4.0 L Oil pressure drops after warm up

[98Chero]

Jeep lovers I cant figure this one out.. its a 1990 4x4 4.0 L Cherokee, Even after an oil change the oil pressure gauge says 40+ psi when I first start the motor but after warming up to 185 degrees the oil presure drops to around 20 psi and alittle lower at idel speed. The engine dosnt use oil and there is no rod knocking sounds. Im clueless to why this engine dose this.Help Hints or Answers are very appreciated.

 

[SteelHeadz]

If there's no engine knock after the oil pressure gauge drops, it's likely that the oil pressure sending unit is bad.

 

[98Chero]

Thanks SteelHeadz
I ordered a sending unit today. I will post the results when I get a chance to put it in. I drive this jeep everyday to work and back and Im trying to restore it as Im using it. I cant recall ever having a oil sending unit act like this before, Im even using 20/50 castrol and a bottle of STP oil treatment. I guess if the new sending unit reads the same its the oil pump or ?

 

[thedevil1236]

at idle, my jeep has run about 20 psi for as long as i can remember. i was having a problem with the gauge for a bit to where it wouldnt work at all. i replaced the sending unit and nothing happened. i then just got a sandwich adapter and put in a manual gauge. a little while later the oil pressure gauge started working again...go figure. I was ready to swap out the oil pump and such. i was talking to a few friends of mine that are mechanics and they explained to me that if your main bearings are slowly going, which they will as time goes on, that the oil pressure will drop lower and lower. from the crankshaft, they help create a lot of the pressure apparently. you could also have an old crank shaft too. if its a 1990 unfortunately it could be any of the above. i bought an oil pump from advanced for like 80 bucks and another 20 or so for the gasket...and the problem kinda solved itself. so now i have a high flow oil pressure pump thats sitting around thats ready if any more symptoms pop up that points to it. anyways i hope this helps. but remember when the engine is cold the oil pressure will be higher, and as it warms up, it will drop. if it drops below 10psi or so, id be a little worried...

 

[flatie46]

It sounds normal to me, they all drop a bit as the oil gets warmed and viscosity drops. Engine heat expands tolerances, the wider the crank and cam bearing tolerance the less press you will have. 10psi per 1000rpm is pretty much the rule of thumb. I forget what the 4.0 is suppose to idle at something like 650 rpm, so that'd be 6.5 psi. Don't let it worry you untill you hear a chatter, then you have a problem. Personally, I like Lucas products myself. I add their oil stablizer every oil change. I wouldn't use a Fram oil filter either. I put one on my CJ and noticed a pressure drop, switched back to Wix pressure came back.

 

[98Chero]

I installed the new oil sending unit ....and.... Moderator plz insert a drum roll
(here)


My old sending unit wasnt bad, got the same psi reading from the new one. Actualy I think that the frequent oil changes and additives have washed away all the gum and gunk that was holding the engine together. I think the engine is going downhill fast because the lifters arnt getting enough oil pressure.... I think im beating a dead horse here..
:deadhorse:

Thanks for everyones input and advise... Im not going to kick this 1990 limited to the curb but rather start from the ground up... after i run the wheels off it.

gotta love them jeeps !

 

[SteelHeadz]

The only thing it could be after replacing the oil pressure sending unit are the main bearings, camshaft bearings and or the oil pump. The lifters would not affect oil pressure but will be affected by low oil pressure by making noise. The bearings are part of what creates pressure in the oil galleys of the engine other than the initial pressure created by the oil pump. If the bearings are worn, the pressure will drop. I'd say it's time to rejuvinate your engine with new bearings, pump and other parts that are severely worn down.

I installed the new oil sending unit ....and.... Moderator plz insert a drum roll
(here)


My old sending unit wasnt bad, got the same psi reading from the new one. Actualy I think that the frequent oil changes and additives have washed away all the gum and gunk that was holding the engine together. I think the engine is going downhill fast because the lifters arnt getting enough oil pressure.... I think im beating a dead horse here..
:deadhorse:

Thanks for everyones input and advise... Im not going to kick this 1990 limited to the curb but rather start from the ground up... after i run the wheels off it.

gotta love them jeeps !