jonathan909 Posted February 8, 2023 Share Posted February 8, 2023 This one's odd. I did a rebuild on it a year or two ago (spun a rod bearing), it wasn't running well after I put it back together, it sat for a bit, and it finally settled down after I stuffed in a pair of fresh O2 sensors. After that, it ran great through the spring, summer, and fall. Then it started acting up in a strange way: The engine temperature isn't as stable as usual, and though it was running a little on the cool side, it overheated on my girls on a day ski trip to the mountains. The first and easiest thing that sprang to mind that might be causing this was a thermostat, maybe not staying closed tightly when it should (hence running cool), but maybe being generally sticky (which might cause both the under- and over-temp conditions. So it got a fresh OEM 'stat, but has still been running cooler than normal. What next? The pump? There's no evidence of leaks, and when it overheated the coolant loss was through the overflow jug. I'm a little puzzled. Quote Link to comment Share on other sites More sharing options...
idosubaru Posted February 8, 2023 Share Posted February 8, 2023 Headgasket. check other things for proper function. Look for bubbles in the overflow while it’s running or after turning the engine off. If you had the engine apart it seems like bad water pump vanes would have stood out. Quote Link to comment Share on other sites More sharing options...
azdave Posted February 8, 2023 Share Posted February 8, 2023 Do you know for sure it is running cool or is that just what the gauge tells you? I've had sensors that looked fine but had moisture inside the plastic housing giving me all kinds of erroneous readings. It's obvious your engine got hot when it boiled over but after you changed the thermostat you say that is no longer an issue. So how do you know it is cooler than normal unless you have a second data point to confirm? In my case, I got a cheap remote BBQ thermostat temp gauge and taped the sensor tip around the heater hose. While driving, I could see the heater hose temperature rise and stay fairly steady while the dash temp gauge wandered all over the place during a 30-minute drive. A gauge like this is what I used. https://www.amazon.com/ThermoPro-Wireless-Digital-Thermometer-Grilling/dp/B014DAVCP4/ref=sr_1_50?crid=36MF7MSFMYGK&keywords=remote+temperature+monitor+bbq&qid=1675857591&sprefix=remote+temperature+monitor+bbq%2Caps%2C112&sr=8-50 1 Quote Link to comment Share on other sites More sharing options...
lmdew Posted February 8, 2023 Share Posted February 8, 2023 My 2000 Impreza will not handle a heavy load when it's hot. Towing or mountain driving. Otherwise the temp is steady as can be. I've replaced the radiator with a full aluminum radiator. Thermostat replaced. Water Pump is good. I'm going to flush the coolant system this spring. Run cleaner in the engine a time or two. Quote Link to comment Share on other sites More sharing options...
jonathan909 Posted February 8, 2023 Author Share Posted February 8, 2023 3 hours ago, idosubaru said: Headgasket. check other things for proper function. Look for bubbles in the overflow while it’s running or after turning the engine off. If you had the engine apart it seems like bad water pump vanes would have stood out. Like most of us, I've had my share of bad HGs, so I rather doubt that's the case, but I'll double-check. Quote Link to comment Share on other sites More sharing options...
jonathan909 Posted February 8, 2023 Author Share Posted February 8, 2023 3 hours ago, azdave said: Do you know for sure it is running cool or is that just what the gauge tells you? I've had sensors that looked fine but had moisture inside the plastic housing giving me all kinds of erroneous readings. It's obvious your engine got hot when it boiled over but after you changed the thermostat you say that is no longer an issue. So how do you know it is cooler than normal unless you have a second data point to confirm? Daughter is reporting the cabin heater (normally very efficient) is blowing cooler than usual as well, which just adds to the weirdness. BBQ thermometer an interesting approach, though. Quote Link to comment Share on other sites More sharing options...
