1999 Cherokee 4.0

[wn1998]

Well I just changed my parents over to 5w40 Rotella T-Syn with an oversized napa gold oil filter after I did a cycle of Auto Rx.

The oil pressure drops below 40 during idle and rises up over 40 while accelerating. Is this normal? I have a 99 Jeep Wrangler TJ with the 2.5L and my pressure never drops below 40. My 2.5L stays right about at the 43-48psi and i don't think I have seen it drop at idle.

 

[michaelc80]

That's normal for that engine.

 

[JonS]

My 1995 4.0 idles at 35 psi hot, and 65-70 psi hot highway speed with Mobil 1 5w-30.

 

[Ryan00TJ]

Perfectly normal. I've noticed the 4.0s seem to run slightly lower oil pressure than our 2.5L's.

 

[MNgopher]

Sounds normal. My 88 Cherokee with the 4.0 idles at 30 psi hot, 55-60psi hot cruise, and 60 psi when cold. This running Rotella T Syn 5w40 and a regular (metric thread on this oldie) Napa Gold filter.

 

[99]

Perfectly normal for pretty much any engine. My 4.0s fit in the readings above. If the needle pegs itself at the high end or low end or starts reading really weird, the sending unit is bad.

 

[Gary Allan]

I did an informal survey on one of the jeep boards that I belong to a few years back. The 4.0 can have a wide variance in oil pressure ..even in the same model year. My wife's 99 peaked at 42 cold ..regardless of oil weight ..would drop to 13 at hot idle. This is with the OEM oil pump. Some, also with 99 model year engines, reported 20+/- idle pressures. My 02 2.5 stays in a relatively narrow range ...somewhere between 42-48 (guesstimate) regardless of temp or visc.

Many report reaching up into the 65-70 psi range ..others lower. The spec is so broad the FSM spec's for 99 read:
At idle (600 rpm) 13 psi
At 1600 rpm or higher: 37 to 75 psi
Oil pressure relief: 75 psi

The 2.5 is the same for the 99 model year.