I have a 1998 range rover 4.6 hse with 84000 mile. I bought it thinking it had a misfire! But now I am sure that it is not a misfire. Just before I bought it it had a complete tune up, new cats, new 02 sensors. I have checked wave forms on the ignition and injectors all good. No codes. 02 switching looks good also. It runs fine under a load and at highway speed. It just has a shake at 800/850 rpm in gear or not does not matter. Now Im thinking harmonic balancer or motor mounts. any thoughts?

 

I would lean towards the motor mounts. Do you see any rub marks on the insulation pad under the hood. That is usually where you see bad mounts showing up as they allow the engine to rise up and rub.
Mike

 

Yes there is a rub spot under the hood! Today while under the hood replacing the alernator[:@][:@]I saw a weight on the balancer, like a pound on wheel weight. Is this something LR did toget rid of a vibration?

 

I doubt it is a pound, probably a couple of ounces.
You may want to have the balance checked on all four tires to make sure they are not causinh a problem.
Mike

 

Sorry I should have been a little more clear. When I said pound on,I meant with a hammer! It looks like it is less than an ounce of weight. Today I had one of my guys check tire bal.and do a front end alignment,new water pump, fan clutch and fuel filter. We spent a little time on the vibration but no luck. You dont have to drive it to feel the vibration, it will do it in park. Tomorrow we are going to unbolt the torque converter and slide it back and start it to make sure it is not a trans problem.

 

Check the motor mounts. They should be easilly visible from the underside.
If weathered, powdery or cracked you will want to replace them.
When worn they will allow excessive side to side rock in the motor.
This is the cause of the hood rub.
When driving; the engine will torque to one side on take up the slack in the bad mount(against your hood).

 

I am going to replace the mount, but that is not the problem. To give an idea of what this feels like, it would be like taking 2 plug wires off and only running on 6 cyl. but it only does this at idle, bring it up to 1000 rpms and it is smooth. Going down the road and climing hills it is also smooth. I am new to rovers but not to fixing cars. This rover has been to a couple of other shops and the dealer for this problem and no one has found the problem. I have checked waveforms on the ignition and injectors all are good. Compression is 155 / 160 across all cyl. resistance test on coils found no problem. I have found a tsb for carbon build up on the valve guides so I may pull the heads and have then done just to eliminate them as the cause.

 

Gotcha.
With compression that even; it probably isn't piston slap, rings or bottom end.
Also sounds like you have been through all of the external issues.
When the heads come off; check the cam lobes for flat spots, mushroomed or bent pushrods or a stuck lifter.

 

I also found a tsb for the rocker arms, they have a new design so I will replace them aswell.

 

Bingo! be sure to follow the latest info on head reconditioning.