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Old 08-30-2013, 10:10 PM   #1
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Oil Pressure Drops When I Brake

So I just bought a jeep wrangler and drove it home 2 hours. Soon I discovered when I apply the brakes, my oil pressure gauge goes from the middle to around 10.

I have a 1998 Jeep Wrangler Sahara.

I got out checked and I found out I am NOT leaking oil and I also checked my oil dipstick and found I have plenty of oil.


I'm thinking its the oil sending pressure unit but I don't know.

Anyone know? Thanks

P.S I joined the forum today and till have plenty of questions to ask.

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Old 08-30-2013, 10:26 PM   #2
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It's supposed to drop when the rpms drop. If it goes down when you accelerate then Id worry

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Old 08-30-2013, 10:49 PM   #3
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It's supposed to drop when the rpms drop. If it goes down when you accelerate then Id worry
But when it drops to very low it's still fine? I have a friend who knows a lot about cars and he says he thinks the oil pressure sending unit is going
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Old 08-30-2013, 11:13 PM   #4
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Originally Posted by pipps2 View Post

But when it drops to very low it's still fine? I have a friend who knows a lot about cars and he says he thinks the oil pressure sending unit is going
Is it only dropping when you apply the brake only or is it dropping because your comming to a stop?
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Old 08-30-2013, 11:14 PM   #5
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Could be the unit. It will drop low when stopped at idle. Rpms keep the oil pump turning thus creating pressure. Slower the pump turns lower the pressure
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Old 08-30-2013, 11:18 PM   #6
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Originally Posted by Lando25 View Post

Is it only dropping when you apply the brake only or is it dropping because your comming to a stop?
It's when my jeeps slowing down. It goes from 40 to 10. I just thought it was abnormally low.
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Old 08-30-2013, 11:18 PM   #7
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My last 04 had a similar issue. The pressure would be fine until the Jeep was warmed up, then it would drop at idle to about 10. When I revved it, the pressure went back to 60-70.

It turned out to be a cracked head... Hairline crack that only appeared when it was hot.

Might be that.

Also, check the oil pan. If it got a good bang, it could have damaged the pick up, although I think that would cause pressure issues across the board.
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Old 08-30-2013, 11:23 PM   #8
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My last 04 had a similar issue. The pressure would be fine until the Jeep was warmed up, then it would drop at idle to about 10. When I revved it, the pressure went back to 60-70.

It turned out to be a cracked head... Hairline crack that only appeared when it was hot.

Might be that.

Also, check the oil pan. If it got a good bang, it could have damaged the pick up, although I think that would cause pressure issues across the board.
Ill take a look tomorrow for any hairline cracks. Thanks for the info
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Old 08-31-2013, 09:38 AM   #9
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My '99 TJ fluctuates between 25 and 50 according to the dash gauge, but only after the engine heats up. If I'm on a long trip, the gauge drops to 20 aat idle. The other day I tried replacing the oil pressure sender gauge when I did my oil change, but even with the new sensor the readings are the same. I've read that the PSI fluctuations are normal with the RPM fluctuations, but it's still a little disconcerting and doesn't seem to be universal, which to me indicates some kind of issue. I will be keeping an eye on mine to see if it gets worse with time. Hopefully it's not a cracked head.
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Old 08-31-2013, 09:56 AM   #10
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Mine sits at 45-50 when cold and then will drop to 40 at idle and back up to ~50 when driving. I would guess it's normal...
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Old 08-31-2013, 10:02 AM   #11
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Just FYI, per FSM, normal idle oil pressure is 13psi. I wouldn't worry about it if it's mechanically sound and no other issues. It should climb up to a driving range of 40-50psi.
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Old 08-31-2013, 10:04 AM   #12
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It's normal for your year. But, it was a big concern for customers, so on later Jeeps, Mopar changed the sending unit so customers would stop complaining about it.
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Old 08-31-2013, 10:08 AM   #13
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It's normal for your year. But, it was a big concern for customers, so on later Jeeps, Mopar changed the sending unit so customers would stop complaining about it.
I just thought when it was that low it was a bad. Ill take a look for any head hairline fractures but I hope it's a universal no-problem. I'm not going to replace the sending unit for now
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Old 08-31-2013, 12:28 PM   #14
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It's normal for your year. But, it was a big concern for customers, so on later Jeeps, Mopar changed the sending unit so customers would stop complaining about it.
Yeah I just popped the hood and looked for any issues. No hairline cracks. It's gotta be a model thing. Thanks for the notice

Also is it normal for jeeps to idle a little high?
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Old 08-31-2013, 12:35 PM   #15
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10 psi per thousand engine rpms is a good rule-of-thumb for acceptable engine oil pressure. The oil pump is engine driven so the faster the engine revs, the higher the oil pressure will be.

