98 Jeep TJ 4.0 High Oil Pressure

[Jms76]

Hello -

Researched this but was unable to find a solution or recommendation that was not related to low oil pressure or sending unit.

Have a rebuilt engine in my jeep that was done by one of the major engine rebuilders. Has run great for nearly 25,000 miles. The other morning started it up as usual (was in garage) and the check gauges light came on. Noticed the oil gauge was reading way low. Very uncommon, and I always keep a close eye on pressure and temp. Immediately shut it down, restarted same issue. Shut down again and restarted and oil pressure was then reading beyond the 40 mark pushing 60. Typically, when i start it cold, it runs slightly 40 PSI. Figured it was the oil sending unit. I did install a new one years ago when installed the engine.

This evening, pulled the sender unit and installed a gauge to read the pressure. At idle, it read 65-70PSI and as the engine heated up remained at 60-65PSI. As i manually revved the engine, it remained pretty much around the same mark, not increasing greatly with more RPM. When i installed the engine about 4 years ago, during initial start, I had the same gauge on the engine and took a pic. Going back to when the engine was new, it read around 50 PSI with the same gauge - now like mentioned fluctuating between 65-70.

I always run Pennzoil conventional 10W30 since I've owned it. That is what it currently has in it.

Did the following with no change in oil pressure.
- Checked oil level, right at full line. This oil has about 2000 miles on it.
- Removed, cleaned, and checked both breather elbows and pipes
- Replaced oil filter just in case there was some relief issue - used different brand as well,
- Inspected the filter internals after removal, no debris. Pretty clean actually.
- Checked antifreeze overflow, radiator, and valve cover cap for any signs of water into oil. The oil has no signs of emulsification. There is no sign of coolant loss or contamination.
- Idle is smooth and jeep sounds the same as normal. Distributor is tight, no signs of issue. Idle is not elevated at all either on start up. Very consistent with where it always is.

The oil pump and pick-up pipe were new at time of engine install and have about 25,000 miles on them. Oil pan was new as well at time on engine installation.

I change the oil every 3500-4000 miles. I could not imagine there being sludge build-up with 25,000 miles and pretty much all new parts when rebuilt engine was installed.

Puzzled by this, just though that my sending unit was on its way out. Unfortunately, maybe not that simple.

Any help would be appreciated, thank you.

 

[macleanflood]

There's a spring and check ball that controls pressure in the oil pump. Probably failed somehow.

As easy as it is to drop the oil pan ...skid pan and exhaust...I'd probably pull the pan and replace the oil pump.

I used a Melling normal volume and pressure in my 97 a couple years ago and it's doing great.

-Mac

 

[Jms76]

Update and outcome.

- Drained the oil and strained with fine paint strainer as it exited the oil pan. Clean, no particle debris collected. Minimal( almost none) particles even when check with magnet.

- Pulled oil pan, no large debris or concerning finds. There was a small collection of debris, less that 1/4" round of particles just below the oil pan drain plug which had some metallic characteristics. Very similar to what I've seen in pans and gearboxes.

- Performed crankcase inspection, no signs of bearings wear, physical damage, excessive heat, etc. checked everything - cam journals, rod caps, main bearing caps, timing chain, thrush bearings, etc.

- Allowed oil to sit and checked for water or fuel, no separation, thinning, color and smell normal.

- Removed oil pump, nothing overtly wrong with pump or strainer. No obvious blockage of pick-up strainer. Did not dis-assemble any further.

- Inspected distributor gear, camshaft interface gear, and distributor oil pump drive - no damage or concern.

- Called Melling and spoke with a tech. He informed me that this does occur on occasion where the relief valve becomes contaminated especially after a rebuild. Explained the scenario I experienced, he felt that when I started and oil pressure was low the relief was stuck open. Then after restart, it freed up and become suck in the closed position. This may be the reason the oil pressure did not vary with rpm.

- Replaced oil pump with standard Melling pump and strainer, re-assembled, primed, etc.

