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Unread 03-26-2010, 06:10 PM   #1
JeepNoma
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Oil pressure drops? 4.0L

So my 1998 oil pressure drops when at idle and the minute i accelerate the needle jumps back up to 40psi. it happens maybe once a month..any ideas?

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Unread 03-26-2010, 06:48 PM   #2
cruiser54
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Check your oil pressure with a manual gauge. It is possibly the sending unit. Exactly which oil filter are you using?
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Unread 03-26-2010, 06:52 PM   #3
JeepNoma
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where is the manual tie in? and the thing that sucks is it rarely does it so id have to check it manually on the road.
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Unread 03-26-2010, 06:55 PM   #4
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does it do it when it gets hot? mine does it only when i idle and when its already hot. it SUCKS and i run 10w-40. Im wondering if it my sender. we might be in the same boat/.
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Unread 03-26-2010, 06:56 PM   #5
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what is your pressure at idle?
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Unread 03-26-2010, 07:02 PM   #6
CJ7-Tim
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Oil pressure is directly related to engine RPM's. With a warm engine, oil pressures should be 10-20 psi at idle and 40-60 psi at 60mph.

If your gauge does not register these changes, you may have a bad oil pressure sender, or a wiring problem.
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Unread 03-26-2010, 07:15 PM   #7
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What CJ-7 Tim is suggesting is what I was getting at. If it's that intermittent, and knowing the high failure rate of sending units, I would probably bite the bullet and replace the sender. As to your question about where to hook up a manual gauge, it is the same place your sending unit is threaded into now. You would unscrew the factory sender and screw in the manual gauge hose.
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Unread 03-26-2010, 08:19 PM   #8
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My oil pressure always increases with increased engine speed and the opposite.When my sender went the gauge just spiked.Thats all I know about that.
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Unread 03-26-2010, 08:28 PM   #9
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I've had a couple occasions where the oil pressure will swing wildly from idle to acceleration. It always happened after changing the oil and filter. Each time I grab a new filter and swap it out and it goes away. I've heard the cheap oil filters can cause this type of thing and in my experience it seems to be true. I've also replaced the oil pressure sending unit twice over the years and in those cases the pressure just goes to zero and stays there.

Try a new filter. The $3-4 might be worth it over the $40 for the sending unit.
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Unread 03-26-2010, 08:32 PM   #10
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just tap the gauge like everyone else does unless your engines about to die which i doubt being as its the 4.0L and killing those is difficult to say the least. my guess bad gauge mine drops off to nothing all the time till i tap it
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Unread 03-26-2010, 08:48 PM   #11
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The cheapie oil filters should not be used. The 4.0 oil filter mounting arrangement necessitates using an oil filter with a high quality anti-drainback valve. Use either the factory filter from the dealer or Wix or Napa Gold.
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Unread 03-26-2010, 09:55 PM   #12
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I used to buy whatever filter was cheapest and that was when I had the fluctuation issues. I started buying Mopar filters at Walmart and have never had the issue since.
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Unread 03-26-2010, 10:00 PM   #13
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GearGuy10 qualifies for the "been there, done that" award and I thank him for his response and support.
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Unread 03-26-2010, 10:29 PM   #14
bmac13sv
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i have a mobil 1 filter on there now. I just replaced my cracked 0331 head and thought that was the issue because the crack would propigate when the engine was hot. but it seemingly did not solve that problem. i no longer lose coolant though but when im at a hot idle it will definitely drop to under 10 and the check gauges light will come on. im gonna check it with a mechanical gauge sometime soon here.
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Unread 03-26-2010, 11:18 PM   #15
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Quote:
Originally Posted by JeepNoma View Post
So my 1998 oil pressure drops when at idle and the minute i accelerate the needle jumps back up to 40psi. it happens maybe once a month..any ideas?
This is totally normal - you should have more oil pressure with higher RPM. You should be above 13 psi at hot idle, and above 30 psi at hot when driving.

Most cars have "idiot" gages that don't move according to the actual oil pressure. The Jeeps do, which is a benefit. When the oil is hot and your idle pressure is above 13 psi then you are fine.
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