jonathan909 Posted February 8, 2023 Author Share Posted February 8, 2023 2 hours ago, lmdew said: My 2000 Impreza will not handle a heavy load when it's hot. Towing or mountain driving. Otherwise the temp is steady as can be. I've replaced the radiator with a full aluminum radiator. Thermostat replaced. Water Pump is good. I'm going to flush the coolant system this spring. Run cleaner in the engine a time or two. Yeah, but it's winter here too, so obviously there's no towing involved, and since we live in the foothills, a day trip to ski in the mountains isn't any kind of serious load - I'm not talking about the middle of summer with a full load of passengers and camping gear and towing a boat. Quote Link to comment Share on other sites More sharing options...
idosubaru Posted February 8, 2023 Share Posted February 8, 2023 2 hours ago, jonathan909 said: Like most of us, I've had my share of bad HGs, so I rather doubt that's the case, but I'll double-check. I've seen those exact symptoms but of course rule out simpler things. Check coolant levels, I've seen cool running due to inconsistent coolant levels...though only in EJ25's with known headgasket issues though no overheating (external leaks). Can the fans run at the wrong times (flaky fan sensor?) causing over cooling sometimes and overheating others? Never seen it so it's just a wild guess. Quote Link to comment Share on other sites More sharing options...
1 Lucky Texan Posted February 8, 2023 Share Posted February 8, 2023 clogged heater core? the flow thru it is important for t'stat function. a piece of tubing or 2 and some hose and clamps and you could 'bypass' the heater core for testing purposes I guess. Quote Link to comment Share on other sites More sharing options...
89Ru Posted February 9, 2023 Share Posted February 9, 2023 +1 head gasket. Less heat in cabin. Coolant overflow tank rising. Temperature fluctuations. Listen for gurgles in the dash. Exhaust gas in the coolant. All +signs. 1 Quote Link to comment Share on other sites More sharing options...
Daskuppler Posted February 9, 2023 Share Posted February 9, 2023 What brand and style gasket did you use for the rebuild? Quote Link to comment Share on other sites More sharing options...
jonathan909 Posted February 9, 2023 Author Share Posted February 9, 2023 (edited) Ain't my first rodeo - when I do HGs I go with known quality. Not sure exactly, but over the past couple of years I've generally been using Mahle MLMs (most of which have actually been Subaru OEM), and they hold as they should. The one exception was the (2019?) rebuild on my '99 DOHC in which I used the gaskets supplied with the EngineTech kit - that was purely an experiment, and they failed after about a year (during which I made a nine-day road trip to NY and back). This car hasn't seen nearly that many miles since the rebuild. So I have confidence in both the gaskets and my work. Edited February 9, 2023 by jonathan909 Quote Link to comment Share on other sites More sharing options...
Daskuppler Posted February 9, 2023 Share Posted February 9, 2023 3 hours ago, jonathan909 said: Ain't my first rodeo - when I do HGs I go with known quality. Not sure exactly, but over the past couple of years I've generally been using Mahle MLMs (most of which have actually been Subaru OEM), and they hold as they should. The one exception was the (2019?) rebuild on my '99 DOHC in which I used the gaskets supplied with the EngineTech kit - that was purely an experiment, and they failed after about a year (during which I made a nine-day road trip to NY and back). This car hasn't seen nearly that many miles since the rebuild. So I have confidence in both the gaskets and my work. Fair enough. Just have to check haha Quote Link to comment Share on other sites More sharing options...
GeneralDisorder Posted February 10, 2023 Share Posted February 10, 2023 Radiator cap or damage to the filler neck. If that's not it - HG failure. GD Quote Link to comment Share on other sites More sharing options...
idosubaru Posted February 11, 2023 Share Posted February 11, 2023 On 2/9/2023 at 10:07 AM, jonathan909 said: So I have confidence in both the gaskets and my work. My lack of confidence has to do with EJ25 HGs generally, not your work. Haha. As GD said The radiator caps can have compromised gaskets. Also the radiator necks can having pitting or chips that prevent proper sealing of the cap gasket. Id just expect (maybe incorrectly) some other symptoms like coolant loss at the cap or improper levels/flow/bubbling in the overflow if that were the case. But I haven’t seen too many symptomatic radiator caps. Quote Link to comment Share on other sites More sharing options...
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