XJ Oil Pressure Low: Why only the XJ's?
09-25-2019, 07:33 AM
#46
CF Veteran
Join Date: Aug 2019
Location: East Tennessee
Posts: 1,578
Received 217 Likes
on
185 Posts
Year: 1996
Model: Cherokee (XJ)
Engine: 242 cui
Bumping this. Remember as the OP here I ask, is there a difference in the xj that makes the oil pressure worse, or seem worse. So far it looks like xj's run a bit hotter, Mine runs around 205F. 155,000 mi. Oil pressure starts over 40 and dips to 18. My zj holds around 194. 212,000 mi, Oil pressure starts over 40 and dips to 33 ish.
As Waynerd points out, water boils at 212. Pretty close.
So, when oil approaches the boiling point of water, does it do anything special.. like thin out?
My oil pan on my xj is shabby and leaks alot. So am wondering if I might attempt replacing it and replacing the rod bearings to help the oil pressure.
That is, if I can't cure it by making it run cooler.
*** UPDATE Did I answer my own question? 205F? I just checked rockauto and the xj takes the same T stat as the zj, 195F. So, I have never changed the T state on the xj. Given that it takes a 195 stat, it seems to me to be not cooling properly. I had it in my head that the xj's were JUST supposed to run a bit hotter.
As Waynerd points out, water boils at 212. Pretty close.
So, when oil approaches the boiling point of water, does it do anything special.. like thin out?
My oil pan on my xj is shabby and leaks alot. So am wondering if I might attempt replacing it and replacing the rod bearings to help the oil pressure.
That is, if I can't cure it by making it run cooler.
*** UPDATE Did I answer my own question? 205F? I just checked rockauto and the xj takes the same T stat as the zj, 195F. So, I have never changed the T state on the xj. Given that it takes a 195 stat, it seems to me to be not cooling properly. I had it in my head that the xj's were JUST supposed to run a bit hotter.
You should not be running distilled water at one atmosphere of pressure. You should be running a coolant mix under pressure, which makes the boiling point higher and freezing point lower than distilled water.
The oil pressure issue seems to be related to where the pressure is sensed. However, if there is some Wrangler out there that has the same oil filter adapter and sender setup as an XJ, that reads different than an XJ, then sensing ain't it.
09-25-2019, 08:26 AM
#47
CF Veteran
Thread Starter
Thanks, but I have never been dumb enough to run straight water. I was wondering out loud if 212 was a point that would make the oil do something like thin out more.
The following users liked this post:
318SixPack (09-25-2019)
09-25-2019, 09:04 AM
#48
CF Veteran
Join Date: Aug 2019
Location: East Tennessee
Posts: 1,578
Received 217 Likes
on
185 Posts
Year: 1996
Model: Cherokee (XJ)
Engine: 242 cui
I'm not an oil engineer, but pretty sure Xw30 is going to be nearly identical viscosity at 195F as it is at 220F. And I doubt that a 242 engine is going to heat the oil and coolant at the same rate either. If someone installed an oil cooler that used a circuit off the radiator, which would have both fluids heating and cooling at roughly the same rate, then we could tell if 200+ is some kind of tipping point. OR if someone had an oil temp system installed, that would tell us something too.
09-25-2019, 09:10 AM
#49
CF Veteran
Thread Starter
I'm not an oil engineer, but pretty sure Xw30 is going to be nearly identical viscosity at 195F as it is at 220F. And I doubt that a 242 engine is going to heat the oil and coolant at the same rate either. If someone installed an oil cooler that used a circuit off the radiator, which would have both fluids heating and cooling at roughly the same rate, then we could tell if 200+ is some kind of tipping point. OR if someone had an oil temp system installed, that would tell us something too.
My zj is always right at 195 .
09-25-2019, 10:24 AM
#51
CF Veteran
Thread Starter
09-25-2019, 10:28 AM
#52
CF Veteran
Join Date: Jun 2017
Location: Hangover, PA
Posts: 2,216
Likes: 0
Received 627 Likes
on
467 Posts
Year: 1998
Model: Cherokee(XJ)
Engine: 4.0L
No the thermostat opens up when the coolant in the engine and heater core reach the temperature the thermostat is rated to open at. After that the radiator and fans work with the water pump to keep the coolant circulating and the temperature regulated.
