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Old 03-25-2016, 03:29 PM
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Low eng oil pressure

Hi, after the engine has warmed up to operating temp and under 900 rpm my psi gauge shows nothing. I hooked up a mechanical gauge right after and shows 10.5 to 11 psi. I also pulled the filter with 2k mi on it and cut it open some metal but small and few between. What is the tolerance pressure at idle for the engine oil? 2005 4.0 120,000 mi.

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Old 03-26-2016, 08:10 PM   #2
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What oil weight are you running, you may pick up a few pounds of pressure running 20w50 Castrol, in my engines if it doesn't have great pressure, I pull the bottom end open and start looking at clearances. And by good pressure, after you get off the freeway and sitting at the stop sign, at lease 25, I prefer 35+

Drop the pan, one at a time pull the bearing caps off and put a piece of plasti-gauge and measure the clearance. Napa does or did have 0.001 oversize bearings if necessary.

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Old 03-26-2016, 08:19 PM   #3
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Hope this helps. I had the same problem. Tried the same things you have done and finally gave up and took tj to my mechanic expecting for him to tell me I'd have to spring for a new engine. He put a Napa gold filter in it after flushing engine, and filled it with synthetic oil and voila! Oil pressure again! Good luck!
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Old 03-27-2016, 10:33 AM
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I'm running 10 30 synthetic mobile 1. I know a thicker oil will help bring my pressure up but I'm not sure about doing that. I Had not thought of using plastigauge I may try that out. But is that accurate that you want 25 to 35 psi at warm idle?
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Old 03-27-2016, 10:47 AM   #5
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Your problem is very likely just a bad oil pressure sender, replace it with a Mopar and avoid the cheap aftermarket oil pressure senders like Autozone sells. Odds of getting a good aftermarket sender from Autozone are not good, it took 3 in one day from Autozone before I finally got a good one years ago.

Stick with the factory recommended 5W-30 or 10W-30 viscosities. I'd run a higher oil viscosity oil if the engine was on its last legs and it was burning through oil, but a higher viscosity is NOT the fix for low oil pressure which is a different kind of problem altogether. Not to mention a high viscosity oil is harder to pump which means it takes longer to get into all the engine's nooks & crannies during a cold start. Cold starts are where the most engine damage is done so it's best to have an engine oil that can circulate and get into all engine parts the fastest.

And NO, you don't need or want 20-25 psi at idle. A good rule-of-thumb for acceptable oil pressure is about 10 psi per thousand engine RPMs. But on newer TJs, they only show a mid-scale reading if the real oil pressure is ok so a newer TJ won't show that low of a pressure at idle RPMs.
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Old 03-27-2016, 06:54 PM
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Thanks Jerry. Since I have two 05s I went ahead and swapped the senders from the jeeps. I still get the gauge to show low needle pointing all of the way left. I think my idle is at 850 rpm when hot and the needle drops left but when I put another 50 Rpm she goes back to mid gauge. I also hooked the mech gauge to it and showed 10.5 to 11 psi. Is there anything I should do or just check it with the mechanical gauge every oil change? Or should I change the brand of oil filter synthetic Dino ect? Thanks
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Old 03-27-2016, 07:00 PM   #7
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Going to a synthetic oil won't change anything, perhaps your oil filter's check valve is leaking. A filter with a known good heavy silicone check valve is Fram's Tough Guard which is an excellent quality oil filter with a 99% filtration rating. That's their upgraded oil filter in the silver can. Fram makes several oil filters but their Tough Guard is the only one I run. I switched to that filter several years ago after figuring out the anti-Fram crap I used to believe was pure BS.
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Old 03-27-2016, 07:30 PM   #8
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I had a similar problem, changing oil pressure sending unit worked for me. I use 5W 20 oil
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Old 03-27-2016, 09:01 PM
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I will try the tough guard filter. Thanks again.
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Old 04-20-2016, 05:51 PM   #10
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I'm new to the forum and was about to ask the same question, glad I searched for it first! Thanks Jerry, I'll be taking mine to get a new sender put in next week

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