Knock under deceleration???

[SRBURG13]

Hey all,

Well, I've been having a weird issue with my motor. Here is the timeline of what happened:

I had the car idleing in front of someones house, the water temp hit around 92 celcius. When i went to drive again, i noticed the knock sensor on the pfc hitting around 25. But it only happened when i let off the gas between 2000 and 4000rpm.

When I get on the gas, knock is near zero, at operating temp idle is near zero. On a cold start and idleing knock is steady at 20, then warms up and idle levels off to zero knock again.

I figure it has something to do with the coolant/system, cause I was low on coolant that day it first happend, and i filled up the rad. The knock went away for the weekend. Then after idling for a while again in the heat, it came back slowly, then on fully again (only under decel) a day later. So I checked the rad again, and its full.

I'm not leaking any fluid, and I'm not blowing white. It's very weird. We are going to do a hydrocarbon test on the rad fluid and see if I'm leaking into the comb chamber, causing improper mixture and the knock. The only thing we can think of, is that I'm leaking rad fluid through the headgasket into the combustion chamber. But that doesn't explain why it's only between those rpm on decel, and nothing at all on acceleration or idle (when warm).

Retarding the timing doesn't change anything, and neither does upping the fuel.

Here is the setup:

Redtop SR
Power FC with Enthalpy's maps
gt25r .86
topspeed intake mani
272 cams
stock rad
fmic


We'll do the hydrocarbon test, check for any weird leaks (maybe frost plugs), and swap water temp sensors. I don't hear any knock, and the car runs and pulls just fine. No noticibility under driving at all.

Any thoughts?

Thanks.