Hi, I’ve got 600 miles on my engine after completely rebuilding It. After about 40 miles it starts overheating at slow speeds or idling. If I rev the engine, it cools down.
I put a new asin water pump in and new radiator hoses during the rebuild. Since the overheating problem, I have put a new radiator in and a new Subaru OEM thermostat.
When I start the car cold, it gets up to normal temp. I can feel the lower radiator hose get hot when the thermostat opens. Both fans turn on at 203 degrees. The heater blows very hot air. Driving at a steady 60mph, the temperature fluctuates between 190 and 220. The hottest I’ve let it get is 135.
After it starts overheating, the lower radiator hose is cold. I occasionally hear water churning in the dash which makes me think there is air trapped in the cooling system.
I have put a spill proof funnel on the radiator with the front of the car elevated with the heater on low, let it run until the radiator fans turn on and off twice, rev the engine a few times, and squeezed the radiator hoses. I’ve done that at least 5 times and am not getting any more air bubbles.
I don’t know what to try next. Any ideas?
When it warms up (but before overheating) does the system build pressure and make the rad hoses get firm?
If it doesn’t build pressure you can get boiling and steam bubbles.
A new rad cap came with the new radiator, I presume.
A bad head gasket is a possibility, even though new.
Did you replace the head bolts with new ones??
Did you have the heads machined?? Or at least check or properly for warpage?
I’m not sure if it’s building pressure. The hoses are new. It does feel like they get firmer as the car warms up. I did replace my radiator cap , I also put the old cap on to make sure the new cap wasn’t the problem. Today when I parked, the temp got up to 130. I shut the car off and could hear and feel bubbling in the top radiator hose. The overflow tank rises by about a liter when it’s hot, and goes back to normal as it cools. I don’t think I am losing any coolant.
I did reuse the head bolts, putting them all back in their original holes. The block and the heads were machined and cleaned by the most recommended (and expensive) shop in town. They came back looking brand new. I used mls fel-pro head gaskets and followed carefully the tightening sequence in the factory service manual.
Make sure the thermostat is installed right side up and check your belt routing. Some belts can be put on wrong and the water pump can spin backward. Too late to use new head bolts.
If you pull the Thermostat to check for proper installation; before reinstalling it, drill a 1/8" hole in it. The hole facilitates refilling the cooling system as it lets any trapped air in the block to bleed out…
Aren’t those torque to yield head bolts???
Did you not read anywhere that said do not reuse head bolts??
Any chance your machine shop might have mentioned that you must replace the head bolts due to one time use only??
Machine shop said it’s ok to reuse head bolts because they are not tty. They did give me some torque grease to put on the head bolts. They went in smooth and I was confident it was a good install.
I double checked the thermostat is installed correctly with the spring up. The OEM Subaru thermostat has a bigger spring and bigger jiggle pin. I thought the Subaru thermostat fixed the problem because after I installed it we drove almost 200 miles before it started overheating again.
I think the belt is correct. Half the time it idles normally, the fans come on at 203 and the temp drops back to 180 and the fans stop.
I cannot believe that head bolts on a 2002 Forester are not torque-to-yield, and even on older engines where the head bolts were not intentionally TTY, the service manuals always recommended new head bolts. For such engines, the service manual would typically suggest to replace the head bolts, however if you measure the existing bolts and they are less than a certain length, and the threads are all on a straight line, then you can reuse them.
I like saving money, but not on something like that. A set of new bolts costs less than $50. My time is more valuable than that.
It sounds like you have inadequate flow through the cooling system, which could be caused by a plugged-up radiator, plugged-up heater core, or too much air (or possibly combustion gases) in the system. Is the water pump turning in the proper direction?
Everything I have read including parts houses including rockauto states the head bolt must be replaced… I am no expert on Subarus, but TTY head bolts have been around a lot longer than 2002…
Maybe @Tester, who has evidently replaced many Subaru head gaskets, can confirm or deny if it calls for TTY or not…
You should not be hearing the “churning in the dash” sound.
