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2008 Forester AT
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Discussion Starter · #1 ·
Hello everyone,

I have a 2008 Subaru Forester; about a few months ago my head gaskets were severely leaking (mixing with the oil etc, but never overheated) so I had to have them replaced by the dealership. Ever since having them repaired I have noticed my car seems less peppy than it used to be, rattles on start up and gets awful gas mileage, and makes a high pitch whistle when under load (ie getting onto the highway entrance ramps or speeding from a stop). This never happened before so I brought it back to the same Subaru dealer and they investigated the issue and came back to tell me it is my rear catalytic converter that is responsible. I looked further into this and it seems to be common when head gaskets leak for the coolant to get into the exhaust system and cause problems with the cat converters. However, despite all this I do not smell the typical ‘egg’ smell and do not have any check engine lights.

I asked my ‘mechanic’ ( he is very knowledgeable about cars and works on them all the time but isn’t employed as one) to replace my rear cat converter for me. But he insists that I need to replace BOTH the front and rear converters because his logic is: if one is having issues downstream that means the one upstream (the front) probably is too and probably has coolant inside of it; and if I don’t replace the front one with it it will not make a difference and will actually affect the quality of the hypothetically replaced rear converter. I’m sorry if that sounds hard to follow; but I want to avoid replacing the front converter as it is extremely expensive compared to the rear one. He claims to hear rattling from both the converters so he is really dead-set on me buying both the converters before he does any work.

I need advice- what do you guys think? Would you bite the bullet and replace both converters? Or would you replace just one, or none at all? Have you ever had to replace your cat converters and what luck have you had? Any brand recommendations?

On a side note having my converters replaced by the dealership is out of the question, they wanted $2,500 to replace the rear one. Any helpful knowledge is appreciated! :grin2:​
 

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whistle under load is often a vacuum leak and can be easy to happen when Head gasket replacement is done, poor quality hg job, vacuum line not tight,...... do a vacuum line leak test and or compression test before you buy new cats
 

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2008 Forester AT
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Discussion Starter · #3 ·
They supposedly did check for a leak in those areas and diagnosed it as a cat converter, but maybe it would be good to get a second opinion. I'm guessing the poor fuel economy and lack in performance must be related to the cat converter too so I would agree with them in that sense;' but then again I just do not have the codes or much of a typical smell. Maybe it is just not bad enough yet...but I would rather stay on the ball and replace them and get the car back to the way it once was. Just need to figure out how to go about it
 

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2012 Forester 4 speed auto
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1,128 Posts
O2 sensors

I would replace the O2 sensors to restore the power and performance of the engine. The sensors will then also tell me if the Cats are still good. You will get a code if they are not. If the Cats are clogged then a vacuum gauge connected to the intake manifold will indicate it. The engine vacuum will slowly decrease because the exhaust pressure is building up in the exhaust system. Replace one cat at a time and check if the vacuum returns to normal. Good luck.
 

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2006 Forester
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131 Posts
Not sure about the whistling? I just had mine done too, and my CEL has been on a while before, AND still on now after the change. I want to just remove the whole system, but i don't know how the engine would handle free flowing cats?

Good luck with your project.
 

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2008 Forester AT
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23 Posts
Discussion Starter · #6 ·
@konaid That is a bummer your CEL is still on after that, maybe it just needs to reset...
Well I decided to buy two new Denso O2 sensors, and a new Bosal rear cat converter off of Rock Auto's website. I am going to replace those things and see if I get a boost in fuel economy and power just have to wait for my parts to come in. I really hope this works!
 

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2006 Forester
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131 Posts
Definitely following. There are some decent headers and cats on Rock Auto i was looking at. Contemplating just ripping everything out and starting over. I just do not know if that would fix the CEL.
(I need that fixed... I keep getting pulled over... I blame CEL and lack of cruise control ;))
 

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2008 Forester AT
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Discussion Starter · #8 ·
So, I finally was able to get the work done and replaced my rear cat converter, and replaced both O2 sensors. It took about 50-100 miles for me to notice a change, but I did notice the fuel economy did improve. As far as being "peppy" I am not sure if I notice a difference or not... But it seemed to be alright. We are going to try and clean the air flow sensors at some point. No check engine lights came on after the work was done and I've driven it about 250 miles since the repair.
The Bosal converter did fit pretty good, except a seal on one end wasn't 100% but I think with the heat and driving it for a bit it was able to melt on here somehow and conform to it better. The first 50 miles I noticed a smell but it was probably all the parts getting adjusted to being used.
We re-attached the heat shields to the converter but now those are rattling quite a bit...
But bottom line is- it did improve fuel economy and the new O2 sensors (I got denso ones) did not find a problem with the front one we didn't replace. The fuel economy could've just been due to changing the O2 sensors though.

Maybe in the future I will replace the front cat to see if it improves performance but its not throwing a code or giving off any smells so its not needed right now.
 

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2006 Forester
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131 Posts
so you just did the rear? Hmmm... Might have to try that. If only they would ship free to Ak...
 

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2001 Forester S, 4EAT
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I just had HG done on my 2001. Couple of days later it threw up a P0131. In the end, since both O2 sensors were from the factory and had 213,000+ miles I bought 2 new Bosch O2 Sensors and replaced them. I have now been driving it for a couple of days now. WOW, the car handles completely differently. Takes off from the light it never used to. So for $65 for each sensor and about 1 hour work to replace both sensors, I would do that first before messing around with the CAT.

Further to this, the general consensus regarding HG issues, is that if coolant does indeed make it into the exhaust it will likely contaminate the Upstream O2 sensor. Which in turn causes the engine to run rich all the time which would result in poor MPG.
 
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