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02 Impreza RS
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Discussion Starter #1
Hey guys and gals. So I have read through most of the other threads on overheating issues and have read the Everything you Ever Needed to Know About Cooling Systems thread. And I'm still a little unsure to what my problem is.

So here is the problem, my car started overheating a couple of weeks ago on a longer highway drive (about 2 hours). The car's temperature was fine but then after about an 1 & 1/2 hours of driving on the interstate my car started to over heat. The temp gauge would slowly start to raise and then it would go down a little. Then back up again and then back down. When I noticed this I got worried and cranked the heater up to try to dissipate the heat. This didn't work. Then I downshifted to 4th gear and got the revs past 3500rpm and the temp gauge went right back to it's normal level. Then when I put it back in 5th gear it started to overheat again. So after this happened I took the nearest exit and stopped for a little while. I notice the water in my overflow tank was much higher than normal and there were bubbles coming out of it. I sat there for probably 5 minutes and then started the car and drove the rest of the way to my destination without another problem.

Then on the way back from this destination, which would be another 2 hour drive, nothing happened. The car never once overheated. So I didn't worry about it too much since then.

So I had driven the car for 3 weeks and never had this issue again, but all this driving was for an hour duration or less.

Well last weekend I went on another trip that took 2 & 1/2 hours to get there and my car started to overheat again. Just like before, after an 1 & 1/2 hours the car started to overheat. Again if I downshifted the temp gauge would go right back to normal. I pulled over, looked in the overflow tank and again it was higher that normal and had bubbles coming out of it. I waited 5 minutes and continued on my journey without any more problems. Until I was driving back from this destination and this time the car didn't start to overheat until about 2 hours of driving. Again with the same symptoms. As long as I kept the revs up the temperature was fine.

The car is a 2002 2.5RS with 108,000 miles on it. Performance modifications include Cobb spicy cams, full exhaust, intake, pulley, and a lightweight flywheel. There is no leaking, at least that I can see. The headgaskets were replaced about 15,000 miles ago by a reputable dealer with a mechanic that I fully trust. The timing belt was replaced about 8,000 miles ago, along with a coolant flush. Again by the same mechanic.

So can someone please help me figure this out?
Thanks
Sorry for such a long post. Hope it's clear enough to understand.
 

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GF OBS-T, GM RS EJ20, SF EJ20
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Had the same problem.

Scangauge water temp would creep up towards 200* on highway cruise, temp gauge would not move because I pay attention to my gauges - but downshifting & flooring the car would bring temps back down to the 170*-180* range.

I put a BIG Koyo Radiator in & STILL had the same problem.

Then I changed the Crucial Racing thermostat in my car for a stocker & problem was Solved.

Now HW cruise @ over 50 mph will have coolant temps in the 120* range which is not good - so I have to put a piece of cardboard over the radiator ice road trucker style to keep coolant temps up.
 

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2005 Legacy GT Wagon
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1,021 Posts
Had the same problem.

Scangauge water temp would creep up towards 200* on highway cruise, temp gauge would not move because I pay attention to my gauges - but downshifting & flooring the car would bring temps back down to the 170*-180* range.

I put a BIG Koyo Radiator in & STILL had the same problem.

Then I changed the Crucial Racing thermostat in my car for a stocker & problem was Solved.

Now HW cruise @ over 50 mph will have coolant temps in the 120* range which is not good - so I have to put a piece of cardboard over the radiator ice road trucker style to keep coolant temps up.
What temp Crucial did you use?
 

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2001 Subaru 2.5 RS-T BRP
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359 Posts
When you changed your timing belt... did you change your water pump as well?

The only thing that I can think of is your water pump going bad. Given it's belt driven, it could be going bad and when at higher RPMS it manages to work a bit better and actually circulate the coolant. At low RPMs, if your pump is going bad, the coolant will barely circulate, cause it to overheat, and that could explain the excess on the overflow tank and the bubbles..

Just a thought...
 

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01 rs coupe. BRP
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1,558 Posts
To double check the pump take off the cap make sure the system is full run the car with out the cap and watch down the fill hole once the thermostat opens you should see flow.Also once it opens actuate the throttle by hand or have some one else do it the flow should increase with the throttle.

It could also be sign of a bad head gasket if your getting bubbles. If the bubbles are only their when the car is running especially and the higher amount of coolant then normal. Check for oil in the coolant and coolant in the oil.
 

