Got my scanmaster installed today and took the car for a spin. The car has a bad exhaust manifold leak (which will soon be replaced by some lpp longtubes) so I knew the computer was probably pulling timing some. Just didn't expect to be up to 12 degrees .

Can a exhaust leak cause that much false knock alone?

 

I've looked at an awful lot of scan logs from LT1's, and I can't ever remember seeing 12-degrees of knock retard, except from engines that had a code for the knock sensor. I'm not even sure exhaust leaks cause significant knock retard. Maybe it its the passenger side manifold, and the leak is right over the knock sensor.

Under what conditions did you see 12-degrees of retard - idle, part throttle, WOT?????

 

I see this at about 1/2-3/4 throttle. It is a bad leak on the drivers side. When I first heard it I thought a rocker had fallen off. I will try to get a vid of it.

 

Are you sure a rocker isn't making the noise... that would account for the knock retard???

 

I'm sure its not a rocker. I had to put a hose to my ear and search around the drivers side of the motor to find where the noise is coming from. Its definately the manifold. The rear bolt broke off and its leaking there. Sounds horrible.

 

Hey man i dont see it being your exhaust unless like injuneer said its right over your knock sensor and still i dont see it doing that much damage. Before i got my headers i had a leak at the top on one of my manifolds and at the bottom of both of them where they join the y pipe and the only thing that happened to me was that i lost a little torque didnt really see anything else happening.

 

If the fuel trims get way out of whack the pcm will start pulling timing very aggressively. An exhaust leak would certainly play hell on the fuel trims. I can check a stock tune file and see if its possible to have that much retard if you can tell me about what rpm you're seeing it at.

Edit: Just as a general rule, when working on an EFI controlled engine fix all known problems first to the best of your abilities. That way if both issues disappear after fixing the known problem, you have not wasted your time chasing an unknown cause.

 

If it was a rocker wouldn't timing be pulled constantly? At idle or deceleration the scanmaster shows no timing being pulled. It looks like it's being pulled back that far at about 3500-4k rpm but I'm not 100% sure on that. I mounted my scanmaster temporarily by the aldl connector till I can find a better spot. It's not easy to view from there.

 

OK here is a couple vids.

http://s20.photobucket.com/albums/b2...t=SANY0002.flv

It's hard to hear exactly where it's coming from in the vid but it is coming from the back exhaust port. I put a hose to my ear again and I'm 100% thats where it's coming from. It about blew my eardrum out lol.

http://s20.photobucket.com/albums/b2...t=SANY0001.flv

 

I drove the car to work this morning. On the interstate I see at least 1.5 degrees of retard. Going up a small hill it seems to max out at 12 deg. I guess I'll find out next couple of weeks when I install the headers. Might look into a lt4 knock module also.

 

Well a lt4 knock module did absolutely nothing. However the exhaust leak is worse than ever now. I got everything together for my header swap I just need a free weekend to do it.

Looks like I'm going to have to replace my intake manifold gasket (leaking oil from the rear somewhere) soon so I will take off the valve covers and confirm all rockers are in place.

 

What does your timing look like at idle? I once scanned a car with bad knock sensors and it held a constant 12* spark retard at idle.

 

A faulty knock sensor/circuit will set a code, and the PCM will start using variable knock retard, based on internal calculations from engine operating conditions. I pointed this out in my first post that this condition could exist, but so far the OP has not indicated whether he has any codes or not.

 

Nope, no codes. 0 retard at idle.

 

Changed my intake manifold gaskets today. Verified rockers look fine.