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1999 Forester S MT
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Discussion Starter · #1 ·
99 forester s, 5 spd, 252k miles

overheats under specific conditions—hot outside (90*+), cruise set above 80 (I tend to run cruise at 83/84), and A/C on. Normal commute has some pretty big up and down hills, only has issues going up hills with above conditions met. Started last week. Changed tstat thinking that was issue. Bled cooling system several times as instructions here mention. Happened once a year ago, thought it may have been bad gas, but this is same issue with more persistence.

A/C works pretty well for cooling. I did head gaskets, timing, tstat, water pump (cast type with solid fan vs stamped steel), which then required a new radiator (that was previously cracked before I got car), all at 195k miles.

Around town, under 80 mph, morning commute, or when temp is only in mid 80’s—doesn’t seem to have any problems, temp gauge sits touching left edge of two little lines on image. Even towed a trailer full of dirt around for several miles and up some hills at residential speeds with no issues. I’ve had a headwind during commute that is strong enough to kick cruise control off climbing 4 different hills with A/C on (and off) and no overheating (but cooler outside temps).

Today I turned off A/C when it started to overheat (took a bit, it had cooled off a little outside by time I left work), it cooled down and temp never moved again (even up hills at same speed).

I’m stumped. Any ideas?
 

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2001 Forester S, 4EAT
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IMHO it's your cruising at 80+mph. That puts you in the 4000 RPM range. That's more that what Subaru designed the vehicle for.
 

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2001 Forester S, 4EAT
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2,901 Posts
There was another post by someone else a while back. Same issue as yours, just slow down. The other owner wanted install extra oil coolers, better radiator so he could drive across the US at those speeds carrying lots of cargo. The fact you noticed it's not happening when you slow down says the cooling system is working and not defective.

Have you checked for head gasket leaks again? My only other suggestion would be to do what the other poster asked about which is put in an additional oil cooler.
 

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1999 Forester S MT
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11 Posts
Discussion Starter · #5 ·
I’ve considered upgrading to a wrx radiator. I’ll drive at 75 mph and see if it happens this afternoon, supposed to be hot today (98*).
 

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2001 Forester S, 4EAT
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Where about are you in the US? I have driven my 2001 from Philadelphia to Austin in 2001 towing a 3300 lb trailer, and at highway speeds it would stay in the 3500 - 4000 RPM range and never make it into 4 gear on the 4EAT, ie would stay in 3, if it was a down hill slope it would pop into 4th gear, I ended up keeping my speeds low enough to keep the revs down in the high 2000 or low 3000 RPMS. And 98F is nothing. Here in TX we are fast approaching 100F in 1-2 weeks time. The last time my son and I drove from DFW to Amarillo it was probably in 98F range as well, but we never drove past 65-70 mph even though other drivers were definitely doing 80+. The drop in MPG for us just was not worth it. Speed limits on the road we drove were 75 MPH. Hey it's an old vehicle designed and built during the tail end of 55 MPH nationwide speed limit.
 

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2012 Forester 4 speed auto
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Going 85 mph isn't that hard on a cooling system since air is traveling through the radiator and passing by the engine block. Your themostat may even be closing to keep the engine above 170F. However, going up steep hills at 85 mph is expecting too much.

Since the engine is overheating I suspect the radiator is partially blocked. Just touch areas of the radiator with your hand when the engine is off and look for cold spots.
 

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2008 LL Bean (4EAT)
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4,995 Posts
... The other owner wanted install extra oil coolers... My only other suggestion would be to do what the other poster asked about which is put in an additional oil cooler.
The other owner had installed a transmission fluid cooler in front of the radiator. In spite of partially blocking the radiator with another hot fluid radiator, he felt that reducing the temperature of the ATF was the best way to reduce the temperature of the coolant.
I think coolant temperature is best handled by the radiator. For overheating problems, I would look first to the condition of the cooling system. If the stock radiator is inadequate, I would replace it with a larger one rather than add an ATF cooler in front of it.
 

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1999 Forester S MT
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Discussion Starter · #11 ·
Drove home with cruise set to 75, ran ac whole trip, temp gauge never twitched, outside temps were 90-92*
 

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2001 Forester S, 4EAT
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Since yours is the 5 MT, your radiator is only doing 1 thing, "Cooling engine coolant". Unlike the 4EAT vehicles that have to also cool the fluid from 4EAT. Does you also have the oil cooler sandwiched between the oil filter and engine block? That engine oil is also cooled by the engine coolant as well. So the only thing you can really do to enhance cooling is to add an extra external oil cooler, to try and enhance overall cooling. But a better question is do you have a OBDII scanner that actually gives you a digital display of the coolant temperature vs the Subaru idiot needle gauge?
 

