Oil pressure in a 4.0

[catfishblues]

My dad's XJ just started losing pressure. When it's warm, it fluctuates between 2 at idle and 20 or so when you rev it. Yesterday, it held 40 or better. A fresh oil change made no difference. It's not knocking yet, but we don't want it to, either. It's got about 175k on it and it's a 99. Thanks

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[NickMaul]

Could just be the sender?

 

[71chevytruckguy]

Sounds like a sender or a gauge. Really strange for a motor to lose 20lbs of oil pressure over night.

 

[catfishblues]

It starts at 40 cold, then gradually drops. It really doesn't seem like a gauge or sender problem, but I haven't put a mechanical gauge on it yet, either. It may also be electrical. There is some corrosion in the terminals, but it isn't acting erratic. It seems to be reading like it should.

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[transman731]

a mechanical gauge will tell you all you need to know

 

[catfishblues]

Yeah. That's the next step. Is it 1/8 npt?

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[gt500swim]

mine did the same thing... might be time for a new oil pump

 

[Jeff B]

These have a classic sending unit issue. located at the filter head, replace it & your problem will be gone. (Usually)

 

[catfishblues]

Jeff, it was the sender. Sorry I hadn't checked back in. Thanks for all your help, guys. I didn't realize how common this was with the 4.0.

 

[JMOliver]

Yea common thing on the xj 4.0s

 

[flyn289]

Had the same problem on my xj...unfortunately mine is a 2000 and i lost oil pressure due to a cracked head toasting my bearings...