Why You Should Never Use Stop Leak in Your Subaru


Don't use Stop Leak in Your Subaru

Subaru Radiator Hose Full Of Stop Leak

Pictured above is an upper radiator hose on a 2002 Subaru Outback as you can see in the picture it is full of stop leak. The radiator was over 60% restricted after installation of the stop leak by another repair shop to the point where the vehicle overheated where as it never had any overheating problems prior to the stop leak being introduced into the cooling system. The car now needs a radiator, head gaskets, attempt to clean or replace the  cylinder heads, most likely a engine block, all cooling system hoses and a heater core.  A very unfortunate set of circumstances.    As soon as the Technician removed the upper radiator hose we knew it was going to be bad.

I have tried numerous time to suggest that Stop leak is a terrible idea, and will continue to try and post pictures as to why.

Pictured below is the same stop leak crystallized in the cylinder head passages, and engine block passages there is now good way to remove it all is literally in solid chunks through out the cooling system.

Stop leak in a Subaru

Stop leak in a Subaru

Stop Leak In A Subaru Engine

Stop Leak In A Subaru Engine

Thanks For reading


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30 Responses to Why You Should Never Use Stop Leak in Your Subaru

  1. Eric L February 25, 2011 at 11:50 pm #

    What’s your take on the coolant conditioner in the blue bottle that Subaru sells? Subaru has a TSB out that uses it to fix some small HG leaks. It also says to use it in the SOA service manual for my 06 Legacy GT. I used Subaru coolant and haven’t put in the conditioner yet; so far there have been no leaks.

    • Justin Stobb February 28, 2011 at 6:03 pm #

      Hi Eric,

      My take has always been that it was a cheap way for Subaru to try and get some of the vehicles out of warranty with out replacing the gaskets. We have seen quite a few vehicles with multiple doses of the “Subaru coolant conditioner” end up with restricted radiator creating over heating situations ultimately damaging the head gaskets and having an internal leak( typically around 150k).

      At least The Subaru Stuff doesn’t seem to crystallize like the stuff we found in this poor Subaru and it is a “measured amount” to be added. If it wasn’t for the fact that Subaru wont honor Power train warranties on cars that do not have it put in, well above the ones listed in the link you sent me, we would never use it. On all Subaru’s still covered under Subaru’s power train warranty we do in fact put it in every time we service the cooling system. Since Subaru has now also changed the coolant service intervals its going to be interesting to see what the stance is in a couple more years with the Subaru Super Coolant and the newer design engines with the MLS type gaskets from the Factory.


  2. Jack February 26, 2011 at 2:10 pm #

    Justin, regarding not putting stop leak into the engine,
    is the “Subaru approved” stop leak an exception or do all stop leak products create the same clogging issue?

    • Justin Stobb February 28, 2011 at 6:05 pm #

      Hi Jack,

      We do see some Vehicles that have multiple doses over the years with the Subaru Conditioner have clogged radiators.


      • Steve March 14, 2011 at 8:02 pm #

        So, with warrantee issues aside, when you guys do HG’s on phase 2 Subaru’s, do you add the Subaru conditioner?

        • Justin Stobb March 14, 2011 at 8:50 pm #

          Hi Steve,

          We use a MLS gasket on the SOHC 2.5l Currently and do not use any Subaru Conditioner as a result.

          When we have done the H6 or Turbo Models with the MLS gaskets from the Factory we have omitted the Conditioner as well.

          Any vehicle still under Subaru’s power train warranty gets the conditioner whenever we service the cooling system.


          • Steve March 15, 2011 at 9:54 pm #

            Thanks Justin,
            You are doing a great job here.
            I just did the HG on my 2000 OBW, with 106k miles. Its kinda shocking to see the original graphite layer peeling off… mine went on the lower side of #4, and was obvious. you could see the staining from the fire ring thru into the fwd coolant passage, but the head was not warped at all! The inside of the engine was clean, and a sweet golden hue. No blocked coolant passages, and you can still see all the bore hone marks.
            After reading and absorbing your site here, I used Subaru Gaskets, replaced the waterpump and thermostat too, as well as the Timing belt, tensioner, and pulleys.
            I will not put the Conditioner in now, but will do a flush and fresh coolant every 30k from now on!

  3. Erik October 5, 2011 at 10:59 pm #

    Thanks for your very helpful website!
    I have a 2003 Forester, original head gaskets. Should I continue using the Subaru conditioner with each coolant change? From you website, it seems my car will most likely leak oil instead of coolant from the head gaskets.

    • Justin Stobb October 11, 2011 at 4:21 am #

      The 2003 are more apt to leak oil externally and there is no amount of coolant conditioner that will address oil leaks. I am really not a fan of the Subaru Stop leak as I have seen the clogged radiators on the vehicles with multiple doses end up blowing head gaskets due to an over heat which defeats the original intent of the stop leak.

      I dont use the Conditioner on any of our loaners or my personal vehicles either I will add.


  4. Catherine Turner February 6, 2012 at 5:24 pm #

    my subaru forrester is blowing hot air but it smells like coolant and the windows keep fogging up? What should I do…..I drove it to work and I have an hour commute. I am worried. Please help me if you can.

    • Justin Stobb February 6, 2012 at 8:33 pm #

      Sounds like a heater core failing.

      You need to have it repaired as soon as you can.


