pocket.

Hi there, thank you for the reply, yes I burp the whole system, then go for a drive, as I am driving along the highway the temperature will start to rise up again and I need to burp it again. I have repeated the same process over and over. After I burp it the coolant fans will come on as they are supposed to but then after a drive do not come on until the temperature is up over 100 celsius where at that point even turning the air conditioning on does not seem to lower the temperature..

Sorry, I just realised I may have made that confusing...After I burp it, the fans come on as they should and temperature seems ok, then I go for a drive on the highway, the temperature will start to rise, fans don't come on as they should, then it needs burping all over again. I have to keep repeating the process.

Check radiator gases for hydrocarbons to rule out percolation from blown head gaskets.

I used one of the blok tester kits and that showed no change in the liquid color but was also told they are not 100% accurate, I saw your post about "high-pressure head gasket percolation" in another thread and was hoping you might respond as you are very knowledgeable. Thank you.

I bypassed the heater today and drove it for a while, it seems better but fans still not coming on when they are supposed to, taking a lot longer to heat up but still heated up. I flushed the heater core and made sure there were no blockages in the heater core.

Check fan connectors?

Purge system fully. If unable to, attach plastic funnel to radiator orifice tightly, and fill funnel half-way with water. Warm engine fully, topping funnel as necessary. Turn on AC to get fans running (hopefully). Watch funnel's large "puddle" surface for bubbling gases once fully warmed and purged. If bubbling continues unabated, that's when you use a hydrocarbon sniffer to determine if you're just purging vestiges of air vs actual percolation of combustion gases through head gasket leak into coolant. If the the latter you're looking at a $1.5k HG job, as no amount of manipulation (open t-stat, fans wired on all the time, or other tricks) will work.

I have checked the fan connectors, they seem fine. Thank you so much for the advice. I have done the funnel thing you just explained but still get a few bubbles, they are not frequent though.the bubbles will come up every couple of minutes or so, I will try a hydrocarbon sniffer. Thanks again.

I tried burping the system again today, this is happening after trying to burp it for 30 minutes or so. Does this look like trapped air or could it be head gasket bubbles? thank you again for the help. https://youtu.be/IyleToUF1JY https://youtu.be/fYvAy5jTstEburping

Could be either. Site wouldn't load.

https://youtu.be/IyleToUF1JY

that should work

This issue gets more and more bizarre, I have a spare temp sensor laying around so I plugged it in and heated it up with a hairdryer whilst holding it in my hands, the fans come on perfect every time when they should when doing this, I tried this 20 or so times. I plugged the temp sensor back into the crossover pipe start the car and again the fans don't come on, it is almost like there is no coolant touching the temp sensor inside, but on my obd2 scan tool, the temperature shows it is quite high and that the fans should be on. I would have thought that if the scan tool was showing the temp that is coming from the ECM then the fans would be coming on at that temperature. I have tried a few different obd2 apps as well that all show the same temperature.

Great differential diagnosis! What's happening is that trapped gases (either unpurged air or exhaust gas percolation) are "stuck" in the cooling system NEAR the sensor, so that it doesn't get hot enough (liquid transfers heat 50x faster than a gas) to trigger the fan system. Chase the gas problem, then eliminate the fan issue.

Thank you so much for the reply. That makes sense.