I have just bought a 2010 MT Sport with 8k miles on it.

Last week we had a big storm went through lot of rain with an inch or so of standing water on the freeway. When I drove home I thought that the car had blown an exhaust. Had a very loud exhaust note and engine running rough. It cleared the following day so I ignored it.

Yesterday the same deal. Sounded loud and rough and high engine vibration but no CEL so I assume no misfire.

This is RPM and not speed dependent. I can't say if it was down on power as I had no intention of joining the list of crashed vehicles!!

Any ideas as to what might be causing this?

It may be unrelated but my gas mileage is dreadful, even by RX-8 standards. I am getting 15-16 with mixed traffic and freeway driving and only got 17-18 mpg on a freeway run of 100 miles with the car at 75mph in cruise control. Using Premium gas.

Thanks in advance for any help.

 

what exactly do you mean when you say it had "blown an exhaust"

It sounds to me like you might have a clogged cat

 

were you getting a flashing cel? maby your ignition got wet?

 

My bad. It sounded like it had blown the exhaust (i.e. the exhaust system had blown a hole in itself and not being silenced.). I had a deep resonating exhaust note that also resonated through the body. The sound you get when an exhaust has broken.

 

The CEL was not lit or flashing. So I assume that the engine was not misfiring. If it was the misfire monitor would have set a code and lit the CEL.

 

You may want to double check with ASH8, but IIRC there was a bad "batch" of S2's that went out with defective Cats. I think if you do a search you will find some people that had their cat go before 10k.

 

I haven't personally experienced this phenomenon but some owners say it's pretty normal in heavy rain.

https://www.rx8club.com/rx-8-discussion-3/heavy-rain-%3D-loud-exhaust-154869/

 

Thanks for that link. Seems that a) I am not alone and b) need to search better before posting

 

There were a few very early 2009 S2 owners that had a CAT issues, but cant say there was a problem at all, or a faulty batch.

Now the S2 CATS are not the same as S1.

S2 CATS have not changed.

OP's is a 2010, I suggest you take it to a dealer IF you believe you still have a "noisy" exhaust..

Having said that if you believe your MPG is that bad then possibly your CAT is defective/blocked, again take it to a good Mazda Dealer...you still have a B2B Warranty.

 

That sounds a lot like misfiring due to moisture in the ignition wire connectors, but that really should be accompanied by a flashing engine light. So maybe it's not misfiring, or maybe just not bad enough to trigger the CEL. If it isa misfire, it will melt your catalytic converter. Definitely get it checked out before you drive it any further if it still sounds like that. You can try popping th plug wires off the plugs to let them dry out.

 

That was my thought but if the misfire monitor doesn't pick up that level of misfire then what would it pick up?

I started to wonder about too much water getting through the Air Filter and getting water into the combustion but again misfire monitor should pick that up, unless the motor isn't misfiring but just firing with a high water content.

I seem to remember that either Saab or Volvo tried some sort of water injection as it increased power by getting more expansion of water droplets to steam than just expanding air.

I don't think that the CAT would like that too much!!

It is in the dealer next week for other stuff so I'll ask them to check the plugs and leads out at the same time.

Andy