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Oil Pressure FSM Spec Questions

T

tn_outside2003

NAXJA Forum User
Location
Tennessee
2000 XJ with a 4.0. 175,000 miles. New head.

FSM says HOT idle oil pressure at 11 psi is within specs.

Chilton says HOT idle oil pressure at 13 psi is within specs.

(I got this data form forums, NOT the actual manuals.)

My question: Is that number the recommended factory expectation in a good motor, OR is that number the MINIMUM before the motor needs to be rebuilt?

My XJ, when quite hot, after a 35 mile, 80 mph interstate haul, when idling at a red-light, will drop oil pressure down to about 13-20 psi. (It has a new sending unit). Idle at about 600-700 rpm. This is within the specified numbers. If I give it the SLIGHTEST amount of gas (to like 800-900 rpms) the pressure jumps immediately back to 40 psi or above. It runs between 40-50 psi on the Interstate, when driving at 80 mph. There is no engine noise other than a slight lower-end rattle right when started up. Within a couple of minutes it purrs like a kitten. It has a rebuilt, 2005 TUPY head from a Wrangler on it. My mechanic checked my main bearings. They looked smooth and no copper was showing through. They said "your bearings look pretty new." They did not spec them with a micrometer or anything. Their point being....."They look fine....if you lose pressure you will need a rebuild. No reason to spend all day speccing bearings. Could be rod bearings, cam bearings, shaft seats, main bearings... or any combination. So drive it until it loses pressure."

Does this mean that I drive it and forget about it......or consider the pressure at the low range and start saving for a rebuild?

Again...to be clear. My question is: Is my HOT oil pressure in a completely normal, "don't even think about it" range, OR is it indicating a worn engine and a rebuild in the next year or two?"

Thanks,

JG
 
Sounds what I would call normal.

What Oil and filter do you use ?

Mine used to idle scary low, like 13 PSI. Changed sending unit, went to wix oil filters and 15w -40 or 20w -50 for oil and its solid. Did one thing at a time to see what helped and by how much. Wix filter was good for a few PSI just on its own. that was like 6 years ago. still going strong. I would not fret over it.
 
I just put a Wix on it a couple of days ago. I have run Castrol 10W-30 since I got it at 53,000 miles.

I was considering a heavier oil. The Wix got me about 3-5 pounds.

Thanks for your tip.

JG
 
I seem to go about ~15-55 with new oil depending on revs, and 15-40ish before an oil change. Seems okay to me?
 
I really don't see how your gauge would be accurate enough to differentiate 11 and 13 PSI
 
Thank you for your replies. Exactly TWO people attempted to answer me with the data I requested. The others went off into a tangent regarding how accurate I am able to read my gauge. Not helpful!
For the two people that attempted to actually offer me advice based on actual oil pressures and experience...Thank You.

However...my original question was NOT answered by anyone:

ARE THE NUMBERS SPECIFIED (11 AND 13 RESPECTIVELY) THE NORMAL NUMBERS FOR HOT IDLE OR THE MINIMUM NUMBERS?

It is really a simple question. The answer would read something like this: "Those numbers are the recommended numbers, by the factory, for a new, good engine." OR "Those numbers are the MINIMUM safe numbers recommended by the factory and your engine is worn."

Anyone who actually KNOWS??

I am really not interested in a discussion of whether or not my gauge is accurate within 1 or 3 psi. That was NOT the intent of this thread.

It was a simple question. Are these numbers NORMAL or MINIMUM? I am referring the numbers specified by FSM and Chilton.....NOT your opinion on the accuracy of gauges.

Any help is appreciated. Hopefully a person who works on Jeeps for a living or a person who understands the FSM will step up and offer the actual answer to the question.

Thanks in advance to anyone who can offer any more useful answers.

JG
 
Approximately 13 psi warm idle is the minimum spec in the FSM for a used engine, and is also very commonly seen with older 4.0L engines.

Unless you are experiencing other oil pressure related issues, 10-13 psi does not mean a rebuild is required. If you have adequate oil pressure at 55 mph, stop worrying and enjoy the drive.
 
tn_outside2003 said:
Thank you for your replies. Exactly TWO people attempted to answer me with the data I requested. The others went off into a tangent regarding how accurate I am able to read my gauge. Not helpful!
For the two people that attempted to actually offer me advice based on actual oil pressures and experience...Thank You.

However...my original question was NOT answered by anyone:

ARE THE NUMBERS SPECIFIED (11 AND 13 RESPECTIVELY) THE NORMAL NUMBERS FOR HOT IDLE OR THE MINIMUM NUMBERS?

