Why Do I Have Engine Knock

[wyochimneysweep]

I think you still got something mechinacal going on. if you look at your fuel trims one bank was +13 and the other side was -13. I would suggest to take off the cats and put in a pipe and drive it and see if it makes a diffenece one of the cats could be plugged and my experiance with plugged cats the warmer they get the more they cuz problems.

[wyochimneysweep]

maybe the noise you think is lifters is back pressure on the exhaust on the one side. I would expect the passenger side cat beginning to fail more and more if i had to guess and that would also lower the vacuum

[knowledge]

Hi wyochimneysweep

I agree with you. The more I look into things the more I lean towards the cats being blocked up. Particularly bank 2 which I believe is the passenger side. Which by the way, is the cat the got red hot for a minute or so a few weeks ago. I think the drivers side cat was beginning to turn a little red but the front half of the drivers side cat was glowing bright red.

Anyways, I do have cause to believe that cat and possibly the other one is partially restricted.

I have a pressure tester on its way so that should tell me for sure here. I sure hope I don't have to take the exhaust off...

As for the noise/tapping. Being I have never adjusted the rocker arms while the engine is running nor to where I was satisfied, I am leaning towards just needing adjusted. For example, when I did them last time. For some odd reason I was able to jiggle almost every rocker arm on the drivers side and almost none on the passenger side. And the drivers side is the noisiest. Its just weird because I did everyone the same.

I will keep a look out for an exhaust leak but it sounds to me at least that its certainly coming from under the valve covers. I know its easy to get them mixed up so I will keep checking.

Also, for the tapping to get louder after going on a road test would make a lot of sense if it is an exhaust leak. But a good buddy and I am pretty sure its rockers from under the under the valve covers

[wyochimneysweep]

i was so much thinking a ehaust leak but exhaust not being able to leave the clynider and possilbely making it harder for the vavles to open cuzing more noise. just a theory. good luck but I do think your going to have to remove the cats. if they got red they are probably trashed.

[knowledge]

HI wyochimneysweep,

Yeah I wish it was just an exhaust leak too and its definitely something I need to make sure is not going on ( and this is the kiss of death so as soon as I say this I am guaranteed to have an exhaust leak ) but I can hear that it is under the valve covers.

The odd thing about the cats is though they weren't red hot for more than a minute and actually it was really only one at that. However, it was the one for bank 2 which is the side where the fuel trim readings caught your eye.

The stupid thing is the exhaust lay out. To replace the one I am going to have to remove then entire exhaust from the manifold back on the passenger side.

I am probably going to take the "opportunity" to convert this to dual exhaust and get rid of the crappy system where two pipes go into my muffler and only one comes out. Like seriously why the hell would they do that.....

[knowledge]

Also, it never occurred to me that the back pressure would actually hold up the valves. I just figured the exhaust was backing up into the cylinder and it was getting half filled with exhaust and half filled with new air/fuel. Either way its the same result.

Now I have to wait for the damn pressure tester to come in later this week.

Unfortunately, its the thing I want the most and of course its the one coming in last... Go figure I didn't even have the option to do anything other than standard shipping.


But in the mean time, I am going to try an vacuum test everything I can. Adjust my valves and maybe even hook up a timing light to see if its matching what the computer thinks it should be.

Other than that, what are you thoughts on just bumping up the spark Minimum table to go no lower than 20* advance?

I believe I know the answer but will it still register knock with burst knock disabled and the MIN table bumped up? Even if its not retarding the spark as much as it should I can still keep my eye on how much knock I have.

[wyochimneysweep]

i wouldnt bump up timing till we find the source of the trims going haywire. one step at a time

[sarg]

In for answers on this one. Seems quite puzzling.

[knowledge]

Hi wyochimneysweep and sarg,

I see so what I am seeing on the LTFTs is not normal then. I am guessing then that basically the LTFTs need to be more or less the same, certainly not negative 15 on one and positive 15 on the other.

Just to note I have never reset the fuel trims. I probably should do that huh??

Well I will keep plugging away as much as I can while I wait for the pressure tester and scrap valve cover to adjust the valves. In the mean time I hope to eliminate or confirm vacuum leaks and the timing.

I will keep you posted. I am torn because its going to be a real pain in the butt to do the cats but if they aren't my problem I am not sure what the heck else it might be.

Stay tuned!!

[knowledge]

Hmmm does anyone else think that the cats being backed up could cause my IAT temp to be as high as it is?

That sensor has to be in the throttle body so I always just thought it was the heat of the engine causing that but maybe not??

[wyochimneysweep]

The iat wouldn't cause your fuel trim issue and plugged cats will create a little more under hood heat also

[knowledge]

Well life keeps throwing one me one curve ball after another here. Looks like Ill have to wait yet another day before I can do anything worth while.