- The oil pressure was reading cold at 50-60 PSI (original pressure when engine was installed was 40-50). The fully hot oil pressure at idle read was 35, original pressure was about 28.

- The oil pressure did vary now with RPM.

- The situation is improved, it’s possible with a little time the oil pressure will drop as pump wears in. Its higher than original, but lower and fluctuates with RPM now.

- Obviously was unable to inspect the block internal oil passages, but there were no signs of bearing material, heat marks on caps, or concerning cam wear. The lifers are quiet and fully sound like each had good oil flow. Engine runs as it did prior with no indication of any cylinder mis-function. I’ve put around 200 miles on the Jeep since the oil pump change.

 

[Steel City 06]

Consider using 5W-30 in lieu of 10W-30 for your next oil change. That will help reduce your high pressures when cold, but won't affect your hot pressures. It's also a bit more energy efficient.

That said, don't bother changing it out now. The actual difference is fairly small, so no need to throw out good oil just yet.

You could also install an oil to coolant heat exchanger which will heat the oil up to operating temperature far faster:
https://wranglertjforum.com/threads...ost-oil-cooler-heater-in-a-4-0-and-why.73481/
It also serves as an oil cooler once everything is up to temperature. Basically it clamps oil temperature to within 10 degrees or so of coolant temperature.

 

[TJ Jim]

And @macleanflood does it again. Way to go Marc!

 

[JKP]

One thing to be aware of is that anything below 40 weight has had a large portion of the ZDDP removed due to EPA regs unless you get a specialty oil.

Been running 0W-40 in my 4.0s for this reason.

 

[bohuggabee]

Hello -

Researched this but was unable to find a solution or recommendation that was not related to low oil pressure or sending unit.

Have a rebuilt engine in my jeep that was done by one of the major engine rebuilders. Has run great for nearly 25,000 miles. The other morning started it up as usual (was in garage) and the check gauges light came on. Noticed the oil gauge was reading way low. Very uncommon, and I always keep a close eye on pressure and temp. Immediately shut it down, restarted same issue. Shut down again and restarted and oil pressure was then reading beyond the 40 mark pushing 60. Typically, when i start it cold, it runs slightly 40 PSI. Figured it was the oil sending unit. I did install a new one years ago when installed the engine.

This evening, pulled the sender unit and installed a gauge to read the pressure. At idle, it read 65-70PSI and as the engine heated up remained at 60-65PSI. As i manually revved the engine, it remained pretty much around the same mark, not increasing greatly with more RPM. When i installed the engine about 4 years ago, during initial start, I had the same gauge on the engine and took a pic. Going back to when the engine was new, it read around 50 PSI with the same gauge - now like mentioned fluctuating between 65-70.

I always run Pennzoil conventional 10W30 since I've owned it. That is what it currently has in it.

Did the following with no change in oil pressure.
- Checked oil level, right at full line. This oil has about 2000 miles on it.
- Removed, cleaned, and checked both breather elbows and pipes
- Replaced oil filter just in case there was some relief issue - used different brand as well,
- Inspected the filter internals after removal, no debris. Pretty clean actually.
- Checked antifreeze overflow, radiator, and valve cover cap for any signs of water into oil. The oil has no signs of emulsification. There is no sign of coolant loss or contamination.
- Idle is smooth and jeep sounds the same as normal. Distributor is tight, no signs of issue. Idle is not elevated at all either on start up. Very consistent with where it always is.

The oil pump and pick-up pipe were new at time of engine install and have about 25,000 miles on them. Oil pan was new as well at time on engine installation.

I change the oil every 3500-4000 miles. I could not imagine there being sludge build-up with 25,000 miles and pretty much all new parts when rebuilt engine was installed.

Puzzled by this, just though that my sending unit was on its way out. Unfortunately, maybe not that simple.

Any help would be appreciated, thank you.

Im going of basics. I didnt read responses or your entire post. When i have oil pressure issues. Remove the oil pressure gauge. Clean it. Put it back on. Use synthetic oil.