The catalytic converter on a 2007 Mazda RX-8 is full of engine coolant. Is there any way this can be saved or do I need a new cat? It seems to me that coolant won't melt the cat like unspent fuel or oil would, And water molecules pass through here all the time. The big worry for me is the anti corrosion stuff in the anti freeze, and how it reacts with platinum/paladium/ceramic,etc.

I was thinking maybe wash it out (pressure washer? Distilled water? soap? hold it out the window going 150? joking) and let it dry. I'd like to re-use this thing if at all possible; cats are expensive.

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EDIT: I just ran the garden hose through it for a few minutes, both directions and let it sit in the sun for a week. It passed emissions (just an OBD check, they didn't use the little sniffer wand) and hasn't thrown any codes or acted weird. Haven't noticed any exhaust leaks. 02 sensors have looked normal ever since it finished burning all the vaseline.

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    This Q&A might help. Commented Mar 7, 2017 at 18:44
  • I'm curious how you got coolant in the catalytic converter in the first place? Unless the engine blew a rotor housing seal i can't think of how that would happen.
    – vini_i
    Commented Mar 8, 2017 at 2:05
  • @vini_i You can read more about it here: chat.stackexchange.com/rooms/54466/rx-8-restoration-room
    – Zshoulders
    Commented Mar 8, 2017 at 3:57
  • The short answer is: I have no idea, I am assuming it is one of the interior coolant channel seals (which I think is the same thing as a rotor housing seal?). The cat is the low point of the exhaust system, and since it is just a hole in the side of the engine and not a valve.... I assume it just flowed out the exhaust port and settled here
    – Zshoulders
    Commented Mar 8, 2017 at 4:01
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    Coolant is inexpensive enough that there's no point in saving coolant from your cat.
    – cory
    Commented Mar 8, 2017 at 13:55

3 Answers 3


The concern is valid. Liquid coolant is not of particular concern when at ambient temperature. At higher temps the Phosphorous, used as a corrosion prevent agent in coolant, is a concern. It binds with the Cerium used as an Oxygen storage agent in the catalyst substrate to form CROP, Cerium Oxide Phosphorous. This makes the Cerium unavailable to passing Oxygen. This then reduces the amount of available Oxygen to react with fuel molecules. Phosphorous does not bind with the more noble metals, Platinum, Rhodium and Palladium. Note that the above is a much simplified version of the catalyst contamination processes.

It is acceptable to rinse out the catalyst with water. Reactions to not take place below 350 degrees F. It is acceptable to use a mild detergent although it is unlikely to help much unless oil based products are present. Coolant is water soluble and should rinse away with little effort. Rinsing may also remove some of the other contaminates commonly found to be clogging the pores of the substrate which can improve catalyst efficiency. Ash is usually found in catalysts and mild agitation will dislodge some of it. This is good thing.

After cleaning dry completely before installation to avoid unlikely, but possible, thermal cracking. Install with new gaskets, confirm no exhaust leaks. Run as normal, see if the on-board tests pass or fail the converter. It is my usual practice to not give up on a converter until the on-board test sets a failure trouble code.

I have cleaned failed converters with a low pressure pressure washer in an attempt to revive them. This has resulted in a few that worked for a few more months. I do not recommend this cleaning method unless one is already committed to replacement if needed.

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    Would "drying" constitute letting it sit in the sun for a week, or do I need to force air through it?
    – Zshoulders
    Commented Mar 7, 2017 at 17:17
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    @Zshoulders A time in the sun would work. You can also put in a oven on warm. Cats are made to handle heat... Commented Mar 7, 2017 at 17:42

The catalytic converter is best washed in a solution of citric acid. One scientific study found that 80% of the damaging impurities such as phosphorous can be removed using citric acid. You can purchase citric acid on eBay or elsewhere using Google.

Many users found it works best in a warm solution, about 165 degrees F. Submerge the cat. completely, for about seven hours, swooshing it around occasionally.

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    That is a very interesting article. I wonder what prompted somebody to sink a cat in citric acid in the first place
    – Zshoulders
    Commented Mar 8, 2017 at 15:26

Best / easiest way to deal with this, assuming that you've only just discovered this and it hasn't been sat like this for ages, is to find another car that is working and allow it to warm up fully. Then, wearing some sturdy gloves and being careful not to burn yourself, place the entrance to the cat over the exhaust of the running car. This will dry the cat out perfectly and ensure there are no nasty substances left in it.

This method also works for drying out empty fuel tanks when you want to be sure there are no combustable remnants (for example if you have to weld up a fuel tank).

  • It has been sat like this for, at worst, 6 and a half months. I like the idea but I don't know of anybody nearby who has one of these, much less would let me try this.
    – Zshoulders
    Commented Mar 7, 2017 at 16:31
  • Anyone nearby who has a working car? You just sit it at the end of their tail pipe. Commented Mar 7, 2017 at 16:33
  • Oh like just set it up against ANY running car? Sorry misread your comment; yea I could manage that XD
    – Zshoulders
    Commented Mar 7, 2017 at 16:39
  • Yes, just so the exhaust fumes coming out are directed into and through your cat. Commented Mar 7, 2017 at 16:40
  • If you are welding a fuel tank then the "best" method is to fill it with water first and have an air bubble or small air pocket in the region you want to weld. At least, that is how petrol tanks were repaired by the craftsmen I worked with - never had one ignite like that.
    – Solar Mike
    Commented Mar 7, 2017 at 18:54

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