My 98 wrangler has a lower end knock but only when cold! I replaced rod bearings, did not help!!!! I herd about undersized pistons could this be the problem or could it possible be main bearings????

check oil level-condition -pressure===knocking is usually damage to lower end --rod knock --piston slap--failed bearings

Tow it. No product is gonna help you with this. You could replace the bad rod bearing with new one, it might last 40 miles, but this is fairly big job and not for amateur.
Tow it.

images for 2001 ford ranger 3.0 engine

you wont see anything from there --lower knock usually called rod knock caused by a bad bearing or wrist pin [piston pin]to inspect the lower end is by the oil pan you can inspect the crank and rods from there

Rod/main bearings are bad. This is an engine rebuild or replace. You could throw an additive in it and drive it till it dies. I have seen engines last two weeks and up to 2 or 3 years, with rod knock. The ones that last a long time usually throw a piston rod out the side of the block.

More than likely your crankshaft will be as bad as the rod insert.

Very first thing you need to do is verify the actual oil pressure using a mechanical gauge. Lowest pressure at idle, with a warmed up engine would be 20psi. I'd rather see it at 30.
If the pressure is good, then you could have a valve lifter or rocker arm that has worn. Those cause tapping noise, not knocking. Knocking is usually caused by a worn rod or main engine bearing. When either wear, oil pressure gets lower (why I said to check that) Knocking can also be caused by an undersized piston or a cracked flywheel (common on automatics). Piston noise generally only happens cold and is much less common than it use to be in Jeep engines.
So, everyone that gave you info on your problem may be right...The only way to know for sure would be to have someone that is experienced and honest check it out for you. How expensive the repair is will entirely depend upon what the actual diagnosis is.

Hi...I responded to your earlier post.
I asked if you had plasti-gaged the crank journals before putting it back together. Sometime s one or more journals are more worn than the others and require a slightly over-sized bearing. If one journal is undersize it will cause a knock. Additionally I mentioned that the wrist-pin may be worn (the pivot pin that connects the piston to the connecting rod) An experienced tech can hear the difference...it is a slightly different, double knock. Less likely, but can happen, a piston skirt may be worn. That can sound like a knock. Older 4.0 engines were famous for that but haven't seen one recently. (usually noise subsides a bit as the engine gets hotter) What does your oil pressure look like? sometimes the camshaft bearings can wear enough to lower the crank pressure and cause knocking as well. (unusual but if engine was starved for oil before replacing the crank bearings it is a possibility.)

There are several reasons why you would hear a loud knock from the engine, and the knock will usually be either in the top end or in the lower or bottom end of the engine.

The first thing that should be done is to drain the engine oil, and then pour the oil filter into a pan where you can see it, and if there is a lot of metal flakes, or a fine cloud of brass floating in the engine oil, you will not want to waste the time or the money trying to repair that engine by dropping the oil pan and replacing the piston rod bearings, it will only be a very temporary patch at best, that is if the patch even lasts for a day.

If you do find metal flakes in the engine oil and oil filter, the piston rod bearings will not be the only bearings in the engine that will need to be replaced, and the crankshaft main bearings can only be replaced by removing the engine and placing it upside-down on a good engine stand. The biggest problem would be that even if you did successfully replace the piston rod and crankshaft main bearings, it would only take one piece of metal flake that was left over in the engine block to find its way to one of your new bearings and then it will take no time at all before that engine will be right back where you started, and that is how important just the cleaning process is in repairing or replacing internal engine components.

If you do not find any metal in the engine oil or oil filter, then you should attempt to find the actual cause of the knock before you decide how to repair the engine, and you will have to remove the engine oil pan to inspect the rod bearings, and if you do remove the engine oil pan make sure that when you inspect the rod bearings that you only do so one at a time because you can not mix up the rod caps, and be very sure that when you remove a rod cap that you do replace it the same way that it came off, and if you turn around the rod cap and install it the wrong way or scratch the crankshaft bearing surface the rod bearing will fail.

If you can hear the knocking louder from under the vehicle then the most likely causes for a lower end engine knock are a damaged piston, worn out piston rod bearings, a broken piston rod, a broken flex-plate, and sometimes loose torque converter to flex-plate bolts, and the only parts that you can really check out without opening up the engine would be a broken flex-plate, or for any loose torque converter to flex-plate bolts.

If you can hear the knocking more from the top of the engine, then the most likely causes for an upper engine knock would be a faulty lifter, or broken rocker arm, or a broken valve spring allowing the valve to contact the piston, and it will require the removal of the valve covers and possibly the intake manifold to inspect for the problem.

If you have any doubts then it would be a good idea to consider a good used engine that you can hear run before you buy it, or a rebuilt engine for that vehicle.

I hope that this information will help you out and save you some time and a lot of money.