Oil pressure drops when idling

[mike_b]

I know this has been discussed here many times, but I went through a good number of the posts, and I am still wondering ...

So, I have 2006 TJ 4.0 with around 220k miles. A years back I had the same problem: Engine runs fine when cold, Oil pressure just above the center on the gauge (no actual pressure values),. But after a longer drive (when the engine is warmed up) and coming to a stop (for example a traffic light), I see the oil pressure drop below the center position, slightly fluctuate a bit, and then drops to zero, sometimes with the "Check gauges" light coming on. As soon as I rev the engine again, the meter moves up to the middle position + and stays there as long as I am revving the engine just a bit over idling.
A year back i took the car to an oil change place and asked what that could be. They suggested an additive "for older engines", and first I thought that's just a gimmick to pull more money from my pocket, but to my surprise the problem went away. I had another oil change just a month ago, at the same place, and now it is back.
So, it seems that the additive did its job. I guess I will go back to the place and see if they can tell me what they did this time.

Absent any resolution of this, Is this something I should worry about, or should I ignore it? If the latter, what do I need to pay attention to keep it from causing a problem down the road?

 

[Duck Doctor]

Could be a faulty sending unit, which you can verify by testing oil pressure with a mechanical gauge.

 

[mike_b]

Sorry, should have mentioned that in my post. I tried a different sending unit when this happened the first time. Did not change anything.

 

[DuckNut]

You need to compare the dash gauge with a mechanical gauge as @Duck Doctor said. Then keep in mind a lot of oil pressure gauges are just dummy gauges, meaning they only show if there is pressure or not. They do not show actual pressure. Many people install aftermarket gauges so they know what their pressure is.

I believe in 2003 they switched to the dummy gauge. It shows 40psi and if pressure drops below 13 the gauge drops to 0.

 

[hear]

Sorry, should have mentioned that in my post. I tried a different sending unit when this happened the first time. Did not change anything.

I had to replace my daughter's twice before I got a good one. It was doing exactly what you described when I bought it, so I just replaced the sensor as a matter of habit. But then it still did the same thing so I put the mechanical gauge on it to verify I didn't have a bigger problem. Once I saw it was good, I tried another sender and now it reads perfect. I'm sure the mopar sensor will solve this for all time & eternity....

 

[mike_b]

Ok, so I hooked up a manual gauge, and it showed me 40 psi when the engine was cold (at idle), which then dropped to 20 psi when the engine was hot (2010 deg). As soon as I rev the engine a bit, pressure jumps up to 60 psi. Minimum at idle as per repair manual is 13. So, I guess it is either the sensor, or perhaps the electrical contact. I will clean the contacts and put the sending unit back in and if it happens again, change the sender.

 

[mike_b]

@DuckNut: you mention aftermarket oil pressure gauges. I might consider that, but if I use a different sender, would that not permanently lit the "check gauges" light with the annoying beeping? Or is there a way to override that?

 

[BlueC]

I might consider that, but if I use a different sender, would that not permanently lit the "check gauges" light with the annoying beeping? Or is there a way to override that?

You should be able to install a T and run two separate units.

 

[MBDIAGMAN]

Thanks for giving us the numbers. Keep doing frequent oil changes, drive it and enjoy it.

 

[astjp2]

Ok, so I hooked up a manual gauge, and it showed me 40 psi when the engine was cold (at idle), which then dropped to 20 psi when the engine was hot (2010 deg). As soon as I rev the engine a bit, pressure jumps up to 60 psi. Minimum at idle as per repair manual is 13. So, I guess it is either the sensor, or perhaps the electrical contact. I will clean the contacts and put the sending unit back in and if it happens again, change the sender.

Worn journals or oil pump. Drive it with 15w50 and you will see a pressure difference, normal part of wear. Not worth a rebuild yet till 10 psi or lower at idle.

 

[jeepguy03]

Ok, so I hooked up a manual gauge, and it showed me 40 psi when the engine was cold (at idle), which then dropped to 20 psi when the engine was hot (2010 deg). As soon as I rev the engine a bit, pressure jumps up to 60 psi. Minimum at idle as per repair manual is 13. So, I guess it is either the sensor, or perhaps the electrical contact. I will clean the contacts and put the sending unit back in and if it happens again, change the sender.

That is good pressure and within spec. These sensors are notoriously unreliable. I've owned several XJs, and worked on a few other 4.0 powered vehicles as well as my current TJ. The sensors are always going bad. I have a Mopar sending unit in my TJ right now, and the first time I drive my Jeep after winter storage the gauge is pegged at 80 for the first mile, then does some crazy sweep back and forth, then reads normal for the rest of the year I drive the Jeep. No idea why, but my pressure is fine via mechanical gauge. I wouldn't sweat it.

 

[jeepguy03]

Worn journals or oil pump. Drive it with 15w50 and you will see a pressure difference, normal part of wear. Not worth a rebuild yet till 10 psi or lower at idle.

That isn't worn out, it is within spec per the factory service manual.

 

[MBDIAGMAN]

Normal pressure as seen in a new or low mileage engine is, of course, a good thing. When bearings and oil pumps wear, the pressure will reduce and it indicates that there is some wear in the engine, but it doesn’t necessarily mean that it needs attention. Oil pressure can be significantly lower than that of a new engine without causing any further engine damage or wear.

In the seventies the oil embargo’s brought lots of attention to fuel economy, especially in big rigs. In the course of all that research and engineering Cummins regulated oil pressure to, if I recall correctly, 15 PSI. They claimed that any oil pressure above that level was unnecessary and all it was accomplishing was a small loss in fuel economy because it took more energy to turn the pump at higher pressure.

 

[astjp2]

That isn't worn out, it is within spec per the factory service manual.

Still lower than new, part of the wear process. I didn’t say worn out just worn.

 

[Woodrow]

Still lower than new, part of the wear process. I didn’t say worn out just worn.

Yep. My '97 4.0 did the same thing at 155K (around 10-12 psi with a mechanical gauge at warm idle and around 40psi when cruising). The factory gauge would drop to 0 and sometimes the check gauges light would come on. The engine ran well and sounded good. My solution was a Golen stroker which, with the same sending unit and factory gauge, shows 40psi at idle and just under 60psi when cruising. Problem solved.