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:

  1. Flush cooling system. (done)
  2. Remove thermostat.
  3. Fill system with water, leaving room for Blue Devil.
  4. Start engine while cold and turn heat on max.
  5. Pour product in slowly (30 sec for half bottle) then idle for 50 minutes.
  6. 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...

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    Is this an update or a question? Just follow the instructions they should know their own product the best. Yes the thermostat should be removed to let the product immediately start circulating they say it bonds to metal so you dont want it staying in your radiator for 15 minutes before it actually gets to work on the head gasket. It seems like the product takes time to work so maybe leave it in for a couple days before writing it off as a fail. Let us know how it goes.
    – Mike Saull
    Apr 11, 2013 at 20:33
  • It is a question, yes - I was just asking if there's anything else I should know to do to get the best chances of success. Apr 11, 2013 at 20:53
  • By the way, if it really is better at bonding to metal surfaces, that sounds like a good reason to make sure to get the inside of the system really clean before using it. I +1'd your comment but I think it might even make a good answer in itself. Apr 11, 2013 at 20:55
  • So far, mixed results. :( The overflow tank level did not rise during the 50-minute idling step, but high pressure did build up, and moved coolant into the overflow tank after I took it for a fairly light test drive. I only used half the bottle originally, so after letting it cool down I've added the other half and running it again. We'll see what happens but I'm not too hopeful right now... Apr 11, 2013 at 23:01
  • Was the coolant level in the overflow tank supposed to raise?
    – Mike Saull
    Apr 12, 2013 at 2:12

1 Answer 1


I figure I'll post my own answer to the question today since it's the end of my 60 day window to request a refund. After all the things I tried, considering the Blue Devil attempt a failure, and starting the process to request a refund, I finally had success; the seal has held for over a month now.

What it took in the end was driving 4 hours at 70 mph on a cold day, where the temperature gauge on the dash stayed all the way down on "C" for the entire drive (made possible by the thermostat having been removed). At the end of the drive, I expected to need to refill the coolant, but only a small amount had been lost, and there was no pressure on the top radiator hose after the engine had cooled.

My best guess for what happened is this:

  1. To seal, the product needs a temperature gradient at the point of the leak.
  2. This is supposed to be provided by adding the product to the system immediately after starting the engine, while it's still cold.
  3. But the leak only happened under high engine load.
  4. Active cooling via cold air across the radiator provided a cool enough system to get the necessary temperature gradient at the point of leak while it was actually leaking.
  • It's weird- this whole head gasket black magic fix thing is sort of speculative at best. I can't argue with any of your reasoning, but I also wonder whether the high engine speed/medium light load (low cylinder pressure) of your long drive might have helped. If the engine isn't loaded too heavily, the combustion pressure might not be high enough to chase the product out of the leak every cycle, allowing it to eventually coagulate enough... Feb 7, 2014 at 22:40
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    For what it's worth, I need to revisit and update this answer. After 6 months it started failing, and by now it's undrivable. I've read up a lot more on the chemistry of HG sealing products and I'm left pretty skeptical that there's any benefit to the more expensive ones. I also don't think my guess at how it worked before fully makes sense, so I'd like to make a new shot at trying to explain it. Feb 7, 2014 at 22:54

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