No oil pressure and loud knocking
 

I have a 98 Dakota with a 3.9l and 126k miles. A couple of months ago, I was driving down the interstate. The check gauges light came on and the oil pressure gauge was at zero. I immediately pulled over and turned off my truck. I restarted and crawled to the next exit. The pressure gauge stayed at a normal reading. I assumed I was low on oil. I added a quart, checked the level and it was fine. I didn't think twice about it. The same thing happened a few more times. However, the oil level each time was fine. I would restart my truck and find that the pressure was also fine. A week ago, I let my stepson borrow my truck. He called to let me know that the truck had started making a loud knocking noise. I told him to stay put. When I arrived I started the engine and found that it was in fact making a loud knocking and there was no oil pressure. Any ideas where I need to start to find the cause? Any help would be appreciated. Thanks

Welcome to DF
I would hook up an oil pressure gauge and verify no oil pressure. You could have a bad oil pump or a camshaft bearing coving the oil passage hole. I will move your post to the 2nd gen Dakota section.

First the knocking sounds Bad

Second the oil pressure sending units do go bad this is a normal problem with them replace the sending unit and see what happens but the knocking sounds bad like your oil pressure is indeed low.

You may have sludge and the oil pickup is plugged up with the sludge

Why people put off checking this kind of problem ASAP I will never understand. If its a real problem you could loose your engine... not a cheap fix

With any old engine if you used Dino oil sludge is a potential problem unless you use a cleaning oil most now are cleaning oils but some don't have the cleaners.

Dino oil will always give you sludge no matter what they say even the ones with the cleaners it just slows the sludge down is all.

if it's knocking, you've already lost the motor. Just a matter of which bearing. IF you are lucky, you just need to replace the oil pump and all the crank/rod/(maybe cam) bearings. May have to have the crank (and maybe cam) re-ground depending on how bad it damaged from no oil. Would have been much cheaper if you'd dropped the pan/checked with mechanical gauge the first time. We're putting a 20psi shutdown switch on the dragster, the camaro has a 15 pound switch. Would rather lose a round than a motor.

First, welcome to the forum & we feel bad for you. No pleasure in telling someone that either they have to spend a bunch of money or they should send their old truck to the wrecking yard.

I'm with the other posters who think the engine is toast. You can pull it out and have a look, but it's at least going to need to have all the crank bearing (mains and rods) checked, pull the cam & check that for damage, and even pull the pistons and rods out and check the piston pin bushings.

I don't know what caused the failure, but I'd suspect a clogged oil pickup screen. Oil pumps almost never fail these days, even with cruddy dino oil. But a bunch of floating debris in the engine can make the screen start to clog, and then it's just a matter of time.

Your alternatives are to rebuild this engine (expensive,) get a reman'd engine (expensive,) or find a good used one (not as expensive but risky.)

Sorry, man.

Another vote here for a clogged oil pump screen. As unlikely as it may sound, I have personally seen it happen, as have many others.

Sounds similar to the problem I had and I just finished replacing my engine.

I spun a crank bearing resulting in having no oil pressure and thus developing a knock in cyl 2.

So unfortunately I think your motor is toast too, sorry man!

OP said it would drop, then come back up after a restart. restart = trash falls off the pump into the sump...then gets sucked back up.