That is a sure sign you still have air in the cooling system. Likely in the heater core.
Maybe after you burp it, air is getting back in the system. That would likely be from the combustion chamber. Do an exhaust gas analysis test on the coolant.
The only way for the lower radiator hose to be significantly cooler after engine is fully heated up is that no coolant is flowing from the block through it or if there is a malfunctioning bypass in the system.
Try burping the system again…this time without raising it since the place where you put your funnel is already the highest point in the cooling system
Seems pretty likely the engine rebuild was done correctly and isn’t the cause of the overheating. Assuming that, and presuming there’s no air pockets in the system blocking the flow, the first tests are to make sure
- the water pump is pumping coolant in the correct direction and at the correct flow rate
- the cooling system is reaching & maintaining the correct pressure
- the engine is maintaining the correct air/fuel ratio, and ignition timing
The symptoms are pretty consistent with number two on the list, cooling system is not maintaining the correct pressure.
130 Celsius is 266 F, is that the coolant temperature?
My mistake. I meant 230 F.
I’m going to pressure test the system tomorrow and try to burp it again.
I left the radiator outside while I was rebuilding the engine. I wonder if a mouse or squirrel could have packed some crap in there and now it’s stuck in the block.
I pressurized the radiator and it held 15psi for over an hour. I did a combustion gas test and it was negative. I burped the system a couple more times. Got a few small bubbles, but still overheats.
I got a leak down tester and pressurized each cylinder to 100psi. They were all less than 12 percent, and no bubbles came out the radiator. I verified the belt is on correctly, I removed the thermostat and put it in hot water to verify it opens at 180. I back flushed every passageway I could find in the cooling system, radiator, engine block, heater core, oil cooler. Everything looks clean and correct. I got a small amount of crud from the heater core, less than half a teaspoon.
The timing is at 15 degrees at idle, and afr stays right around 14.7.
I’m stumped. I removed the engine plugs in the cooling jacket to check for any blockage, and verified the water pump propeller is turning with the engine.
Have you used a coolant funnel method?
Yep, funnel hooked up to radiator. I’ve tried it with the car flat on the ground, slightly raised, and on a steep hill.
Original radiator? I’d replace it.
Hi Deegore:
Given all the checking that you’ve done, maybe it’s time to ask about the rebuild.
Earlier you stated:
Hi, I’ve got 600 miles on my engine after completely rebuilding It. After about 40 miles it starts overheating at slow speeds or idling. If I rev the engine, it cools down.
Can you say a little more about what got done during the rebuild?
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Did you have your cylinder heads machined (in case it was warped)? If so, how much material was removed?
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Did you have the cylinders bored with oversize pistons installed? If so, how much were they bored?
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It may be too late to answer this now, but could there anything in the replacement head gasket that might be obstructing coolant passages?
I have very little Subaru rebuilding experience, but here’s an interesting video I found about Subaru cooling system issues with the EJ-series engine. It may interest you.
If you take the temperature of each head, is one head running hotter than the other?
I don’t know if you can even determine, using an IR gun, if one cylinder is running hotter than the other on a given bank, but if you can, and they are very different, that would be very informative.
It seems you’ve already addressed most of the possible causes for the overheating. But what about the cooling system’s pressure cap? Are you certain it is working correctly? If not sure, probably makes sense to purchase a replacement and try that.
You’re using the traditional 50/50 coolant mixture, right? Too much water or too much coolant in the mix can cause overheating. Are you able to peer into the top of the radiator at a time the engine is starting to overheat (open cap when engine is cold only, & wear eye protection of course)? You should see a lot of coolant pouring into the radiator once the thermostat opens. Similar to how much water comes out of a garden hose at full “on”. If you are only seeing a trickle, faulty water pump, water pump turning wrong direction, or coolant system blockage possible.
15 degree advanced at idle seem like more than I’d expect. But I have no experience with this engine. Suggest to double check. Info is often on emissions-label attached to engine or underside of hood.