Knocking only when accelerating

I have a '99 XJ that makes a knocking noise only when I accelerate. When I let off the gas and just let it roll it's fine but once I accelerate it knocks. When I put it in park and step on the gas the engine sounds normal, no knocking of any kind.

I can't imagine it's a rod or valve knock because I had a remanufactured engine (all new internals) professionally installed about 8 months ago. The only aftermarket product I have on the engine is a cold air intake and a stainless steel header that had already cracked but was rewelded and re-installed along with a flex exhaust pipe section to ensure it wont happen again. A new muffler and high flow cat was also installed professionally at the time of the flex pipe. The knocking only started once I received the xj back after the re-welding of the header and installation of the new exhaust components.

Remedies I have already tried:
- New spark plugs
- New spark plug wires
- Premium gas
- Oil is changed every 3,000 miles along with zinc additive

SOunds like your header cracked again to me.

Pull the inspection cover for the flexplate and look for cracks near the bolts.

Is it a knock or valve tap, from crappy fuel?

sounds like a cracked header to me. some sort of exhaust leak. if it was a rod knock or valve tap it will most likely be constant

Thanks for the input. I've tried premium gas heyhar. I can't visually see any cracks in the header. I guess that's what i get for buying a cheap ss one. I'm kicking myself now - should have kept the OEM on. Guess I will live with it until it gets worse.

Busted motor mount?

Are you sure the knock is coming from the engine compartment? If u-joints are getting sloppy, they will make a knocking sound when accelerating. It will help to try to identify the location of the knock and if you can feel any vibration along with the knocking sound.

Put it in drive, stand on the brake and bring the RPMs up to 2000 or so for no more than about 30 seconds. Does it still make a noise then?

I'd agree with Rockslut, it sounds like loose flexplate bolts or a cracked flexplate to me.

I had the broken motor mounts replaced. I will take a look at the flexplate and try the standing on the brake up to 2,000 rpm.

Talyn said:

Put it in drive, stand on the brake and bring the RPMs up to 2000 or so for no more than about 30 seconds. Does it still make a noise then?

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Yes - It still makes the noise when I do this.

wildbill1 said:

Are you sure the knock is coming from the engine compartment? If u-joints are getting sloppy, they will make a knocking sound when accelerating. It will help to try to identify the location of the knock and if you can feel any vibration along with the knocking sound.

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Already replaced front drive shaft U-joints and swapped the front axle shafts for new CV shafts.

Ryan-MI said:

Yes - It still makes the noise when I do this.

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That rules out any thing axle and drive shaft related.

Since it doesn't do it in park or neutral its going to be hard to diagnose with out a helper. You will probably have to have someone stand on the brake and pull the RPMs up while someone checks the engine bay.. Be careful though.

I would check the flex plate first though. And the motor mounts. Possibly the engine is shifting and the fan could be hitting something.