The 1998 Subaru Outback I've been having cooling system problems with turns out to have a combustion gas leak into the coolant. I suspected this but couldn't get a test to turn up anything; taking it to a good radiator shop, they were able to detect it. (pH test fluid in the block check kit turned from blue to green.)
If there's no other option I'll tear open the engine and replace the head gaskets (there are two because it's a boxer engine - bleh!) but I'd like to see if I can get by with a temporary fix for a while. The mechanic seemed pretty knowledgeable about this kind of issue, and when I asked him about using Blue Devil to seal it, he agreed it's better than the cheap leak-stop products, and gave it about a 50% chance of success. He told me he'd used it on one of his own vehicles and it worked, but overall sometimes it works and sometimes not.
Since then I've bought a bottle and I plan to try it, but I want to make sure I follow best practices using it, especially since I've had clog/blockage problems with leak-stop products I used in the past. The procedure on the bottle is:
- Flush cooling system. (done)
- Remove thermostat.
- Fill system with water, leaving room for Blue Devil.
- Start engine while cold and turn heat on max.
- Pour product in slowly (30 sec for half bottle) then idle for 50 minutes.
- Test drive, let engine cool, re-install thermostat and refill with antifreeze mixture.
Any ideas why the thermostat needs to be removed? I plan to do it in any case. Is the reason to make sure the product circulates through the system before it gets hot? This seems to be the intent of step 4 requiring a cold engine.
Aside from that, are there other things I should make sure I do right using this stuff? I want the best chance of having it work (I really don't feel like taking the heads off this thing -- take a look here for why: http://www.youtube.com/watch?v=yakX-GAV9uE) and not ruining the radiator or heater core in the process (although those are lesser concerns).
Update:: I've used the sealer with some partial results. Idling, the coolant doesn't move into the overflow tank at all. However, I still have high pressure building up in the cooling system when driving the car. At least once, I got coolant to move back from the overflow tank into the cooling system after it cooled down, but most of the time it's staying pressurized basically indefinitely, indicating that there's still a leak.
As discussed in the comments, I tried running the car heavy to heat/pressurize it, then removing the spark plugs to try to get the pressure to force the sealant back across the HG towards the cylinders. (I later found this youtube video that recommends roughly the same thing: http://www.youtube.com/watch?v=YL9QjN7AcW8 -- though I did not try actually running the vehicle with spark plugs out) I expected pressure in the cooling system to drop when I removed the plugs, but it didn't. So it seems the leak is extremely intermittent and dependent upon temperature, pressure, or some other condition not always present, and this is making it really hard to get the sealant into the places it needs to be.
Any ideas on making it work? I'm thinking I might try running it with spark plugs removed, one at a time, or if I feel crazy enough even loosening one while the vehicle is running at high rpms... but unfortunately that's really difficult with the plugs way down on the sides...