Baystate Jeepers

Noticed that my TJ's oil pressure gauge drops dramatically when braking from 40 to 10. Doesn't stay at the low point, it just drops, then goes back to normal.

See video attached. Anyone ever have this? I'm hoping its nothing TOO serious.. hoping maybe a bad oil pump or the sensor? or maybe I should change the type of oil?

ps- sorry for the music in the background

http://smg.photobucket.com/albums/v355/ ... EO0007.mp4

When does it go back to normal? When you are stopped or when you start to speed up again?

My first suspect would be the oil pressure sensor.
Can be verified by attaching a mechanical pressure gauge... or just swap the pressure sensor and see.
Not that expensive or hard to swap.

My 97 did the same thing. Dropped so low the Check Gauges light and chime would go off.
Swapped in a new oil pressure sensor... All good again.

It drops when breaking, and goes back up to normal when speeding up. I was thinking of maybe trying to swap a sending unit on Friday morning. Cheap and easy swap that might work?
It drops to about 10, so its not turning on the dummy dash lights on yet.

I'm assuming when you say "type" of oil, you mean viscosity. A 5W30, 10W30, or 10w40 viscosity oil are all fine if your oiling system is in good shape.

Jeep oil pressure sending units are not known for their longevity. I would start there and plumb in a mechanical gauge as Kurt suggested. Off the top of my head, Jeep 2.5/4.0/258's are spec'd for 37+PSI of pressure at 1600 RPM. If it does come down to the oil pump, they are definitely not hard to replace. I recommend a Melling stock replacement or high volume pump. I've always felt that Jeep oiling systems are just OK when it comes to valvetrain lubrication, so I like the HV models.

I’m assuming the oil is clean and topped off. I’m also assuming with the radio and brakes applied you’re at 600-700 RPM. If this is true everything is working as it should. A factory new 6/8 cyl has a PSI of 13 at 600 RPM.

I hope this helps

To me, the normal range it 20-50 depending on the way Im driving.
I will drive it to CT tomorrow and see if it's still being the same low pressure.

Kurt wrote:
My first suspect would be the oil pressure sensor.
Can be verified by attaching a mechanical pressure gauge... or just swap the pressure sensor and see.
Not that expensive or hard to swap.

X2. Mine dropped so low I got a "check gauges" light, at idle, after she warmed up. Cold, she was running close to 40 el-bows

R/R the sensor and it's just like it should be. If you're going to remove the sensor to plumb in a mechanical gauge, you may as well just replace it (first) and see what happens

We shall see how it goes tomorrow!

Posimoto wrote:A factory new 6/8 cyl has a PSI of 13 at 600 RPM.

The Coyote 5.0L in my truck has much higher pressure than that to operate the intake and exhaust VVT. The 2.5L in my Heep idles about 800 @35-45 PSI depending on the oil's temp.

Don't forget too, that when you hit the brakes, the ECM raises the idle.

I'm willing to bet it's a sensor going bad.
Ol' Yeller was showing good pressure cold or hot with the mechanical gauge hooked up at idle. With the sending unit it shows good when cold and low to the point of the check gauges light going off when hot at idle.

Let's face it. Sending units suck. I used to have a "T" for both the factory and mechanical, but now I just run the mechanical with full copper tube for durability (not the cheap plastic tube most gauge's come with). I might have some 1/8" NPT T's and pipe pieces hanging around. I can toss some in a baggy and give them to you the next time I see you - whenever I can make a run again. I think I keep a few 1/8" plugs in my repair kit too (sending units can sometimes leak when they go bad).

The Depot or Lowes would have the tOOb and fittings too. Don't forget to tape the threads.

ZAEDOCK wrote:

Posimoto wrote:A factory new 6/8 cyl has a PSI of 13 at 600 RPM.

The Coyote 5.0L in my truck has much higher pressure than that to operate the intake and exhaust VVT. The 2.5L in my Heep idles about 800 @35-45 PSI depending on the oil's temp.

Don't forget too, that when you hit the brakes, the ECM raises the idle.

The factory service manual states 13 PSI at 600 for 258, 304, and 360 AMC engines.

I'm guessing the engine is an inline 6 the OP never mentioned the engine size.

I set the above engines to 700 rpm at idle

andd I have a 242 AMC engine (4L I-6)

Posimoto wrote:The factory service manual states 13 PSI at 600 for 258, 304, and 360 AMC engines.

I'm guessing the engine is an inline 6 the OP never mentioned the engine size.

I set the above engines to 700 rpm at idle

I'm a donkey. I thought you meant 6/8 cylinder engines in general. (In my defense, it was pre-coffee).