4.0 oil pressure/rebuild?

[Fester]

The 4.0 in my YJ has decent oil pressure at start-up (about 40 psi), but after a few minutes it starts dropping to 10 - 5 - and sometimes 0 (just reading the stock gauge). Is this because when everything warms up it expands and the oil leaks through bearings etc.? Time for a rebuild or is it something else? It is possibly a higher mileage engine (not real sure). I verified the gauge readings by pulling the valve cover........I could see plenty of oil at start up but after a few minutes it didn't seem like there was much new oil coming up.

If it's time for a rebuild please recommend a decent machine shop.

Thanks in advance.

 

[Badcop]

Or you could buy my XJ 4.0 and do a swap with it. Even comes with an AW4 auto trans....

 

[Brett]

It could possibly be your oil pump is just worn out. My '89 4.0 pushes around 60 psi at startup and higher RPM's as well after it warms up. That's with 177K miles on it too.

 

[Fester]

It could possibly be your oil pump is just worn out. My '89 4.0 pushes around 60 psi at startup and higher RPM's as well after it warms up. That's with 177K miles on it too.

I thought that might be it too, but after it warms up for a minute and I rev it the pressure drops faster?

 

[mowlasley]

I had the same problem with mine. When I pulled the oil pan to have a look at the pump, I found one of the bolts missing that holds it to the block, so the passages were not even lined up correctly! I went ahead and replaced the pump and have no problems since (at 213,000 miles).

Did this problem suddenly crop up, or has it been gradually worsening? Mine just out of the blue started dropping.

 

[Fester]

I had the same problem with mine. When I pulled the oil pan to have a look at the pump, I found one of the bolts missing that holds it to the block, so the passages were not even lined up correctly! I went ahead and replaced the pump and have no problems since (at 213,000 miles).

Did this problem suddenly crop up, or has it been gradually worsening? Mine just out of the blue started dropping.

It was this way when I bought it so I'm not sure, but I'll look at the oil pump first. It looks like I will be able to pull the pan and get to the pump with the motor still in the Jeep. Were you able to replace the pump without pulling the motor? Thanks.

 

[Brett]

It was this way when I bought it so I'm not sure, but I'll look at the oil pump first. It looks like I will be able to pull the pan and get to the pump with the motor still in the Jeep. Were you able to replace the pump without pulling the motor? Thanks.

Yeah, you can access the oil pump just by pulling the pan. It's not fun to do, as the pan can be a pain to get off after it's been on the motor for so long, plus working under it isn't fun either. But, you don't have to pull the motor!

 

[mowlasley]

Yeah, you can access the oil pump just by pulling the pan. It's not fun to do, as the pan can be a pain to get off after it's been on the motor for so long, plus working under it isn't fun either. But, you don't have to pull the motor!

I have an XJ with 3.5" of lift, but I imagine with a YJ you could do the same. Just jack it up on the frame rails, let the axle droop as far as possible, and slide the oilpan out.

 

[Brett]

I have an XJ with 3.5" of lift, but I imagine with a YJ you could do the same. Just jack it up on the frame rails, let the axle droop as far as possible, and slide the oilpan out.

Bingo....pull the starter off and depending on the year, pull the bolts holding the exhaust pipe to the exhaust manifold. Makes it a LOT easier with those out of the way!

 

[Fester]

Thanks for the feedback. I'll have to tear it apart and see if I see any visible damage/problems with the bottom end and oil pump. Hopefully I'll find something........don't really want to pay for a rebuild right now.