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Overheating oddities


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15 replies to this topic

#1 havoc

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Posted 29 August 2011 - 10:27 AM

'97 Subaru Legacy GT 2.5

Has weird overheating issues. Very sporadic. Can be fine for weeks, then start heating 15 minutes after driving. Crank heat to HOT on full blast and only warmish air. When at normal operating temperature sometimes only one radiator cooling fan engages, sometimes both.

There is a huge debate on whether a clogged heater core can cause these overheating issues or not. Some say yes, others say the core is not in the normal operating loop and will have no effect on the temp.

Had the timing belt, water pump, and the head gaskets replaced. Still ran hot. Replaced the thermostat. STILL some overheating issues, but behaves slightly better. Tried burping the system by removing air vent plug, filling overflow to full line, and filling rad to the neck then running the car at 3000 RPM for 10 minutes.

At this point, I'm stumped. Ready to tear out the ECT, radiator, and cooling fans and replace everything. Any thoughts?

#2 Rooster2

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Posted 29 August 2011 - 11:57 AM

Non OEM head gaskets are known to be trouble makers that will seal poorly to cause over heating, starting on a sporadic basis. A cheapie thermostat can cause problems too.

#3 grossgary

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Posted 29 August 2011 - 12:56 PM

is the overheating now the same symptoms as before the headgaskets were replaced (sometimes it's hard to remember, i know)?

radiator might be to blame.

if it's predictable - like driving when it's really hot out it overheats and it never does when it's cold or overheats when climbing steep grades, acclerating fast/heavy loads - then those are signs of clogged radiator - it doesn't have enough capacity to dump the necessary heat.

headgaskets can fail again if there was a glitch in the installation.

what kind of headgaskets were used?
were the heads checked or resurfaced?

#4 havoc

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Posted 29 August 2011 - 01:19 PM

Overheating generally occurs when driving for extended periods of time. Hills also tend to be an issue.

With the fans, is it normal for one to engage and the other not to sometimes? It seems odd that both do not turn on at the same time. Wondering if I should check my fan relay?

#5 Fairtax4me

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Posted 30 August 2011 - 12:28 AM

Only one fan normally operates to cool the engine. The other fan is to give extra cooling capacity when the AC is running. SO it is 100% normal for only one fan to run.

Is the heater core integral to cooling the engine? Yes and no. The heater core is part of the bypass loop that routes coolant from the engine through the core and back via external piping to the water pump. (external to the engine block) This loop is designed to return still warm coolant to the back side of the thermostat. This does two main things. It keeps the thermostat open in cold weather when coolant returning from the radiator can be cooler than the 170 degree opening temp of the thermostat.
It promotes more even heating of the block as the engine warms up. It regulates the temperature of coolant coming into the engine by allowing the thermostat to open only partially, then the cold and warm coolant join and mix at a semi-controlled rate until the coolant in the radiator is warm enough to keep the thermostat open. It prevents a rush of cold coolant into the already hot block when the thermostat opens. Cold water on hot aluminum = cracked block, warped heads, bad stuff in general.

The heater core isn't going to cause your temperature troubles though. Running hot going up hills or on long drives is a typical symptom of a clogged radiator or bad radiator cap.

#6 Quidam

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Posted 30 August 2011 - 01:14 AM

"Crank heat to HOT on full blast and only warmish air." This isn't a diagram of your EJ but same principal. The heads are the hottest part of the motor and the heater core gets the flow from there. Your core behaves like it's clogged, or they act like that with an air bubble. I don't believe this being clogged will cause a motor to overheat. I think a through inspection of the radiator would be a good move. hth

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#7 havoc

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Posted 30 August 2011 - 08:44 AM

OK, did a little work on this.

Removed air vent plug, filled rad to very top of fill neck, filled overflow to "full line"

Ran engine for 10 minutes idle, heat at high and blower fan on lowest setting, no overheat. Heat blew HOT, as in so hot you couldn't keep your hands in front of the heater.

Thinking that there was air still trapped in the system after replacing the thermostat. When refilling the coolant I did not run the heat, which I now believe caused air to build up in the heater core since no coolant was running through it.

I do not believe that there is something wrong with the rad. Top and bottom hoses were both HOT when the temp got within normal operating limits. Felt rad and no cool spots, so there I do not believe there are any clogs.

#8 Rooster2

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Posted 30 August 2011 - 09:15 AM

I am still thinking a bad head gasket. With Subaru, it will idle all day with a bad head gasket, and not over heat. Take it out on the highway, or run up hills, and the added stress allows exhaust gas to bleed into the cooling system forming a large hot bubble of air. It is this air bubble that won't allow coolant to circulate through the cooling system, so you get poor heat from the heater, and coolant over heating with engine over heating.

If you replaced the thermostat with a high quality unit, then unfortunately, it is prolly your head gasket causing trouble.

#9 Quidam

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Posted 30 August 2011 - 03:02 PM

OK, did a little work on this.

Removed air vent plug, filled rad to very top of fill neck, filled overflow to "full line"

Ran engine for 10 minutes idle, heat at high and blower fan on lowest setting, no overheat. Heat blew HOT, as in so hot you couldn't keep your hands in front of the heater.

