I recently purchased a used 2006 Rubicon with a fluctuating engine knock that seems to be coming from the top end of the engine. I've had little ride time on it and have noticed a cold start engine knock.

I took the Rubicon to the Honda dealer for diagnostics and said it's a decompression cam problem. The ATV mechanic whom I purchased the ATV from said he would take care of it and returned the Rubicon and said the decompression spring of the cam is free and works as it should and will not proceed further. The original owner of the Rubicon rode in on the ATV and I did not notice the knock at all. Took it home - cold start and there is the knock.

The knock fluctuates when fluttering the throttle but it's mostly knock. I rode it last night for about 2 miles and did not notice the knock once I returned to the shop.

Any ideas what this could be? I had a 2002 Rubicon once and it had no such knock.

Thanks

 

Timing chain, guides, or tensioner? These have been known to be problems. Have them insp. now before they cause serious damage.

 

What oil is in it, and how cold is it?

A mechanic's stethoscope can be very helpful in trying to diagnose something like this.

 

It's Honda's recomended oil. GN4 10W-40 I'm pretty sure.
The temp was about 48 degrees.

 

Probably not the problem then. They can make some bad noises until warmed up if you run 10w40 below zero.

Without hearing it, it is kind of hard to diagnose over the internet.

 

Mabey the mechanics were wrong. Try high octain pump gas. Maby that will solve it.

 

I always use the high octane gas so I think we're good there. I'm going to have to tear into it. First look at the cam shaft and go from there. The only thing that spooks me is pulling the clutch without the special tools but I think I can "MacGyver" something.

For the Honda dealer to go on a fishing expedition is going to start at $500 bucks and go up from there.

Thanks everyone - we'll see what happens.

 

Denguy; here is my take on your deal.

90% of the time, a knock is bad news in an engine. If it is what I call a "true" knock and not just a tap or click it is also almost always in the piston/ rod assembly.
The compression will load the piston/ rod assembly and when that happens you will hear the knock.

Good luck and let us know what you find out.

 

It's a knock at start up and it fluctuates and goes away when engine is well warmed up. At warm up the knock is 80% and subsides when the throttle is released for a fraction of a second. I would think piston slap or a rod bearing would make the knock all the time yet I see what you mean by compression loads on the piston may make a similar sound. Run it down the road a few miles and the sound is unnoticeable?

Regardless it looks like a major tear down to see what's going on. The engine is fairly smooth but with the knock. I heard there was a way to check the timing chain by backing off or applying more or less tension the cam tensioner. Might be a place to start?

 

Yeah, I think I'd just replace the tensioner before tearing down.

Has the machine ever been sunk?