Jeep Guy

I just had my catalytic converter removed and replaced with a straight pipe. It is just a temporary fix until I get another converter. but now that it is removed my engine makes a sharp rattle when I accecerate hard. if I just accelerate normal from a red light engine sounds fine. maybe a bit loud. is there something wrong that I need to give attention to?

ol"blue

It's probably spark-knock. Also known as detonation.

Jeep Guy

what is that exactly and is that a bad thing?

Jeep Guy

had a catalytic convrter put in and engine is still making same noise when accelerating hard. what next?

Firestorm500

Go up a grade on fuel. If that doesn't work, try a different brand.

What is happening is the fuel in the cylinder is self-igniting before the spark plug fires. The octane is too low for conditions. Lower octane fuel burns too fast.

The rattle is the piston vibrating like a bell in the bore. The flame front is trying to push the piston down while the crank is still pushing it up.

Very much of this will lead to engine destruction.

mike37

check the motor mounts I had that type noise it was the fan hitting the shroud when the motor shifted
open the hood and have some one look at the motor and do a power brake

only did it if I hit it hard off a stop

wewsel

check the flex plate

Mike1998XJ

The odd thing to me is apparently this problem wasn't there before his exhaust was worked on. Wouldn't it stand to reason that the problem was created when the work was done? I think he needs to get underneath and see if the work they did left something loose, or bent, or anything along that nature.
Maybe he could take someone for a ride that knows what detonation sounds like to confirm that it truely is detonation...
Mike

Jeep Guy

I know where the sound is coming from. it is definately coming from the distributor and based and some of your input my guess is that the timing got a little screwed up or I need higher octane fuel for my next fill up. I will fill up with higher octane gas today and hope that does the trick by the weekend. if not I will probably have a friend of mine adjust the timing if need be. thanks for your input and I will post back with an update. One bit of good news is that it does seem to have improved since having the new cat installed. still will make the same noise but it takes a bit more of a punch on the gas to create the sound.

Mike1998XJ

ok, you threw us for a loop with the information you originally gave... How does the timing get screwed up while working on the exhaust is beyond me!
Good Luck
Mike

ol"blue

I don't think you can adjust the timing on a 4.0L, but when you remove the cat and change the exhaust it can affect the way an engine performs, making it run lean. That could cause the detonation. Disconnect the bat cables and touch them together to erase the memory so that it can relearn the parameters. JMO

Mike1998XJ

Yep, mabe letting the computer re-learn would do it Ol' Blue. Since he is getting better as time goes on that would make sense.
He can save his money on higher octane gas at this point... Allowing the computer to re-learn and a six pack of beer with the money he saves on his fill-up sounds good.
I wonder how he knows the sound is coming from the distributor? Mechanics Stethescope? Length of hose? You know how those engine sounds can be hard to pinpoint with out some simple tools.
Can the distributor gears wear out on these engines?
Mike

fantic238

the engine torsion at full acceleration makes the exhaust pipe touch some hardware

Mike1998XJ

That would be more in line where I was originally headed since he tied his issue in with the timeframe when his exhaust was changed... My pipe is quite close to the crossmember near the tranny, but my pipe holder fits pretty tight in the rubber piece. I had my Daughter rev it and watched but it was ok on mine. Came close but never touched. He should check that too. Could they have messed the length of the tailpipe up also and the bend is contacting the axel on hard acceleration? Or maybe somewhere else....
Mike

fantic238

yeah, and the corner of the tank sheetmetal or the clamp to the leaf...