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2009 FXT
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95 Posts
Have you ever replaced the rad cap? Bad rad cap or none Subaru OEM rad cap can cause this.
I had a 2005 Impreza with over 400,000km, when I would travel at 140km/hr, after about 30min the temp would rise. Slow down temp would drop.
New rad cap fixed it.
 

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2008 LL Bean (4EAT)
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4,995 Posts
... temp gauge never twitched...
The temperature gauge is not a true gauge. It is an indicator, programmed to hold the needle in one place through the wide range of operating temperatures, and will not move until overheating begins. To see the actual coolant temperature, a digital gauge connected to the OBD2 port is needed. Here is a ScanGuage mounted in my Forester for the past ten years.

555577
 

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1999 A/T - 235,000 mi. WA state
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890 Posts
The temperature gauge is not a true gauge. It is an indicator, programmed to hold the needle in one place through the wide range of operating temperatures, and will not move until overheating begins.
That's a wow for me! But you've got the data set-up to know ...
:oops:
 

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1999 Forester S
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530 Posts
That's more that what Subaru designed the vehicle for.
Hogwash... You are aware that Subaru set a 24-hour land speed record with very similar, as-delivered-to-customers hardware in the early '90s, right? Modern vehicles are tested, during summer, in Death Valley and get driven up and over the Eisenhower Pass under full load. Driving down the road at 80 mph with the A/C on falls WELL within the design limitations of OP's Forester.

OP, check your A/C condenser and radiator for caked-on dirt/bugs/grass/etc. Perhaps check that your cooling system is properly burped by lifting the front of the vehicle off the ground with the engine running, radiator cap off, and wait until the thermostat opens.

Did you replace the thermostat with an OEM part? Your car should NOT be overheating under these conditions. Either something was missed during your HG repair, or there is a damaged/failed component contributing to this.
 

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2001 Forester S, 4EAT
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@soobcrazy "Hogwash?", we are not aware of the exact temperature, the OP is only basing his "overheating" judgement on the needle gauge on the dash. I have asked once myself and also someone else has pointed out, that the OP needs to install some sort of OBDII device/gauge to see exactly what temperature the coolant is reaching. So @soobcrazy please calm down until we get some more concrete facts.
 

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1999 Forester S MT
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11 Posts
Discussion Starter · #18 ·
Unfortunately no obdII to plug in and get real time readings.

I consider it overheating when needle moves past the top of the little temp symbol towards hot and pushes fluid out into overflow tank.

I’ve replaced with oem tstat and still have issue. As stated it only does it under conditions above, and I did head gasket and related a few years ago. No other signs of head gasket issues.

I haven’t jacked front of car up, but did park it on our decently inclined driveway with cap side as high point, with heater on low and ran it without cap until it was at temp before topping off fluid. Still got hot next day.

I have considered it may be related to ac and heat soak of some sort, the radiator is only about 2 yrs old. I’ll glance at both for blockages, and clear any I find. Most definitely can tell when ac compressor kicks on and off.
 

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2001 Forester S, 4EAT
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Unfortunately no obdII to plug in and get real time readings.

I consider it overheating when needle moves past the top of the little temp symbol towards hot and pushes fluid out into overflow tank.

I’ve replaced with oem tstat and still have issue. As stated it only does it under conditions above, and I did head gasket and related a few years ago. No other signs of head gasket issues.

I haven’t jacked front of car up, but did park it on our decently inclined driveway with cap side as high point, with heater on low and ran it without cap until it was at temp before topping off fluid. Still got hot next day.

I have considered it may be related to ac and heat soak of some sort, the radiator is only about 2 yrs old. I’ll glance at both for blockages, and clear any I find. Most definitely can tell when ac compressor kicks on and off.
1. So we are not working with accurate data. Only based on your judgment that it is over heating.
2. Fluid into overflow tank is also normal. Just driving around here in DFW in 45 mph speeds causes my 2001 to push fluid into overflow tank. Coolant expands!!!!


Let's get some accurate data!!!!!
 

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1999 Forester S MT
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Discussion Starter · #20 ·
Does your overflow tank fill all the way up and leave the radiator low after temp climbs above normal operating range in your driving? I’ve had to pour a good quart of fluid back into radiator from overflow

While I don’t believe the gauge is 100% accurate, I do believe it is a reasonable measure of the cooling system if it normally runs at 170* (as per manufacturer), and it has a specific location the needle rests at, then any variance in that position either up or down (yep went bad one winter and was lower than normal, was a cold trip with pauses every 1/4 hr to warm up) would mean a deviation from the specific temperature the tstat maintains. When the gauge is only three hash marks from the H on the top of the gauge, I’d consider the car to be overheating.

However the fact that all all other times the gauge looks like it’s glued in place for it’s normal operating position and temp, when it moves up it indicates to me something is happening to raise the temp of the cooling system—which I then call overheating since its above the normal, and tends to continue rising if I don’t turn off ac and slow car down.

I may try a new cap, has original as far as I can tell.
 
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