If the thermostat determined an engines running temperature then engines would never overheat
If the thermostat determined an engines running temperature then engines would never overheat
09-25-2019, 10:38 AM
#53
CF Veteran
Thread Starter
No the thermostat opens up when the coolant in the engine and heater core reach the temperature the thermostat is rated to open at. After that the radiator and fans work with the water pump to keep the coolant circulating and the temperature regulated.
If the thermostat determined an engines running temperature then engines would never overheat
If the thermostat determined an engines running temperature then engines would never overheat
THERMOSTAT
A pellet type thermostat controls the operating temperature of the engine by controlling the amount of coolant flow to the radiator.
09-25-2019, 12:11 PM
#55
CF Veteran
Thread Starter
09-25-2019, 02:43 PM
#56
CF Veteran
Join Date: Dec 2016
Location: SoCal
Posts: 3,493
Received 391 Likes
on
309 Posts
Year: 1987
Model: Cherokee (XJ)
The thermostat controls the minimum operating temp of the engine, not the max or average. If you have a 195° t-stat, it should technically never get below 195° as thats when it will close. Think about when you are running the AC. The e-fan kicks on. If there was no thermostat at all, the temp would drop below 195° since the fan is on and cooling the radiator. But since there is a thermostat, it stops the water flow going to the radiator making the e-fan useless from cooling the engine since no water is flowing. Then once the water in the engine goes back above 195°, the thermostat opens and uses the radiator to cool the water back down. Its a never ending cycle.
The following users liked this post:
318SixPack (09-25-2019)
09-25-2019, 03:03 PM
#57
CF Veteran
Thread Starter
More than any other component the Thermostat is what is SUPPOSED to regulate the temp. It's not a simple ON OFF switch. It opens to varying degrees based on temp. There may be an allowable range but,
I think my point is that there is a desired operating temperature of 195, not 205, not 210. 195 is the target.
195 is the target operating temperature as measured by the CTS. If your fan clutch is old or your efan broken or radiator plugged you are likely to exceed the target range.
Normal operation temperature above 195 is, I would contend, not what Chrysler had in mind when the built the engine or they would put 205F etc. thermostats in them.
My ZJ never exceeds 195 with its all new components and hot tanked head with the valve job.
Last edited by 97grand4.0; 09-25-2019 at 03:11 PM.
09-25-2019, 03:15 PM
#58
CF Veteran
Thread Starter
The following users liked this post:
318SixPack (09-25-2019)
09-25-2019, 05:37 PM
#59
CF Veteran
I guess I am one of the few who havent had oil psi problems. My last xj, 165k miles, idled warm @ 30 psi. The one before that, which I rebuilt the engine, idled at the exact same pressure. One was a aw4 one was a ax15. So I am pretty sure it has nothing to do with the trans. Oil temp is more important than coolant temp or trans temp when talking about bearings. Highly doubt it's the lifters either, unless they are bypassing where the plunger meets the body of the lifter. Cam bearings seem to wear out faster than anything.
09-25-2019, 05:41 PM
#60
CF Veteran
We can get into the engineering of open and closed loop control system and talk about G(s) of the system and use calculus, but, we are off track.
More than any other component the Thermostat is what is SUPPOSED to regulate the temp. It's not a simple ON OFF switch. It opens to varying degrees based on temp. There may be an allowable range but,
I think my point is that there is a desired operating temperature of 195, not 205, not 210. 195 is the target.
195 is the target operating temperature as measured by the CTS. If your fan clutch is old or your efan broken or radiator plugged you are likely to exceed the target range.
Normal operation temperature above 195 is, I would contend, not what Chrysler had in mind when the built the engine or they would put 205F etc. thermostats in them.
My ZJ never exceeds 195 with its all new components and hot tanked head with the valve job.
More than any other component the Thermostat is what is SUPPOSED to regulate the temp. It's not a simple ON OFF switch. It opens to varying degrees based on temp. There may be an allowable range but,
I think my point is that there is a desired operating temperature of 195, not 205, not 210. 195 is the target.
195 is the target operating temperature as measured by the CTS. If your fan clutch is old or your efan broken or radiator plugged you are likely to exceed the target range.
Normal operation temperature above 195 is, I would contend, not what Chrysler had in mind when the built the engine or they would put 205F etc. thermostats in them.
My ZJ never exceeds 195 with its all new components and hot tanked head with the valve job.