It is really a simple question. The answer would read something like this: "Those numbers are the recommended numbers, by the factory, for a new, good engine." OR "Those numbers are the MINIMUM safe numbers recommended by the factory and your engine is worn."

Anyone who actually KNOWS??

I am really not interested in a discussion of whether or not my gauge is accurate within 1 or 3 psi. That was NOT the intent of this thread.

It was a simple question. Are these numbers NORMAL or MINIMUM? I am referring the numbers specified by FSM and Chilton.....NOT your opinion on the accuracy of gauges.

Any help is appreciated. Hopefully a person who works on Jeeps for a living or a person who understands the FSM will step up and offer the actual answer to the question.

Thanks in advance to anyone who can offer any more useful answers.

JG
Click to expand...

This is a forum, Dude. No point in ranting. You gotta take the good advice with the worthless. Get over it. :looser:

Anyway, you already said what the FSM said. That means that the factory considers 11 PSI normal and they would not rebuild even a new engine if is had 11 PSI.
 
Last edited:
your oil pressure is just fine, drive it.
next time you have an engine apart deep enough to visually inspect the main bearings, dont accept "looks good to me" as anything acceptable out of the mechanic's mouth, even if it's a super low-budget job. new bearings and oil pump at that point were less than $200 away. some of the best insurance you could ever buy.
also, relax a little, you might learn something from the "additional info" you didnt ask for.
 
This response I have written is because I am bored, not because you're being nice and polite and look deserving of help. The guys who have answered you so far have worked on Jeeps for a long time and are used to maintaining them and giving practical advice that is in your best interest. Most importantly they are volunteers who want to help, nobody is paid to wait on you hand and foot. If you want to play entitled asshole with attitude find another forum.

-

Yes, consider the FSM numbers are used by techs in a dealership. I've always interpreted this to mean that no rebuild is warranted until oil pressure reads less than 13 PSI at hot idle when using a stock Mopar filter and a one of the SAE #w30 oils. It's very specific. Generally speaking a healthy 4.0 will show 30-40 PSI on the stock gauge at hot idle. It's sort of like the coolant temperature. Hanging out around the middle is "normal": Should be to the right when cold, and to the left when hot. Note that the engine oil heats up more slowly than the coolant! Exact pressure will vary depending on wear, weight, temperature, and filter specifications.

Oil pressure is frequently misunderstood. You do not NEED a lot of pressure. In reality it's a combined measurement of the internal health of the engine, the temperature of your oil, and the status of your filter. It is not something that needs to be maintained at a specific level by adding thicker oil or whatever.

The oil pressure sending unit is situated between the filter and cam. Most of the oil in the engine flows through the cam. From there onto the crank bearings and into the lifters up to the valve train. Some examples with approximate numbers of changing pressure:

- Cold engine, +20 PSI: Oil is very thick and resistant to flow, resulting in high pressure with little lubrication.
- Colder engine, +15 PSI: Oil slightly thicker than above, so thick that it won't flow through the filter. Filter's bypass valve is in operation and dirty oil is send through the engine.
- Cold engine, +10 PSI: Could be the difference between running 0w30 and 15w40, where 0w30 is much less resistant to flow and providing more lubrication than 15w40 at sub-zero temperatures.
- Hot engine, -8 PSI: Filter full of dirt, bypass valve has opened and dirty oil is being pumped through engine. OR, you just drove for 15 minutes at 4000 RPM and the oil is very hot!
- Approximately +5 PSI for every 1000 RPM

What matters the most is flow. You've seen the impact that the filter and viscosity has on this so now to the FSM's point: If oil pressure is showing up below 13 PSI, the engineers have reason to believe that a component is worn to the point that some part of the engine is not receiving adequate lubrication. That is to say that the giant leak, if you will, is starving other parts of the engine. Increasing oil weight accomplishes exactly nothing here: you cannot correct the difference in wear! You still have the same relative flow to the worn and non-worn areas.

I take it from your description that your 2000 XJ experienced a cracked head, was neglected, and instead of a full rebuild a 0331 TUPY head was bolted on to replace it. The cause of your low pressure is likely the cam bearings. Ethelyne glycol from the coolant is VERY bad when it plays with engine oil, not only does it replace lubricant causing wear, it's also corrosive to the bearing materials.

That said, a guy on another forum has been driving his 2004 WJ 4.0 with 4 PSI for the past 30k miles. No noise, still runs strong. Is there something wrong with his engine? Obviously yes. But it doesn't seem to really matter that much - rebuild it when it breaks!

In the mean time run a quality oil and quality filter and listen for odd noises. The 4.0L will last a long time.
 
holysirsalad said:
This response I have written is because I am bored, not because you're being nice and polite and look deserving of help. The guys who have answered you so far have worked on Jeeps for a long time and are used to maintaining them and giving practical advice that is in your best interest. Most importantly they are volunteers who want to help, nobody is paid to wait on you hand and foot. If you want to play entitled asshole with attitude find another forum.

