Good afternoon everyone, i've just signed in and introduced myself in this forum which has always been a great source for information to deal with common issues that affect this truck, which most of them are just simple maintenance stuff that were needed because of its age. Aside from that lately i've been having some difficulty to try to identify the source of a noise that is coming from the engine area, picture this: im cruising in the highway at approximately 40 mph and i try to pass a car so i tap into the gas pedal just 3/4 way in and the tranny downshifts from O.D to 3rd and of course making the engine rpm's rise at around 2700~3000 rpm's more or less and then after 3000 rpm's. Then i hear like a can of marbles rattling on the lower portion of the engine, i know that sort of noise is mostly produced by pinging or spark knock, BUT here's the thing. after 3000 rpm's the noise goes away and i still have my foot on the gas pedal 3/4 in......I also have a noise coming from the catalytic converters (precats actually), a rattling type of noise that can be heard even from the cab with windows up at idle after driving more than 10 miles (when everything is at operating temp) coming directly from the cats and precats, after i shut off the engine i can hear the cats starting to crack because of metal contracting..

My main question is:

Can a damaged cat or a failing one which isnt completely plugged up cause a rattling spark knock type of noise at 3000 rpm under load and then go away as i described in the text above?

Both cats sound the same when idling and both cats do the same crackling noise after shut down. I just want to know if they also can cause spark knock if they are failing... this issue worries me each time i try to pass someone because i know its not good for the engine as i think its gonna blow up on my face lol.

Things i've done so far:
Cleaned MAF sensor (was a little dirty and improved a little the spark knocking)
Done a simple TB cleaning with throttle body cleaner solvent and checked operation of the EGR Valve
EGR Valve was cleaned with carb cleaner and checked operation of the diaphragm, no vacuum leak of the diaphragm and valve opened and closed as it should, no leaky valve
Checked cats and both of them produce noise at idle, but arent completely plugged up


Also.. I know in the states you would have legal issues by removing the cats because of emissions and that kind of stuff... Not my case here lol, here we dont have any laws enforcing us to have cats.. So in case i need to replace them, wouldnt it be easier and of course cheaper to just delete them and trying to use the Gotts mod? If so how can i remove the precats?

 

What engine? How many miles?

 

The easiest thing to do is gut the cats. With gutted or no cats, you will get a CEL with P0420 and P0430 unless you do the mod.

The rattling may not be spark knock, it's probably the cats. If you think it may be spark knock, use high octane gas, it should go away.

 

Hi, sorry i forgot to put the specs of the truck.

4.6L Modular V8, 4R70W auto tranny and it's 4x2 with stock gearing on the differential, around 70k miles on the clock, it stopped working a while ago.. (broken screw gear). Thanks.

 

The motor is equipped with a Knock sensor to retard the ignition timing when ping or knock is detected.
If there is no code set for a faulty knock circuit, you hearing some other noise.
The Cats have outer heat shields that come loose and rattle. Its a tinny sound as opposed to marbles sound.
Large strap clamps fixes that issue.
If the substrates in the cats are broken they have a more hollow knocking sound that can vary with exhaust flow pressure and RPM.
Good luck.

 

Hi thank you for your reply, as for the knock sensor well I did connect the scanner to check for codes and the only one I got was for a transmission position or range sensor malfunction, not related to the knock sensor.. yesterday I went under the truck and checked if the heat shields we're coming loose and they were tight but I did hit the cats with a plastic hammer and heard the hollow noise you described, however, when the engine is under load and the noise is heard it's not the same as the one i detected when the plastic hammer banging test was done.

But I do believe that maybe at certain rpms (which in this case is between 2800 and 3100) the cats start to rattle with the exhaust flow and pressure that is built under those circumstances..

 

Good morning everyone, just to inform you guys that i have found the cause of my supposed spark knock noise.. Was the cats, gutted them and installed the mod for the oxy sensor code and no more knocking. Thank you all.