I have an 89 Cherokee with a 4.0 inline 6, when its cold ill start it up and the oil pressure will be where its supposed to be at 45 but as it warms up the oil pressure drops till it hits like 5 to 10 or even lower the farther I drive, when I give it gas it goes up to maybe 20 but when I come to a stop it idles around 5. the engine will rattle too, only when the oil pressure is down that low when its cold it sounds like a champ, or even when its warm and the oil pressure is low, and I step on the gas it wont rattle. what could the problem be?

 

Bad oil pump. If you're getting the associated engine rattling when your guage goes low and no rattling when it reads good, I'd say it is reading correctly and you have straight up low oil pressure due to bad oil pump.

 

What kind of oil are you running?
Just sounds like an old, tired motor. Many people just throw a higher grade oil in as a bandaid.
My renix would start around 40 and drop to 20 at 55mph once warmed up and about 10 idle.

 

yup ^ but if that dont work ur in trouble cuz its prolly bad bearings

 

Yeah it sounds like a champ when i stomp on it, and im running 10 40 right now, i just changed the oil like a month ago, thats ****ed if i got a bad motor, tryin to just change the pump and hope it works i guess, thats all i can do right haha

 

x2 on that, sounds like an oil pump. Don't run it with the oil presure that low. If you have to buy your self some time until you can fix it, run some 20-50 weight oil.

 

I'd confirm your actual oil pressure. Get a mechanical oil pressure gauge and hook it up where the oil pressure sending unit currently resides. Then you'll know for sure and can make better decisions.

But if you have noise along with low oil pressure, then you have a "clearance" issue somewhere inside the engine, which can be bearings, an oil pump, or both.

How many miles on this engine?

 

Before you spend money on a pump drop the oil pan and check a couple of rod bearings. see how worn they are. If you get a new set of bearings plastiguage then to see what you clearances are. if it its rattling some that may by you some time. you got to drop the pan anyway. bearings arent that much. May save you from dropping the pan twice anyway.

 

X2!!!


The 4.0 at idle runs at 10psi. That is where it is suppost to run with say a 10w-30 oil. I run 10w-30 in winter and 20w-50 in the summer and that will keep your oil a little thicker on those hot days just to make you feel better... You dont have to do all that pointless work. You might have some noise, make sure its not just lifter tick man, all these old jeeps get that... its the nature of the engine.

 

Running oil that is to heavy can be just as bad, or even worse that running to light a weight oil. Oil thats too heavy can foam due to the added restriction and take the oiling of your engine to basically zero.

Def get the pressure check with a mechanical gauge. It should however drop as the engine heats up, oil flows more easily as it warms up.

 

that is not true. the only thing that "foams" oil is over filling the engine.

 

PS if you here a knock or rattle, make sure its not a crack in your exhaust manifold... on the 4.0 it makes a similar sound unlike other engines.

 

Well, as everyone has said, When you truely have Low oil pressure, you will get a rattling from the valvetrain. My 98 shows to have low oil pressure the warmer it gets but there is no knock in the engine. The sending units from what I have read up on can be faulty. But saddly in your case it sounds like one of three things. The engine is low on oil, the oil you have rather old/used, or your oil pump is dieing.

 

Erra, the thing that foams oil is water/air. Doesn't really matter how much oil you have, over filling the oil will cause it to leak into your cylinders.

 

Mine has been doing the same thing since I bought it 3.5 years and 30,000 miles ago. With 369,000 miles on the clock, I'm just going to keep running it the way I have been until it quits, which, hopefully isn't the middle of nowhere and after I already have a spare engine.