No oil pressure
 

I just picked up a 1997 Grand Cherokee 4.0 for $300. Got it running finally and the engine was knocking and had no oil pressure. Im assuming it may be the oil pump but wanted some opinions before I tear the engine open. What could cause this that I am over looking?

It is most likely a faulty Oil Pressure Sending Unit. If it actually had no oil pressure the engine would seize. The knocking could be any one of many other problems. Get a direct reading pressure gauge to confirm if you actually have oil pressure. The Sending Units are not expensive. NAPA sells one equivalent to the Original Factory Sending Unit and for not much more a better unit.

First stupid question, does it have oil in it? How long has it been sitting before it was fired up? It's possible the pump has lost it's prime and isn't picking it up. See if the filter is full. If not, crank it over without the filter on it. DON'T LET IT FIRE UP when doing this or you might get a bath! Without restrictions, the pump may start pumping if that's the problem. If you have oil flow, I would suspect major bearing damage. If it was just the gauge sender, it wouldn't be knocking. JMHO.

Engine is full of oil lol. Jeep was sitting for about 6-7 months. It was not doing this b4 it was parked. My sister in law own the Jeep before me.

Here's an update there's definitely no oil flow thru the engine. Its got a loud clacking sound when running. I suspect it's the valves from not having oil on them. It's definitely coming from the top end on the driver's side of the engine.

Okay, here's something you can try without tearing apart the engine. Take the oil pressure sender out and force oil under pressure into that fitting. I suggest a tank with oil in it pressurized by a air hose or a pump of some sort. That should fill the oil system with oil, pump, lifters, and bearings. Then replace the sender and try starting it. You may need to pressurize it for awhile because of the check valve in the filter, but that may not be sealing well if the oil drained back out of it.

I would suspect the problem is the pump has lost it's prime by the oil draining out of it and it's worn just enough to not pull a vacuum enough to pick it up from the pan. JMHO.

I have seen this problem in a few Chevy vehicles because the distributor drive shaft has broken below the gear and no longer driving the oil pump. It is kind of uncommon in our type of oil pump driveshaft but I have seen it happen a few times on Chevys with the similar drive system.


Just something else to take a look at. :).

Ill give this a try asap. Thanks for the advice.

I hope it's not that. I'll take a look tho.

I thought about that, but you said it was running when she parked it, so I didn't mention it.

If you pull the drive gear, make sure it's timed right when you put it back in.

Ya it had a lifter tick when she parked it but that was it. Should be fun retiming it lol.

Change oil, check sending unit, and it could be an oil pump as all previous replies have stated

Hoping to work on it this weekend. Will let y'all know what I find.

Um a broken piston skirt wouldn't cause me to have no oil pressure would it? Found the number 6 piston skirt in the oil pan today. What would cause this?

I would think that the no oil pressure problem caused the broken skirt.