Also, as stupid as this sounds I am hoping its my cats because if it isn't then I bet I am going to have much more serious problems.

Everything is pointing towards it being an exhaust restriction and that would explain most if not all of my symptoms. But I am not sure if I have been put through enough torture yet for this to be the last of it. Guilty conscience I suppose.

Also its worth mentioning before I corrected the cam offset problem and I would run the engine, there were really loud bangs coming from the muffler or the cats or both. It honestly sounded like tiger woods was teeing off in my muffler. Obviously that was no good for them but with both the housing getting red hot and those explosions in the exhaust would that make it pretty likely the cats are blocked up.

Finally, I suppose I am going to have to drive around a little to get a proper reading of the exhaust back pressure being it doesn't seem to be too bad in neutral. I just am not sure if I can because last time I disconnected an O2 sensor it got the cats red hot ( but I did have the cam offset at 43.5* as well )

[knowledge]

2 for 2 wyochimneysweep and to have figured it out with just with the fuel trim discrepancy is pretty amazing to me

Anyways, the passenger side cat ( Bank 2 I believe ) has a back pressure of just over 2 psi at idle ( 1000 RPM ) and between 8 and 10 psi at 2500 RPM.

While the drivers side ( Bank 1 ) has like 1.5 psi at idle ( 1000 RPM ) and less than 3 psi at 2500 RPM.

So clearly that is a major problem. Unfortunately, its going to be over a week before I can get the parts shipped here and then installed.

Or I could take a hell of a gamble and tow it to a muffler shop and have them weld in a new one and just hope to hell I can drive it home.....

Im kinda dead in the water until that cat is replaced.

I did check timing too and its reading what the ECM is saying so that checks out and I didn't get to do the propane/vacuum test so there is still that but its looks like I got my answer.

The lack of vacuum is pulling my spark. Hopefully that takes care of the IAT being a little on the high side as well and makes it so I can finally start tweaking tables!!!!

[knowledge]

Oh and I can finally contribute something that everyone on here might not already know but

when checking for bad cats by seeing if the vacuum from the intake slowly starts dropping when keeping the engine at a constant 2500 RPM. It is far from fool proof. Particularly if you only have one bad cat ( or side ). As a matter of fact I wouldn't even waste your time trying it that way. Check it the right way with a tester in the O2 bung and know for certain.

[knowledge]

So here is how my exhaust is roughly laid out. There are flanges connecting the exhaust manifolds to the down pipes with the drivers side down pipe having the drivers side cat on it Then one big flange for both down pipes to connect to two more pipes ( one having the bad cat and the other just straight ) go into a big ass muffler and then only one pipe comes out..... yeah what the hell is right

Oh and from the big flange all that is welded together with no way to separate it without cutting it

Here is link for the passenger side cat assembly ( remember the drivers side one is on the down pipe before the big flange )

http://www.amazon.com/Magnaflow-4547.../dp/B0025Q8QVW

all you need to know about the rest is that the two pipes from the assembly in the link above go directly into the muffler when then only has one pipe going out ( yes my muffler is my Y pipe ) and all of that is welded.

So what I am getting at is, I don't think I can really go about replacing the cat with a straight pipe to make sure there isn't other things going on.

But maybe not. I guess if its not me that would be welding the new cat back in ( and it won't trust me ) then they can weld a straight pipe in and then I can do it all again and they can weld the cat in.

Or I could decided Im feeling lucky and just have them weld a new cat in right away and cross my fingers

Or I could just replace everything from the big flange back with nice shiny new stainless steel and eat Ramen noodles the rest of the year

What would you recommend?

[wyochimneysweep]

Do it right the first time and get what you want. I spent 400 going cheap to only end up throwing it all out after a year and spending 1200 on what I wanted and installed my self. True dual 3 in mandrel bend with x pipe and two glass packs. Nothing like a rumble

[knowledge]

Good advice yet again from wyochimneysweep.

Unfortunately, my mom passed away monday so this has to go on the back burner for the better part of the upcoming week.

I will post any more updates when I do finally get them if anyone wants to check back in a week or so

Thanks again for all the help

[sarg]

We did a little tuning last night and one thing we noticed was timing would go negative when you stomped the gasand the vehicle was downshifting. Disabling torque management in the abuse mode section got rid of this. Anything over 80 degrees showed KR, even on this pretty well stock vehicle. He has removed the cats and no change to KR showing up. Vehicle is a 07 trailblazer ss with a filter and no cats. Pretty darn close to stock.

[wyochimneysweep]

80 degrees iat? mine runs iat over that all the time and no knock retard but i'm also on e85

[sarg]

Yeah, this one is on 93 octane. It is odd, it seems like it stays on the low timing map all the time. It was doing this before the intake and exhaust. It actually seems a bit better since taking off the cats, but still any time over 80 degrees it gets KR happy. I am wondering how much of it might be the IAT timing reduction maps.