I've got an HKS DP, Helix HFC, and an HKS Kansai catback. The cat back didn't have a second O2 bung since the JDM cars don't have one. No biggy, I just burned one in as close to the factory location as I could get.

When I did that I didn't have the DP and more importantly the HFC. Now after about a year with the HFC the CEL for the cat is coming on almost every week. So, I decide to to do the mechanical CEL fix.

Put everything in no problems. It's a little close to the chassis but, not too bad. Drive it around for a couple of nice sunny days and one or two drizzly days everything is cool.

Then we get one hell of a thunderstorm and there is a lot of standing water. I drive through a couple of large puddles and not long after that I hear this tapping under the armrest. At first I was like WTF wondering what it was. I soon realized it was my O2 sensor saying "Hi" in Morse code by tapping on the chassis.

Everything dries up and it goes away. Today on the way in it was raining and there was no noise. So, I'm thinking "cool, just a freak thing". Until I drove through a large puddle and it came back.

I'm thinking the exhaust gets hot, expands, the large amount of water from a puddle soaks the bottom of the exhaust, it contracts which, then makes the exhaust bow up and smack the O2 sensor against the chassis. Has anyone else noticed anything like this? Does this sound possible? I mean it is a long run of large diameter pipe that is mostly unsupported.

Also, my exhaust note changes dramatically when it rains hard (I swear it changes with temp and humidity but, not as noticeable as when it rains). Does anyone else's do this?

 

dude the same **** happined to me a my 95 Acura gsr...its weird

 

Really.

Today it is dry and no noise. I think I'm just going to hammer out a little clearance for it. I would try just one anti-fouler but, I'm in the process of building a cat delete so, I probably will need both for that.