Oil Pressure Question

A

AbkshelbyXJ

NAXJA Forum User
Location
Newport News, VA
The other day I was asking about swapping the engine outta my '00 XJ. My engine has been run hot due to a totally clogged radiator it appears. The temp hangs around 220 or 230 which is not good. The oil pressure is at like 10psi when the engine is hot. I have no type of engine rod knock and only a few seconds of cold start valve tapping.
I plan to swap another radiator in it to hopefully cool it down for now. Why is my oil pressure so low but the engine still runs fine? I changed the oil pressure sending unit. Could my gauge still be reading wrong? What is wiped on my engine causing the low pressure? I know a decent amount about engines but am not yet a 4.0L expert. But you guys are helping that a lot. :D
 
Could be the oil pressure bypass spring lost some tension due to heat.
 
Water boils at 212 degrees, 50/50 coolant mix at 265 degrees.

Did you boil the coolant?

Change the oil and filter, then test your oil pressure with a mechanical gauge.

Let us know what you find.
 
The coolant never boiled and the rusty mess I took out of the radiator had some antifreeze in it. But the radiator is stopped up from rust. I have no idea how long it has been run like this as I just bought the Cherokee. That oil pressure relief spring though in the post above is interesting. Never thought about that. The cooling fan cuts on at 220 and the gauge just moved slightly after that cut on. I am estimating 230. Still not good. Radiator should be tossed tomorrow.
 
joe_peters said:
Water boils at 212 degrees...
.
Click to expand...

At sea level, add 1.5 degrees for every 1000 ft.
Sorry, just had to throw that in...

In case it wasn't mentioned before, flush the system BEFORE installing the new radiator. Flush it with alot of clean water with the radiator out, take the t-stat housing off and run as much clean water through the water jacket in the block, all the hoses, the heater hoses and heater core too or all that crap will end up in your shiny new radiator.
 
ParadiseXJ said:
In case it wasn't mentioned before, flush the system BEFORE installing the new radiator. Flush it with alot of clean water with the radiator out, take the t-stat housing off and run as much clean water through the water jacket in the block, all the hoses, the heater hoses and heater core too or all that crap will end up in your shiny new radiator.
Click to expand...

be sure to flush the heck out of it. since the system is in such bad shape, i would recommend replacing both radiator hoses, all the heater hoses, t-stat + housing as well. it would probably be a good idea to replace the water pump as well.

i am still trying to get out that radiator stop leak that the prior owner put in there. been fighting that battle for 3 years now.

i know it sounds like a lot of work, but replacing all parts of the cooling system is nearly a must. all the crud in there can stick to the hoses, clog the t-stat, clog the heater core and cause a mess of issues latter on. heck, i would even flush it out, have some lunch and flush it again just to be on the safe side. also, i would fill it with the cheap coolant as odds are, your still going to have some debris in there and will have to do another flush in a few months. no sence in spending $$ on the top of the line stuff just to have to dump it out soon later.

hth
stewie
 
To answer your question about oil pressure, normally low oil pressure is caused by excessive bearing clearances (mains, connecting rod, and/or cam). Sometimes it can be the oil pump or pressure relief valve. It would be interesting to start a thread of how many people ever cured an oil pressure problem by replacing the oil pump. My guess is that the oil pump does not normally fix it.
 
I want to completely replace the entire cooling system on this thing in the spring but for now I flushed it for a 4th time and installed a clean radiator I had in stock from my local pick n pull junkyard. Finally got the nasty coolant out and the temp now is back down to 195 finally as it should be. This should help it survive a little longer until I find out why it has the low oil pressure at least. I don't know why people let vehicles go like this. Totally ridiculous. I appreciate everyones input.
 
Your engine should survive with 10 psi at idle. As a general rule you need 10 psi for every 1000 rpm for safe operation. You can also try stepping up your oil viscosity to 10w-40 if you have a higher mileage engine and see what effect it has. May be a very slight mileage penalty for this. Just some thoughts.
 
RandyD71 said:
Your engine should survive with 10 psi at idle. As a general rule you need 10 psi for every 1000 rpm for safe operation. You can also try stepping up your oil viscosity to 10w-40 if you have a higher mileage engine and see what effect it has. May be a very slight mileage penalty for this. Just some thoughts.
Click to expand...

I was thinking of running synthetic 15W-50 after the new whatever oil they put in from the dealer washes some dirt out of the engine. The thing only has 120K miles on it. Hopefully it will not disintegrate till I can find another engine or buy a new one.
 
Have you actually verified the pressure with a mechanical gauge yet? The dash gauge and sender are both notoriously inaccurate. Are you using a Fram? Low pressure and taking a long time to get pressure on startup are common problems with the crappy Frams.

Besides, 15W-50 is awfully thick. 10W-40 might be better? What did the dealer put in there?
 
Pelican said:
Have you told us what the pressure is at higher RPMs? All speculation till you verify with a mechanical gauge.
Click to expand...

Cold the gauge will read 50psi at 1500rpms.
At temp at 1500rpm it will read just over 20psi.
Hot idle reads 10psi and sometimes will just drop to zero but not now that the engine temp is 195 down from 230.
My other '00 Cherokee has 20psi at hot idle.
I think the dealer put 10w-30 motor oil in it.
I stopped using Fram filters many years ago.
Normally use Mopar filters and have never had an issue.
This one actually had a Mopar oil filter on it when I first saw it.
Dealer put some purolater look alike on it when they changed the oil.
 
More than one person has said that my oil pump may be what the problem is mainly the pressure relief spring. I'll likely just swap in a new pump and see if that does any good first. I have found 2 possible replacement engines if I need them. It is till running fine at the moment with very low oil pressure when hot.
 
And more than one person has suggested using mechanical guage to verify the actual oil pressure. The dash gauge is notoriously inaccurate. I seriously doubt the pressure relief spring is an issue.
 
I know the oil pressure is low. How low does not really matter nor do I have a mechanical gauge to hook up to it to verify the factory gauge which is close enough and tested fine. I'll end up replacing the engine at some point just right now I am trying to get it to survive till I can afford to do it right.
 
AbkshelbyXJ said:
Cold the gauge will read 50psi at 1500rpms.
At temp at 1500rpm it will read just over 20psi.
Hot idle reads 10psi and sometimes will just drop to zero but not now that the engine temp is 195 down from 230.
My other '00 Cherokee has 20psi at hot idle.
I think the dealer put 10w-30 motor oil in it.
I stopped using Fram filters many years ago.
Normally use Mopar filters and have never had an issue.
This one actually had a Mopar oil filter on it when I first saw it.
Dealer put some purolater look alike on it when they changed the oil.
Click to expand...

Book spec for oil pressure is 13psi idle, 35 to 75psi at 1600RPM and above.
I normally see (hot) 10psi idle, 30 or so at 2,000rpm. The rule of thumb is 10 per 1K up to the normal PSI.

You just got done overheating the oil. Replace it 10-30W and call it good. Replace the Filter. If the problem is caused by bad oil, this will remedy itself.
 
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