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02 Impreza RS
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Discussion Starter #9
Thanks for the replies. Nope, the water pump was not changed when the timing belt was done. I'll try what you suggested bugman1964. So far I have not seen any oil in the coolant or coolant in the oil. I'm going to be changing the oil soon so I'll know for sure in a couple of days.

As I was thinking about it more last night it seems to me to be an internal leak in the headgaskets. Causing exhaust gasses to get into the coolant which would cause air bubbles and after an 1 & 1/2 hours of driving the air bubbles build up and impede the flow of the coolant. And stopping for 5 minutes allows enough air bubbles to escape to allow the cooling system to function again.
I don't know, does that seem to make sense to anybody?
 

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01 rs coupe. BRP
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yes it makes tons of sense. That why I was curious if the bubbles only came out while the car was running. I forgot to mention in there to check the head gaskets look through the radiator cap hole while it's running. As some one else mentioned you could be boiling your coolant due to a bad rad cap go pick one up for a couple of bucks or have your's tested.
 

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02 Impreza RS
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Discussion Starter #11 (Edited)
No, the bubbles come when the car is shut off. At least that is the only time that I have looked in the overflow tank. Bugman what am I looking for when I look through the radiator cap while it's running? And yeah I was thinking about just replacing the radiator cap since they are so cheap.
 

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01 rs coupe. BRP
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the 2 things your looking for is flow (you'll see the water moving), and also bubbles going through the system. also the bubbles should increase with rpm. Pull the dip stick and look for any white funk on it that will tell you that it's bad gaskets. also crawl under the car and look for oil leaks or coolant leaks. Just cause there aren't marks on the ground doesn't mean it's not leaking. I had a really bad headgasket that was pissing oil but only when I was drivign and never left a drop on the ground.
 

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02 Impreza RS
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Discussion Starter #13
the 2 things your looking for is flow (you'll see the water moving), and also bubbles going through the system. also the bubbles should increase with rpm. Pull the dip stick and look for any white funk on it that will tell you that it's bad gaskets. also crawl under the car and look for oil leaks or coolant leaks. Just cause there aren't marks on the ground doesn't mean it's not leaking. I had a really bad headgasket that was pissing oil but only when I was drivign and never left a drop on the ground.
I just wanted to clarify, but are you saying that bubbles in the system and the bubbles increasing with rpm are a sign of a bad headgasket or are normal?

So far no white stuff on the dip stick. I will be changing the oil tomorrow or friday and I'll get a better look at everything then.

Thanks
 

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'06 LGT
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Odd situation, I'm having the same problem and will be keeping an eye on this to see the result. Although my car doesn't overheat on the highway, but only at idle. And there aren't any bubbles nor sign of a bad hg, it was just replaced.
 

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2000 EJ257 Swapped RS
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548 Posts
Odd situation, I'm having the same problem and will be keeping an eye on this to see the result. Although my car doesn't overheat on the highway, but only at idle. And there aren't any bubbles nor sign of a bad hg, it was just replaced.
If your gasket was replaced recently I would assume that is airlock from when the system was drained. Kind of a messy solution but undo your rad cap when car is cool and let it run and idle until it gets hot. (wrap a rag around the rad cap so you dont make to much mess) Check for flow etc...to make sure your thermostat and water pump are properly working. The system should then kind of "burp" itself by letting the air out the filler tube. You might have to help it out by gently squeezing the hose that runs from the top of the rad to the engine or giving it a little throttle. If the coolant drops to where you cant see it in the top of the rad then you probably had air so just top it off with coolant through the rad cap.
 

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Definetly not air in the system...

I've driven the car without the rad cap to eliminate that case. I also picked up this nifty funnel do dad that had a rad cap atached to the bottom of it.

I'm assumin its a problem with the water pump but was hoping someone would chime in to help the OP and me aswell.
 

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2000 EJ257 Swapped RS
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Definetly not air in the system...

I've driven the car without the rad cap to eliminate that case. I also picked up this nifty funnel do dad that had a rad cap atached to the bottom of it.

I'm assumin its a problem with the water pump but was hoping someone would chime in to help the OP and me aswell.
huh, don't know what to tell ya then hopefully both of you guys find the solution
 

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01 rs coupe. BRP
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check to make sure both cooling fans were plugged back in. either that or possibly the tstat but that is an easy fix on a subaru.May also be a clogged radiator when was the last time your system was flushed? Not just drained and refilled.
 
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