Intermittent vibration/knocking from engine bay at 2100 RPM

[midlifecrisis]

I recently replaced my "test" pipes with a used set of stock catalytic converters. Since then I've noticed 2 new sounds;

1. At cold start, it pops once like a backfire as soon as I turn it over. After that, it runs like a champ. This, I assumed, had something to do with the cats.

2. With car parked and in neutral, if I hold the throttle steady between at 2100 and 2400 rpm there's a noticeable deep knocking sound coming from the engine bay.

I don't feel it or hear it when accelerating, only when there's no load on the engine. Every time I drag my wife out to hold down the accelerator, we can't seem to replicate it. She's getting pissed.

My remedy so far has been to rip through and stay above the troubled RPM range, but don't know how long I can keep it up :smile:

I have aftermarket headers and exhaust, and a Dinan chip, and the oil is topped off.

Any commentary would be greatly appreciated.

 

[C-speed]

I recently replaced my "test" pipes with a used set of stock catalytic converters. Since then I've noticed 2 new sounds;

1. At cold start, it pops once like a backfire as soon as I turn it over. After that, it runs like a champ. This, I assumed, had something to do with the cats.

2. With car parked and in neutral, if I hold the throttle steady between at 2100 and 2400 rpm there's a noticeable deep knocking sound coming from the engine bay.

I don't feel it or hear it when accelerating, only when there's no load on the engine. Every time I drag my wife out to hold down the accelerator, we can't seem to replicate it. She's getting pissed.

My remedy so far has been to rip through and stay above the troubled RPM range, but don't know how long I can keep it up :smile:

I have aftermarket headers and exhaust, and a Dinan chip, and the oil is topped off.

Any commentary would be greatly appreciated.

Off the top of my head, seeing as you said you bought a pair of "used" cats, did you inspect the guts of the cat prior to the install? Sounds like they may be broken or have partially come apart on the inside, so its rattling now.

A goog way to check is to take the cats off again and take a look inside, if its bad, you'll see it right away, otherwise, give them a good shake and see if you can hear it rattle. If it is, you got hosed on them and should ask for your money back

I'm assuming of course, that these sounds did not occur prior to the cats install, and that you reinstalled the cats correctly with no loose bolts or rattling heatshields that could cause a similar sound.


HTH

 

[midlifecrisis]

Turns out the OEM cats were coming in contact with the rear sway bar, which was causing the knocking sound. Perhaps my DC Sport headers don't fit as well as the stock headers, causing the bottom OEM cat to sit too low.

Anyway, I bought the Prospeed High Flow Cats (much smaller and lighter) which have plenty of clearance, and the problem was solved.

 

[goldNSX]

Turns out the OEM cats were coming in contact with the rear sway bar...

I was about to suggest you to have a look there or the harmonic balancer pulley (less likely as there would have been more symptoms). Glad you've found it. It's not the headers only, the rubber of the exhaust mounting 'wears' out in the way it brings the exhaust pipes in the near of the swaybar mountings.

 

[imducati]

Just for historical records I just installed the DC sport headers and the cat attached to the rear bank also hit the bar. looks like the header is just slightly lower than stock.