-

Yes, consider the FSM numbers are used by techs in a dealership. I've always interpreted this to mean that no rebuild is warranted until oil pressure reads less than 13 PSI at hot idle when using a stock Mopar filter and a one of the SAE #w30 oils. It's very specific. Generally speaking a healthy 4.0 will show 30-40 PSI on the stock gauge at hot idle. It's sort of like the coolant temperature. Hanging out around the middle is "normal": Should be to the right when cold, and to the left when hot. Note that the engine oil heats up more slowly than the coolant! Exact pressure will vary depending on wear, weight, temperature, and filter specifications.

Oil pressure is frequently misunderstood. You do not NEED a lot of pressure. In reality it's a combined measurement of the internal health of the engine, the temperature of your oil, and the status of your filter. It is not something that needs to be maintained at a specific level by adding thicker oil or whatever.

The oil pressure sending unit is situated between the filter and cam. Most of the oil in the engine flows through the cam. From there onto the crank bearings and into the lifters up to the valve train. Some examples with approximate numbers of changing pressure:

- Cold engine, +20 PSI: Oil is very thick and resistant to flow, resulting in high pressure with little lubrication.
- Colder engine, +15 PSI: Oil slightly thicker than above, so thick that it won't flow through the filter. Filter's bypass valve is in operation and dirty oil is send through the engine.
- Cold engine, +10 PSI: Could be the difference between running 0w30 and 15w40, where 0w30 is much less resistant to flow and providing more lubrication than 15w40 at sub-zero temperatures.
- Hot engine, -8 PSI: Filter full of dirt, bypass valve has opened and dirty oil is being pumped through engine. OR, you just drove for 15 minutes at 4000 RPM and the oil is very hot!
- Approximately +5 PSI for every 1000 RPM

What matters the most is flow. You've seen the impact that the filter and viscosity has on this so now to the FSM's point: If oil pressure is showing up below 13 PSI, the engineers have reason to believe that a component is worn to the point that some part of the engine is not receiving adequate lubrication. That is to say that the giant leak, if you will, is starving other parts of the engine. Increasing oil weight accomplishes exactly nothing here: you cannot correct the difference in wear! You still have the same relative flow to the worn and non-worn areas.

I take it from your description that your 2000 XJ experienced a cracked head, was neglected, and instead of a full rebuild a 0331 TUPY head was bolted on to replace it. The cause of your low pressure is likely the cam bearings. Ethelyne glycol from the coolant is VERY bad when it plays with engine oil, not only does it replace lubricant causing wear, it's also corrosive to the bearing materials.

That said, a guy on another forum has been driving his 2004 WJ 4.0 with 4 PSI for the past 30k miles. No noise, still runs strong. Is there something wrong with his engine? Obviously yes. But it doesn't seem to really matter that much - rebuild it when it breaks!

In the mean time run a quality oil and quality filter and listen for odd noises. The 4.0L will last a long time.
Click to expand...


Well stated and true^^.
 
First, if you are running a Fram, change it.
 
I know this is an old thread but...

My 99 4cyl has about 275K on it. I bought it at 133K and replaced the idiot light cluster with a gauge cluster at about 160K. I have NEVER seen the oil pressure gauge move out of the 40-50psi range in all that time. Couple weeks ago after changing oil and filter, BAM the gauge is all over the place. Anywhere from 10-30psi at idle to 25-50psi at 60mph. I have even seen it drop to 0 at a stop light.

I was hoping it was just a wonky filter so I just replaced it but still getting the same numbers.

After reading this thread it sounds like I am now seeing what everyone else sees, I am hoping I am still ok. But, just strange how all of a sudden out the blue I am getting such a range of gauge readings.
 
Verify the gauge readings with a mechanical gauge before you start eyeballing the engine.
Totally possible the gauge sender is damaged / defective. It wouldn't be the first time somebody messed up the sender while changing the oil. Hell, I've done it myself.
 
yossarian19 said:
Verify the gauge readings with a mechanical gauge before you start eyeballing the engine.
Totally possible the gauge sender is damaged / defective. It wouldn't be the first time somebody messed up the sender while changing the oil. Hell, I've done it myself.
Click to expand...

My thoughts exactly.

And, what brand oil filter?
 
you should be ok with 11-13 psi. ive got a brand new oil pump/pickup tube on a promar reman short block.. after it reaches full operating temp, at an idle, oil pressure is about 13psi id say. i even t-d' in a mechanical gauge to be sure, and its pretty close to what the dash reads
 
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