My engine knocks hard when its cold, what do I do?

Since you did not state the milage of the vehicle I will assume it is a high milage jeep. Some of the older motors had casting flaws, and this may be one it does not mean the motor is junk, it may just have areas where there is more material in spots and les in others. Since we are not talking about a complete teardown and rebuild, lets try to diagnose it correctly. If there is a worn bearing or timing chain the easiest way to diagnose it is when you do you next oil change try swapping in a heavy duty truck oil.(Delo 400 or Delvac, or Rotella T) the heavy duty deisel oil will exceed requirements of that engine and its detergent quality will clean the motor frim the inside out. also make sure that the coolant is at 50/50 or better in cold climates, as your emmissions system is first generation, it demands that the engine is 165 or better to function. Try a tank of Super Unleaded, to help. If you put on a new cat,make sure there are no air leaks at the manifold to the cat, as this can cause detonation noise to be confused as engine noise. The strait six from jeep never had high horsepower numbers but it was sufficient for offroading because of the torque. good luck and happy motoring. one more question, are you sure the plugs you have are gaped correctly with the right heat range? if not your knocking may be starting there.

"knock" refers to a specified engine situation that happens whilst the crankshaft magazine bearing inserts are excessively worn. This makes the oil hole too large and there may be steel banging in opposition to steel. Verity that engine oil stress is inside spec. Out of spec top oil clearances at best and backside of stroke are what makes the noise. Have the oil stress established with a gage and a hot engine. Drain the engine oil to peer if it has steel flake. Then dip a magnet into the oil to peer if if any metal attaches to the magnet. Low oil stress or steel flake suggests worn bearing inserts. Have the bearing inserts changed if the oil stress is low.

it could be a bearing in the engine starting to give, or a the timing belt/chain is loose you cant really say without hearing it

you might have spun a bearing on your crank. but being that thats a really expensive job you better try what bruddah ubove said and pray that thats the problem

that hard knock is your timing. i am sure that after they adjust it. the car will get it's power back. have fun.