Hello!!
I'm trying to diagnose a problem with my 99 hatch... At WOT, there is a clunking sound coming from under the hood which eventually turns into a fluttering sound... it's as if something is loose and rattling but it goes away when just driving around normally. I hear it primarily when getting on highway onramps and hard acceleration...
The only mods under the hood (D series motor).. AEM V2 and DC ceramic headers.. everything else is stock..

Any help would be great...

Thanks.

 

It seems to happen mainly around 3000rpms and up... also, car seems like there is a boulder attached to it... just seems to be super slow these recently.....

 

Video/sample of the noise? Could be a rod knock.

 

I can try to post one up next time.
I'm also wondering if this is related to why my gas mileage has plummeted. I've never been one to really care too much about it, but from 35mpg to somewhere near 22/23mpg suddenly is a bit alarming..

 

Possibly pinging or detonation? Try knocking your timing back a few degrees and see what happens.

 

one thing i did notice was the crank case breather filter is looking pretty nasty... i'm thinking need to replace this... anyone know what size the stock CX (D16 motor) block crank case breather filter is?

 

yeah definitely replace the pcv. the sound could be spark knock, i doubt it would be a rod knock because you said its above about 3k. no CEL? having no power and the noise could be a plugged CAT which would definitely cause a drop in mileage. mostly because you probably give it more throttle to get where you are going since it feel so slow. after driving it long enough to get everything good and hot feel/look around the cat and see if it feels REALLY hot. it would be hot normally but plugged cats often glow red cuz they are so hot. does the power seem to drop off with rpm? like it starts struggling to get to redline?

 

Have you tried retarding the timing to see if its a pinging issue?