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Old 04-28-2009, 06:30 PM   #1
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oil pressure

i have a 99 wrangler sport, after the engine heats up when it idils the oil pressure drops from 40 to 20, and the rpms go back up so does the oil, i have recently changed the oil and oil and air filters. any recommendations will be helpful,

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Old 04-28-2009, 06:52 PM   #2
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It is absolutely 100% normal for oil pressure to vary in step with engine rpms. The oil pump is driven by the engine so the faster the engine rpms, the higher the oil pressure. A good rule of thumb for typical engine oil pressure is pretty much right at 10 psi per 1000 engine rpms. Your oil pressure sounds perfectly fine.

Be aware that on newer TJs and many cars with oil pressure gauges, they have "dumbed" the gauge down so it doesn't move up and down. The reason being is that varying oil pressure unnecessarily alarms those who don't really understand that it's perfectly normal and they were pestering the dealers with oil pressure concerns. So in their infinite wisdom, the engineers have designed newer TJs to have non-varying oil pressure... but really the oil pressure is varying, it's just being hidden by software programming.

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Old 04-28-2009, 06:58 PM   #3
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IMHO 10psi per 1000rpms is bull. If you have 10psi of psi pressure at 1000rpms something isn't right. Every engine is different. but i disagree with that statement.
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Old 04-28-2009, 07:02 PM   #4
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Quote:
Originally Posted by 4.0l sahara View Post
IMHO 10psi per 1000rpms is bull. If you have 10psi of psi pressure at 1000rpms something isn't right. Every engine is different. but i disagree with that statement.
That rule-of-thumb, which doesn't exactly cover every single engine type in the world but illustrates the issue very well, has been around longer than I've been alive... 61 years by the way.
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Old 04-28-2009, 07:38 PM   #5
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I would like to know of one modern day motor that is around that idles with less than 10psi of oil pressure or has 20 psi at 2k. I'm talking about modern day engines not a model A with a gravity fed oiling system. That rule of thumb isnt even close to what oil pressure should be at certain rpm's
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flowmaster40,hpd30 with aussie locker,currie upper joints,4.88's, lca skids,chromo shafts,vanco brakes,d35 with super 35 and arb locker,re 4.5" springs,currie arms,re front uppers,Re shocks in back,walkerevens shocks up front, Re track bars with currie jj joints,anti rock,ss brake lines,re rear sway bar links,jks 1.25" bl and mml,bfg 35x12.5 km2's,craiger soft 8's,ome steering stabilizer,currie hd steering,rockmen front bumper,emp tank skid,lots more too.
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Old 04-28-2009, 10:51 PM   #6
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I have a 97 TJ with 136k on it. At cold start I have 40+, at operating temp im around 20. Im new to all this but im sure this is normal. Oh these readings are at idle.
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Old 04-30-2009, 12:29 PM   #7
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Quote:
Originally Posted by 4.0l sahara View Post
I would like to know of one modern day motor that is around that idles with less than 10psi of oil pressure or has 20 psi at 2k. I'm talking about modern day engines not a model A with a gravity fed oiling system. That rule of thumb isnt even close to what oil pressure should be at certain rpm's
Damn, I just assumed you knew it refers to what is acceptable and a minimum oil pressure. My 4.0L engine has right at 40 psi at 3000 rpms so that's not 10 per 1000 rpms so I KNOW what is typical. The 10 psi per 1000 engine rpms is just the minimum that would be acceptable... generally speaking. I didn't know you were taking it so literally. That rule of thumb was even mentioned by my HS shop teacher back in the 60's. Oh well.

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