How often have you actually seen exhaust leaks affect fuel trims?

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May 3, 2022
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On one of my cars I have a bad leak at the flex pipe. The only thing is, my fuel trims never really sway more than +/-6%.

Prior to this I was under the assumption you could always tell from your fuel trims, but now I'm starting to think your only clue may sometimes just be your nose.
 
Yes, not always visible on fuel trims. And never if the cat is mounted before the leak/flex pipe as modern cars tend to do


I've replaced 2 flex pipes recently one on the 2.4L Forte and the other on a 2.0L Elantra. I "felt" zero difference in drivability in either case. I do not have before or after fuel trims but the elantra flex pipe was shot and needed replacement as it was getting loud (but no check enjoying light) the forte looked "bad" no sound and also no check engine light- I replaced it always.

I was also under the assumption that any exhaust leak would affect performance for the worse because of more o2 in exhaust thus set a check engine light on my two recent replacements there was no check engine light. On both the H/K the cat is mounted in the "exhaust manifold" right at the head before the flex pipe.
 
It is always a possibility affecting trims by having a leak upstream from the O2 sensor. Having an exhaust leak thus lessoning back pressure can affect EGR operation too which can affect trims as well.
 
Had the bolt rust off and break the seal of the exhaust manifold and exhaust pipe to the cat on my 97 F-150. It was setting a code for too rich I think, because exhaust was leaking before the O2 sensor so it kept trying to richen the mix. I used a c-clamp to clamp the pipe tight and the code went away.
 
Funny this topic come up.

The 2015 Impala 3.6 in the family fleet currently has a cracked exhaust manifold and it hasn’t set any OBD codes. Gotta get it fixed asap, the cabin stinks of exhaust fumes when sitting still.
 
Funny this topic come up.

The 2015 Impala 3.6 in the family fleet currently has a cracked exhaust manifold and it hasn’t set any OBD codes. Gotta get it fixed asap, the cabin stinks of exhaust fumes when sitting still.
Good ol Impala 3.6 manifold cracks. Should've been a recall IMO. Very common. Is it on the Cat? If so you may get that fixed under your 8/80K miles emissions warranty. Those are bad about cracking at the flanges.

And to answer the question if the leak is pre sensor I could see it potentially affecting STFT but overall no I have never seen an exhaust leak affect FT.
 
Really strange. My cat is actually mounted after the flex pipe.

Some cars have smaller pre-cats aswell, and a main under the car.

But as long as there's a higher pressure inside the exhaust, not much if any oxygen will get in to influence the fuel trim. Then there's lean burn engines aswell.
 
Good ol Impala 3.6 manifold cracks. Should've been a recall IMO. Very common. Is it on the Cat? If so you may get that fixed under your 8/80K miles emissions warranty. Those are bad about cracking at the flanges.

And to answer the question if the leak is pre sensor I could see it potentially affecting STFT but overall no I have never seen an exhaust leak affect FT.

Well I did see bigger leaks affect trims, but not pin size holes.
 
It basically boils down to the fuel trim logic employed by the PCM. Some vehicles rely solely on the upstream O2 to determine AF ratio's and therefore adjust fuel trims. Some vehicles, Korean makes especially in my experience, use the downstream O2 to not only determine catalyst function but also provide a "checks and balances" AF ratio signal. Cracked exhaust may not directly affect trims, but it very well can set lean codes.
 
Good ol Impala 3.6 manifold cracks. Should've been a recall IMO. Very common. Is it on the Cat? If so you may get that fixed under your 8/80K miles emissions warranty. Those are bad about cracking at the flanges.

And to answer the question if the leak is pre sensor I could see it potentially affecting STFT but overall no I have never seen an exhaust leak affect FT.
I need to check exactly where it cracked. Thanks for telling me it’s a common issue, I had no idea that it was.
 
Funny this topic come up.

The 2015 Impala 3.6 in the family fleet currently has a cracked exhaust manifold and it hasn’t set any OBD codes. Gotta get it fixed asap, the cabin stinks of exhaust fumes when sitting still.

It doesn't have a exhaust manifold per se, More of a flange bolted to the heads which is part of the Catalyst assembly. I've welded up several cracks & pipes that completely break off.
 
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