by engabe on Fri, 02 Dec 2011 5:01 +0000
Gday akibruce,
2 things come to mind.
Have the dowels that locate the clutch to the flywheel been left out? The clutch will end up misaligned to the crank centreline and cause an out of balance.
Has a conrod of vastly different weight been fitted to the engine. You say that it only appears under load! When free revving does the engine seem especially harsh at this particular RPM?
Did this vibration appear immediately after the conrod replacement?
My 2 cents worth. I apologise for the following ranting!!
It surprises me again and again that dealerships will just replace one conrod without checking to see if the weight is even close to the original. When engines are mass produced originally, conrods are graded by weight. They are normally within 5 - 10 grams and this is considered acceptable by most car companies. I have purchased individual Toyota diesel conrods (mainly 3L and 1HZ engines) and had them vary by more than 50 grams from the original, often requiring swapping for another new conrod to get it closer, so the set can be individually balanced. No single conrod should ever be replaced without match balancing it (if it is heavier) or match balancing the remainder of the rods (if the new rod is lighter).
Not checking the weight of a new conrod is a gamble. 9 times out of 10 the engine will not vibrate. You may be that other 10%.
I hope that I am wrong and you find something simple.
Some additional advice to anyone who is having a diesel engine rebuilt. Pay the extra to have the whole rotating assembly balanced and you will find just how smooth a diesel engine can be (and just how poorly balanced they often are from the factory).
Whenever a flywheel is remachined and a new clutch fitted these 2 items should always also be rebalanced. The shop i work out of gets several clutch/flywheels a week from people chasing vibrations.
By the way i should have introduced my self as a Sydney based engine builder specialising in engine development for circuit racing and rallying. I live and breathe engines. If anyone wants to PM me, I am more than happy to give out my mobile number for people who are having engine related problems.
Regards
Chris