  5. gilbert February 27, 2012 at 2:17 pm #

    Hi Justin,
    My BF3 AWD, 1992, EJ18 seems to overheat quite a bit on long and streneous rides, overspilling the coolant to the reserve tank and eventual overflow leading to less coolant in the radiator. No oil on coolant, and no coolant on oil signs. It did blow the reservor cap off but now heats up persistently on any long travell.
    could this be HG problem too on the 1.8


    • Justin Stobb February 27, 2012 at 11:24 pm #

      Hi Gilbert,

      Yes it could be, but it could be other things as well, all you can do is try to do your best and evaluate the cooling system one part at a time.


  6. KEN HALL June 21, 2013 at 4:42 am #

    Hi Justin,
    2001 outback 2.5 205,000 miles. Had new head gaskets water pump, thermostat, all belts, timing and all pulleys installed at 198.000 miles. drivers side HG was leaking coolant and oil externally before the install. however the car never overheated until about a week after the repair, after a hundred miles into trip, temp gauge went up to the red pulled over put more coolant in. was fine for a while but got hot again. changed the radiator cap and was fine for a month, thought problem was solved not so. the next long trip started to overheat again.let pressure out of radiator with vent cap. temp would return to normal. the faster i drive the more it overheats. i flushed the radiator and block used the special radiator funnel to fill it, making sure to get all the air out of the system. just drove 1400 miles from wisconsin to florida temp gauge went up to the three quarter mark and stayed there most of the trip, but did go up to almost red by forida the faster i go the more it overheats. driving in city traffic in florida with ac on does not overheat at all. 90 degree plus temps everyday.tried three different radiator caps no change. do you think lower pressure cap might help. now the kicker they tried to seal the head gasket with blue devil liquid head gasket repair. before putting hg in. could this be plugging the cooling system. but why does it overheat only at hi speeds especially up hills. it’s got me and two other subaru mechanics baffled. thanks for your help. engine does not use any oil and runs better than ever, and no bubbles in radiator when running. thanks ken

    • Justin Stobb June 21, 2013 at 7:02 pm #

      Hi Ken,

      There really is no way for me to know exactly whats wrong from here, but it is very straight forward to diagnose, and should net be baffling anyone.

      I would start with using an infrared thermometer and measuring the temperature of the coolant in the cooling system in various places such as inlet and outlet of the radiator once the Thermostat has opened, as well as the heater core if there is a suspected clogging do to the Devil being introduced, and yes its well named, we are familiar with the product and witnessed it destroy all aspects of the cooling system.


      • ken hall June 24, 2013 at 6:17 pm #

        Hi Justin
        Car heater works very good. but I do hear like a coffee pot percolator sound in the dashboard sometimes. going to try another thermostat and cap……thanks Ken

        • Justin Stobb July 1, 2013 at 11:31 pm #

          Thats air in the system.


      • ken hall June 27, 2013 at 2:12 pm #

        HI JUSTIN, went to Subaru dealer. Service guy told me to bleed cooling system with front end raised, he said it might take a half hour or more to get all the air out. Did that with the screw on funnel. And it took a half hour to get the final air bubbles out. Then still ran it another twenty minutes. Seems good and problem solved so far. Working on my next 200K…..
        Thanks Ken

  7. Mark otutu August 1, 2014 at 8:09 pm #

    Halo, i want to purchase a second hand subaru legacy bf3, i went for a test drive with it and its strong and stable in the road,there before i checked underneath it and saw some oil leaking from the engine after,replacing the engine oil with a new unused oil,it stoped leaking,is there any proble there?

  8. Sean September 10, 2014 at 2:02 am #

    I was wondering if you would advise someone that just put a new short block and subaru gaskets on an 04 xt with subaru coolant and conditioner to simply refill the system without the conditioner?

    • Justin Stobb September 10, 2014 at 6:04 pm #

      Hi Sean,

      There is no good reason to install coolant conditioner, so yes I would advise to not use it.


  9. Paul October 20, 2014 at 5:47 pm #

    Hi Justin,

    My 03 Forester has blown its 2nd HG (external left side) at 189K miles. 1st one was at 135K miles and did the timing belt and water pump at same time. Can I run without repairing as long as I monitor fluids or will it cause additional damage? No overheating, just loss of compression. Nothing else wrong with the car but I’m worried some other major repair may be lurking since the AC and transmission have never had any problems. Would like to keep the car another 50K miles or so.

    • Justin Stobb October 20, 2014 at 9:21 pm #

      Hi Paul,

      You cant operate a car with a internally failed HG without doing damage, you really need to park it or repair it, Sorry.


  10. Jon July 29, 2015 at 8:24 am #

    Hi Justin,

    My radiator has just gone south on my 02 WRX @ 106000 miles.

    Would you recommend upgrading it to an aluminum rad (if so, which one) or just go with the stock OEM part?

    This car is my daily driver and everything else is stock.

    • Justin Stobb July 29, 2015 at 11:56 am #

      Hello Jon,

      The aluminum radiator for the WRX is a great way to go, provided whomever installs it understands the hoses and overflow will need to be modified a bit. I don’t suggest modifying old radiator hoses but instead only new hoses from Subaru. It’s also much higher in price.

      Or you can just buy a Plastic and aluminum replacement radiator from Koyo and plan on getting another 13 years. Do not buy anything from China.



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