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  #1  
Old September 12th, 2017, 15:54
cgross220 cgross220 is offline
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Low Oil Pressure

So I've seen pretty similar posts about this issue before but wanted some clarification on what might be going wrong before I start throwing money at it.

I recently had quite a bit of overheating issues (though NEVER ran it without coolant or anything) and replaced everything from the radiator to the heater core (water pump, thermostat, tubing, etc). Once I got everything done and took it on a drive, I noticed that after driving for a bit at idle the "Check Gauges" light would come on and the oil pressure would drop to near 0. It happened once at first a few weeks back, but now that I've got it out of the garage from the cooling system repairs it happens at idle whenever it gets warm. At first I figured the engine being hot before might have burned some oil off (I was grasping at straws) and topped it off with some 10W-30. No dice. The weird part is though, at idle once warm the needle will drop down to about 11lbs of pressure, then slowly sink down to about 9, then all of a sudden shoot down to like 2lbs.

In some threads the general consensus seems like it's main bearings, whereas in others the bearings aren't even brought up and everyone agrees that it's likely the sending unit. I'm gonna see if I can get a mechanical gauge from an Auto Parts store and check it and replace the oil with 10W-40 but just wanted to see if there's anything else I'm missing or any reason why I should pull the trigger on a sending unit/start looking for engine swaps now.

Thanks
Caleb
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Old September 12th, 2017, 15:57
cgross220 cgross220 is offline
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Re: Low Oil Pressure

Oh, it'd also be good to say I have a 2000 XJ Sport with the 4.0, A/C, and 4x4. If that changes anything
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  #3  
Old September 12th, 2017, 16:10
SolarBell SolarBell is offline
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Re: Low Oil Pressure

Been there, done that with the exact same model and year. Same symptoms and everything. 10 gets you 20 that the head is cracked and you leaked enough coolant into the oil to fry all the bearings. 15W-40 will help you along for the time being. (I got really good at shifting into neutral at lights and revving the engine to keep the pressure up.) The mechanical gauge is a good sanity check and pretty easy to do, but I'd recommend to start doing research on a long block replacement.
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Old September 12th, 2017, 16:17
cgross220 cgross220 is offline
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Re: Low Oil Pressure

My oil is completely clean though, and the coolant hasn't leaked one bit since I replaced everything (I know I had a crack in the radiator and bad water pump gasket, I wasn't chasing a ghost leak)
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Old September 12th, 2017, 16:38
blakews2217 blakews2217 is offline
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Low Oil Pressure

Quote:
Originally Posted by cgross220 View Post
My oil is completely clean though, and the coolant hasn't leaked one bit since I replaced everything (I know I had a crack in the radiator and bad water pump gasket, I wasn't chasing a ghost leak)


If it's like mine it will start good. Up at/near 40 with it cold. As it gets warm it will go down. 30,20,15,10,8. And then the noises start. Lifter clatter, maybe some knock if it's bad. Me and my dad built the 4.7 stroker I have now. I took the top end and he did the bottom. One beer lead to a few too many and he tried to polish the mains on the crank. It was ground for 10 under. The crank is like 12-14 under. Well now I have the same issue you describe but have the issue of some minor knock. Been driving it sense 2/17 as a DD as I'm trying to make it last by using 20w50 with about 20% Lucas oil stabilizer( I call it thickener because that's about all it does. ) effectively making it almost a 60 weight oil. Maybe it's time to use some gear oil? Just kidding. It wouldn't keep it together long. If I got the cash I'll probably go a high zinc content oil like break in oil.


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  #6  
Old September 12th, 2017, 16:41
cgross220 cgross220 is offline
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Re: Low Oil Pressure

I don't have any clatter or knocks or anything. It runs absolutely perfect. Did the low oil pressure start with the knocking or did the knocking come later?
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  #7  
Old September 12th, 2017, 16:44
blakews2217 blakews2217 is offline
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Low Oil Pressure

Quote:
Originally Posted by cgross220 View Post
I don't have any clatter or knocks or anything. It runs absolutely perfect. Did the low oil pressure start with the knocking or did the knocking come later?


It came later as a result of lack of oil. A general rule of thumb is you want 10 pounds of oil for every 1000 rpm. So 3500 you want atleast 35 psi. I already rebuilt this the first time due to a bad cam gear wiped out a few lifters and sent metal all through the motor. Fun times doing that job in the cold.


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Old September 12th, 2017, 16:48
cgross220 cgross220 is offline
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Re: Low Oil Pressure

So any idea if yours was the heads or the bearings?
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  #9  
Old September 12th, 2017, 17:00
RCP Phx RCP Phx is offline
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Re: Low Oil Pressure

Cam bearings on my 2k!
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Old September 12th, 2017, 17:18
cgross220 cgross220 is offline
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Re: Low Oil Pressure

Damn. Yall arent telling me what i wanted to hear haha. Anyone got an estimate on how long my engine will last with a heavier oil?
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  #11  
Old September 12th, 2017, 21:12
lawsoncl lawsoncl is offline
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Re: Low Oil Pressure

Quote:
Originally Posted by cgross220 View Post
I don't have any clatter or knocks or anything. It runs absolutely perfect. Did the low oil pressure start with the knocking or did the knocking come later?

Have you verified the pressure gauge is accurate with a mechanical gauge? The oil pressure senders do go bad from time to time.
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Old September 12th, 2017, 21:29
RCP Phx RCP Phx is offline
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Re: Low Oil Pressure

Quote:
Originally Posted by cgross220 View Post
Anyone got an estimate on how long my engine will last with a heavier oil?
I only got about another 2 months out of mine before I couldn't take it anymore!
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  #13  
Old September 13th, 2017, 08:07
jerms234 jerms234 is offline
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Re: Low Oil Pressure

Quote:
Originally Posted by lawsoncl View Post
Have you verified the pressure gauge is accurate with a mechanical gauge? The oil pressure senders do go bad from time to time.
I second this, verify with a mechanical gauge, my 99, my brothers 98, and friends 99 have all had bad sending units.

If you verify that the oil pressure is indeed low, and with a mechanical guage, i'd say low at idle is less than 15 PSI. (Perhaps someone can chime in on the FSMs recommended numbers) I'd recommend replacing the oil pump with a high volume pump, you'll be good to go for a long while (years). That is not a simple job, but it is DIY and 150$ vs 2500$+ for a new full motor. Oil pumps are known to wear on higher mileage 4.0s. On mine the oil pressure was electronically indicating 16 -18 PSI hot idle. With 2000 RPM at about 35PSI. I replaced the oil pump with high volume, and did the RMS and fixed the oil leaks while I was in there. The mains all looked great. With high volume pump my 99 now is never less than 35 PSI ever, and 2000 RPM is about 50 - 55 PSI all electrically indicated readings. I've never checked with a mechanical gauge. That is also with 10w-30 oil.

My .02$

Jeremy,

Last edited by jerms234; September 13th, 2017 at 08:12.
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  #14  
Old September 13th, 2017, 09:02
RCP Phx RCP Phx is offline
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Re: Low Oil Pressure

I did the HV oil pump first thinking that would help!
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  #15  
Old September 13th, 2017, 16:36
cgross220 cgross220 is offline
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Re: Low Oil Pressure

Thanks everyone. I haven't checked with a mechanical gauge but will do as soon as I can get over to an auto parts store. I'm also going to throw in some 10W-40 oil either way just since it sounds like the 4.0s like it more anyway. I definitely may try the HV oil pump; it's spent the past 1.5 months out of the 3 months I've had it in my garage so at that point unfortunately I'll probably just unload it and see what I can get
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