How to remove and replace main bearing in campshaft.

A bad engine bearing , rod bearing or a main bearing . Engine will seize up soon or later

loud knock and for how long

is it a diesel or petrol

broken engine mount

loud tapping is tappets ..should go away once oil reaches top of motor
water pump or bearing something running off drive belts

if high milage engine
a loud knock at engine revolution or half revolution
big end bearings or
piston slap or gudgeon wear

Main bearing replacement (there are four main bearings in your V6 engine) is MAJOR work. The engine must be removed from the vehicle and almost completely disassembled to get to the main bearings. If the bearings have 'spun' (likely), the engine will require machining to correct.
Unless you have a LOT of mechanical experience, this is not something a DIYer will be able to do. I'd strongly recommend getting a rebuilt engine and have it installed, if the van is worth keeping. It will probably be cheaper than having yours repaired.

Oh yea, lifters can be replaced , best if you replace cam an lifters together ! Loud knocking is not usually associated with lifters , they usually make a ticking noise ! Knocking noise is most usually in the bottom of the motor , main or rod bearings . most of the time rod bearings !

Have you checked the oil pressure.

Remove the engine drive belt and run to eliminate any accessories causing noise.

Check valve rockers, could have a bad hydraulic cam follower that is not pumping up.

Could be more serious, why did the head gasket fail, might be an associated problem with that if engine overheated ect.

You will have to remove crank to replace and removing motor would be my suggestion...Would also suggest having the crank turned.......hope this info helps and good luck......please rate my post....

hi a loud noise like the one you described can really only be a few things, either the main engine bearing is getting worn and is on its way out or you have a damaged or dirty lifter. The main difference between the two is the bearing will sound like a knock or grinding noise and the lifter will be more like a loud tick from either the left or right head on the motor you may just have to listen carefully to tell but it is possible and they will both speed up with the engine rpm. A bearing will usually cause problems running/starting/idling etc so maybe lean towards a lifter if it is damaged it will need an expensive replacement but try an oil change first as this can also free up dirt etc and solve the prob

It sounds like you have slipped a bearing and you'll need a new engine. You get most of your oil pressure from your connecting rod bearing and crank shaft bearing, you still have some oil pressure. No oil pressure will cause your engines ECM to shut the engine down, but from what you have described. The engine is to far gone for repairs and a engine swap would be the more economical thing to do.
Thank you for using fixya and sorry for the bad news.

hard to tell from limited info need to know more on type of knock possible rod brg.

Four things can give you a knocking noise. 1. failed rod bearing 2.failed main bearing 3. Bad piston wristpin 4. broken flex plate (auto trans)
If It's a bearing, likely you are seeing lower than normal oil pressure. Any bearing repair requires the pan to be dropped but to do the job correctly requires engine removal, crank re-grinding or replacement and oiling system cleanout (expensive). Wristpin (piston/rod assembly) can be serviced without engine removal but pan and cylinder head need to be removed. Flex plate is pretty much a trans remove and replace, with new wheel.
Only a good experienced tech can differentiate between the noises so I suggest you seek aid in your diagnosis, though, if either bearing or piston is the problem I'd likely find another engine before considering the repair.