View Full Version : Heater core noise

03-31-2009, 02:20 PM
Hi all. I have a 1995 Legacy Wagon and whenever I accelerate, I can hear what sounds like air bubbles going through the heater core. The radiator is full, and there are no leaks. I recently installed a new long block, but the noise was there with the old engine also. I tried raising the front of the car to purge any air from the cooling system but it did not help. Any ideas?


03-31-2009, 09:33 PM
Have you tried letting the car idle and running the heater on HI for about 30 minutes to purge any air from the system?

04-01-2009, 08:39 AM
I have run the engine with the heater on and off and it doesn't appear to make any difference. I also have a 1994 Legacy that I replaced the engine on, and I never had any problem with purging the air out of that cooling system.

04-01-2009, 05:11 PM
But do you get heat out of the system? Consistently? Air sounds in the heater core would block general flow of coolant thru the heater core and the heat would tend not to be as constant as a full system. There should be a nylon hose adapter before the firewall where you can separate the heater core coolant tubes and check to see if it's airbound.

04-02-2009, 07:47 AM
Scorpion8, thanks for the replies. I do get plenty of heat from the core. The sound that I hear makes me think that there is flow through the core, but there are small air bubbles passing through as well. Do you think that it's possible that there is blockage in the radiator that could be starving the water pump at higher revs and causing cavitation?

04-02-2009, 07:27 PM
That's entirely possible, but you posted that the engine did not overheat which would happen at reduced flow. Generally any air in the system will get carried along and eventually get purged thru the overflow tank. A pressure test on the cooling system will determine that the cooling system is tight. Does your temperature gauge fluctuate at stops (creep up) and moving (creep back down) or acceleration (creep up), or is it rock steady all the time? What about your overflow tank... does the car use any coolant or does that level stay steady each day?

04-04-2009, 08:19 AM
There are no coolant leaks as far as I can tell, and the temp gauge stays perfectly steady from idle to highway speeds.

I ran the engine yesterday with the radiator cap removed and waited for it to come up to temperature. When the thermostat opened I revved the engine and the coolant level dropped drastically and almost looked like a flushing toilet. When it went back to idle the level came back up to the top of the filler neck. It looked to me like the coolant couldn't get from the return side of the radiator fast enough to keep up with the outlet flow and that may be where the air is getting to the water pump. I have another parts car and will swap out the rad and see if that makes any difference. I'll let you know what happens. Thanks

04-12-2009, 02:47 PM
Maybe some trouble with filters?

04-14-2009, 10:37 AM
I'd love to hear what happens with this... I've got the same problem w/ my '04 Impreza 2.5TS. I've tried burping the system, and it doesn't do anything. In fact, in my rez, I'm about 3 inches over the full line. I've got steady, consistent heat, and my engine temps are good. But that noise is always there at start, esp. when I'm on the accelerator.
Looking forward to more info!

04-18-2009, 09:09 AM
My plan to swap out the radiator didn't fly when I realized that my 92 had a different style rad than my 95. I did pull and flush the radiator and it appears to be clean inside with no blockage. My next theory was that the thermostat wasn't opening soon enough and the coolant was actually starting to boil inside the block causing the bubbles. I changed out the thermostat, but still can't get rid of all of the air. I'm still open to more suggestions if anyone has any to offer. Thanks

04-18-2009, 09:14 PM
Do you have the 2.2 engine or the DOHC 2.5 engine??

04-19-2009, 08:35 AM
It's the 2.2.

04-19-2009, 12:24 PM
The reasone I was asking was the 2.5 would start putting a little air in the cooling system when the headgaskets were on there way out, but 2.2 are pretty much bullet proof. That is strange, I have herd the noise on many vehicles but never have herd of a solution, one thing you might try is a vacume fill on the cooling system, it is about the only way I know of for sure to get all the air out. That is the way the factory fills the cooling systems. Might be something to try.

04-20-2009, 07:21 AM
Holy crap! So my symptoms, very similar to the OP's, could be the head gaskets going?!? That sounds a little extreme for an '04 with 55k miles on it, but I do have the 2.5L SOHC engine. Does that apply for a Suby as recent as mine? Any way to check? Any other solutions? I don't really want to take it to my mechanic fearing that, as I'm sure he'd be happy to confirm and and do a friggin' head job on my engine!

04-20-2009, 10:38 AM
It is not likely that your engine has headgasket issues being that new and with that few of miles on it magic, but it is possible. The best way to check for that is checking for hydrocarbons in the cooling system but you have to have some expensive equipment for that. A quick test is to start your car and check your coolant overflow tank, rev the engine up a few time and see if you get any air bubbles coming up through the coolant in the tank. If you don't than chances are your headgaskets are fine. Subaru's are very difficult to get all the air out of the cooling system and the best way is to fill it with a vacume filler but not too many peaple or shops have one. My advice is to buy a coolant funnel, about $30, it is a funnel that has rad cap fittings on the bottom of them, put it on fill the funnel about half way up and start the car and watch for air. After the engine is fully warmed up rev up the engine a few times. It could take up to half an hour to get all the air out of the system. That should take care of the noise.

04-23-2009, 06:57 PM
Rutech, I tried the funnel method that you described. I didn't buy one, but I found that I had a plastic funnel that fit tight into the filler neck. I did get some air out of the system, this time without spilling any coolant, but I still hear some bubbles going through the heater core. I'm going to try again tomorrow. Also, would you please explain how to do a vacuum fill, just in case I need to try that next. Thanks, Dave

04-23-2009, 10:18 PM
Well unfortunately you have to have a special tool for the vacume fill. Basically what it does is creates a vacume in the coolingsystem so you have no air in it. Then when you let the vacume pull coolant into the system. Usually the funnel methode will do though. You don't have any traces of a coolant leak do you? That is strange. I am out of idea's at the moment but will do some thinking and let you know. Let me know if you figure anything out and good luck.