Thinking that there was air still trapped in the system after replacing the thermostat. When refilling the coolant I did not run the heat, which I now believe caused air to build up in the heater core since no coolant was running through it.

I do not believe that there is something wrong with the rad. Top and bottom hoses were both HOT when the temp got within normal operating limits. Felt rad and no cool spots, so there I do not believe there are any clogs.


Just some thoughts. The coolant goes through the heater core all the time. There is no valve to shut it off. What heat you get in the cabin, that's controlled by a flapper door for air movement.

You can check the temperature drop on the rad with a heat gun. You should see a 20 degree or so drop in temp. Comparing the coolant flowing into the radiator on top, and what's being suctioned out on bottom by the water pump.

Doug

#10 nub

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Posted 30 August 2011 - 08:15 PM

Hi,

'scuse the lurker post - I've been reading these forums for ages but never really posted as I've found all I need in the search :)

Anyway, can't help noticing your issue sounds similar to my old 92 Legacy 2.2. That would also overheat occasionally, like when overtaking and climbing up hill. Only when it had to work though, I could drive around town all day with no problem.

In the end it turned out to be the heater core. Finally manged to diagnose it by locating the pipes from the engine to the heater core, disconnecting them, and hooking up a hose in a u shape between the engine outlet to the heater core and the return. Drove it like this for a week and the issue went away. Eventually pulled the heater core out (what a pain!) and it was 50% blocked.

Can you do the same on the 2.5 easily? That would be a fairly easy way to rule out your heater core I think.

Cheers,

nub

#11 nipper

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Posted 30 August 2011 - 09:02 PM

Have you ever replaced the radiator cap?

#12 Quidam

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Posted 30 August 2011 - 10:50 PM

Hi,

Anyway, can't help noticing your issue sounds similar to my old 92 Legacy 2.2. That would also overheat occasionally, like when overtaking and climbing up hill. Only when it had to work though, I could drive around town all day with no problem.

In the end it turned out to be the heater core. Finally manged to diagnose it by locating the pipes from the engine to the heater core, disconnecting them, and hooking up a hose in a u shape between the engine outlet to the heater core and the return. Drove it like this for a week and the issue went away. Eventually pulled the heater core out (what a pain!) and it was 50% blocked.

Can you do the same on the 2.5 easily? That would be a fairly easy way to rule out your heater core I think.

Cheers,

nub


Hey,

If your car "really" overheated, that would indeed imply that the heater core is vital for cooling the car.

The "hottest it's going to get" coolant coming from that tube would be redirected and problbly influiences the reading on the temperature sending unit. That's what it looks like to me, a possibility.

But if your car really "overheated", I can't see how that could be true.

Doug

#13 Quidam

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Posted 30 August 2011 - 10:56 PM

Hi,

Anyway, can't help noticing your issue sounds similar to my old 92 Legacy 2.2. That would also overheat occasionally, like when overtaking and climbing up hill. Only when it had to work though, I could drive around town all day with no problem.

In the end it turned out to be the heater core. Finally manged to diagnose it by locating the pipes from the engine to the heater core, disconnecting them, and hooking up a hose in a u shape between the engine outlet to the heater core and the return. Drove it like this for a week and the issue went away. Eventually pulled the heater core out (what a pain!) and it was 50% blocked.

Cheers,

nub


Hi nub,

If your car "overheated" with the heater core out of the loop, that implies that it's vital for engine cooling.

I'm thinking the "hottest it's going to get" coolant, once diverted past the temperature sender, is giving a "false" reading.

But if it "really" "overheated" your car, that can't be true.

Doug

#14 nub

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Posted 30 August 2011 - 11:44 PM

Yea admittedly, it doesn't make a lot of sense to me either. The car definitely got really hot without blocking it off though, enough to boil the radiator and send it out the expansion tank and the overflow. It was only because I keep an occasional eye on the gauge that saved it from cooking.

I only mention it because its easy enough to check, and I know the problem confused the hell outta me until I did it.

Maybe it being blocked traps a big air bubble in there also? I do remember hearing water sloshing around when turning (indicating air in there) despite 'burping' it several times.

To be honest I hope it isnt that, because taking out the heater core was a real pain in the a$$.

#15 havoc

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Posted 04 September 2011 - 08:54 PM

So, I've been driving it for 5 days now and I think the issue has been resolved.

I do not believe I added enough coolant to the system. Apparently, when filling the coolant it is necessary to fill it to the very top of the filler neck.

So, what I did was open the air plug (top screw on the passenger side of the radiator) and added coolant to the very top of the radiator fill neck. Then filled coolant to the "full" line of the overflow tank.

Then ran the engine, revving it to 3000 RPM for 5 minutes until fan engaged. While doing this ran the heat at max with blower on 1. Then let the engine cool back to normal levels. Opened the rad cap and overflow tank and checked the levels. Both were at normal.

Drove it at least 100 mi in the 5 days. City, highway, suburban, steep incline, fully loaded, pretty much every instance of driving. No overheating. So, hoping it's been resolved.

Moral of the story: make sure your coolant is at full capacity when filling and make sure you have the air plug open when doing so. Any bit of air seems to cause weird issues which are difficult to diagnose.

#16 nipper

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Posted 04 September 2011 - 09:11 PM

I've never beleived the heater core theory myself, I just think those people never ever got all the air out of thier system.




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