I need somebody - a couple of examples, hopefully - to run the RPMs up to 2000 on your motor while sitting still and tell me if you feel an occilating kind of vibration - like the engine runs smooth then a vibration builds up then it smooths out then the vibration builds up again.

I'm not talking about anything dramatic...just a vibration that is not really pronounced but you know its there. And the cycle is a few seconds - maybe three or four seconds from smooth to smooth.

I've always chalked it up to inline six harmonics but I'm starting to think it is something else. On mine, it is more pronounced under load but you can detect it sitting still, too.

 

Ya' know, that sounds like an interesting experiment and I'd do it if Cherry wasn't at the body spa but it got me to thinking anyway. Lord knows I've spent enough hours listening to it running so I'm going out on a limb here...

I've noticed the same and I always felt it was due to the clutch fan and/or an accessory cycle such-as alternator electrical load/unload or water pump cavitation while the t-stat opens/closes. I don't think I've reached the point where I'd worry about it unless it negatively affects driveability. After all, you don't want to become an autochondriac (I made that up). You could wait until after something breaks (not recommended) - like when the water pump spits out the shaft bushing and plows the fan into the radiator - you know what I mean?

Meanwhile, try using a mechanic's stethoscope (or hold a long extension bar against your ear-bone while probing accessories) - being cautious of rotating parts - on each accessory while you or a buddy hold RPMs near 2K. You may find that the oscillation is due to one of those...

 

I have the vibe, no mechanical fan, high volume water pump, which is just an aluminum pump with an anticaviation plate on the impeller. Only thing out of the last theory would be charging system and it's all brand new and high output too.

 

I like the "autochondriac" thing - we all get that syndrome once in a while.

I've got a stethoscope and can't isolate a particular source that isn't a rotating part you can't put the scope on.

A buddy of mine suggested maybe the header tubes are picking up a harmonic vibe and amplifying it. Or, it could be exhaust downtube and cat, etc. I don't think it is, though.

I'm leaning toward the idea that it is common to inline sixes and hoping this thread gets a few more hits - maybe examples of XJs that have it and ones that don't.

MCSSLS: I thought about the alternator, PS pump, water pump, etc., but if you have the vibe and have new water and charging system parts - and I have a new PS pump - that leaves very little besides the HB and the AC pump.

Let's hear from some more testers. I'm wondering now whether anyone does NOT have this slight occilating vibe.

 

[QUOTE=CAEMI;754105]I like the "autochondriac" thing - we all get that syndrome once in a while.QUOTE]


That is awesome, and soooo true