Oil Pressure Question

K_Dom

K_Dom

Member
Joined
Aug 13, 2021
Messages
62
Location
Springtown, TX
2000 4.0 with 187,000 miles. Here's my issue. I have the check gauge light/low oil pressure when engine is hot and at idle. I replaced the sending unit with a Mopar sending unit and still had the issue. Last night, I put in a T fitting and ran a manual gauge with the dash gauge. The pressures were reading the same between the two. The only difference is the manual gauge raised and lowered quicker with RPMs. At hot idle, my oil pressure is 8 psi. At running speeds, it doesn't get above 20 PSI when hot. On startup my pressure is at 60 and goes lower as the engine warms up. I replaced the oil pump about 5,000 miles ago. There is a slight tick to the engine when cold but quiets down when warm. What are all of your thoughts or suggestions in this situation.

Kevin
 
Might be time for a compression check, a leakdown test and maybe throw a borescope down the spark plug holes just to get feel for things.

When you did the oil pump did you pull a cap and inspect a bearing?

-Mac

PS SandJengines.com has rebuilt 4.0s for right around $2k shipped. Yeah you can buy a used engine for a lot less...but it's a mystery box...
 
The usual rule-of-thumb for acceptable oil pressure is 10 psi per 1000 engine rpms. Since yours gets no higher than 20 psi even when revving the engine I'd say it's time for a trip to a mechanic who knows the 4.0 for his opinion. The idle pressure doesn't sound that far off though.
 
macleanflood said:
Might be time for a compression check, a leakdown test and maybe throw a borescope down the spark plug holes just to get feel for things.

When you did the oil pump did you pull a cap and inspect a bearing?

-Mac

PS SandJengines.com has rebuilt 4.0s for right around $2k shipped. Yeah you can buy a used engine for a lot less...but it's a mystery box...
Click to expand...
No I did not inspect any of the bearing caps. I will have to see about doing those tests.
 
Did you have similar issues before changing the pump? Did the new pump work good until now? What brand pump did you use?
 
Gollywomper said:
Did you have similar issues before changing the pump? Did the new pump work good until now? What brand pump did you use?
Click to expand...
No I didn't have the same issue before. The ears on the drive end of the pump broke so I had to replace the pump. I replaced with a Melling pump. Around the same time I replaced the head due to a crack (0331) head. I'm thinking the internal leak on the head caused some bearing damage.
 
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