Older TJs, like 2002 and older or so, indicate the accurate oil pressure so they will indicate pressures like 10-13 psi at idle which is what it is supposed to be. That scared some TJ owners enough who didn't know how oil pressure works to have complained vociferously to their dealerships who often couldn't convince them that it was normal. So the dealers forced the factory to reprogram the oil pressure gauges to only show a mid-scale indication if the oil pressure was acceptable... that stopped the complaints from those who knew no better than to complain about a non-issue.
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Old 08-31-2013, 12:36 PM   #16
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Glad to see I'm not the only one with this happening. I have a 2001 that does this and I was thinking the gauges were bunk.
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Old 08-31-2013, 04:05 PM   #17
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. That scared some TJ owners enough who didn't know how oil pressure works to have complained vociferously to their dealerships who often couldn't convince them that it was normal.
That's right. Next time I have an issue with the dealer, I will storm in the dealership and complain vocef.,vociferro..,vicofrous..,vicodinferocious,.. anyways, I'm gonna let them have it.
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Old 08-31-2013, 05:07 PM   #18
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Yeah I hear you, that's a funny rarely used word. It's been stuck in my brain ever since a Toastmasters speaking club I used to belong to back in the 70's. Vociferously was our word for the night one night & we all had to use it like 8-10 times in our talks that evening. That nasty word has been stuck in my brain ever since.
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Old 08-31-2013, 06:52 PM   #19
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Originally Posted by Jerry Bransford View Post
10 psi per thousand engine rpms is a good rule-of-thumb for acceptable engine oil pressure. The oil pump is engine driven so the faster the engine revs, the higher the oil pressure will be.

Older TJs, like 2002 and older or so, indicate the accurate oil pressure so they will indicate pressures like 10-13 psi at idle which is what it is supposed to be. That scared some TJ owners enough who didn't know how oil pressure works to have complained vociferously to their dealerships who often couldn't convince them that it was normal. So the dealers forced the factory to reprogram the oil pressure gauges to only show a mid-scale indication if the oil pressure was acceptable... that stopped the complaints from those who knew no better than to complain about a non-issue.
Speaking of this, mine was a slightly different problem in my 1997 Wrangler.
My sending unit would drop to zero after coming off the expressway. My engine check light would come on too. The mechanic said that it was probably just clogged and he said that he cleaned it. But, it still did the same thing. So, I changed mine with a new sending unit, and now it only drops to 10-13 at idle after the expressway drive. Normally, it reads around 40 psi.

Just saying that a new sending unit works the same as the OP.
My mechanic said that if you have a oil pressure problem, you will likely hear it in the engine, or see it on the ground. I trust his opinion because he rebuilt my auto transmission a year ago, and I haven't had any problems with it.

Basically, we are in total agreement, and I was just presenting additional evidence to support your statement.
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Old 09-04-2013, 10:07 AM   #20
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I should clarify that my crack was internal to the head. I found it when I swapped the motor, and out of curiosity did an autopsy on the old one. It was the infamous 0331 casting... But what's odd is the dealer swore it should have been corrected by the 2004 model. Yet my first 04 had the same failure at 45,000 miles.

As for Chas... Your mechanic is probably right. When mine started to go at first it sounded like the light lifter knock when you first start up until oil pressure built up... But it wouldn't go away. Dealer just added a bottle of lucas and said it would fix the "lifter knock".

From there it went downhill in less than 200 miles... In the end it sounded louder than a diesel. Luckily it was just the lifter and no damage to the push rods, pistons, etc. So the block was salvaged, but the head was shot.
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Old 09-04-2013, 10:29 AM   #21
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Originally Posted by Jerry Bransford View Post
10 psi per thousand engine rpms is a good rule-of-thumb for acceptable engine oil pressure. The oil pump is engine driven so the faster the engine revs, the higher the oil pressure will be.

Older TJs, like 2002 and older or so, indicate the accurate oil pressure so they will indicate pressures like 10-13 psi at idle which is what it is supposed to be. That scared some TJ owners enough who didn't know how oil pressure works to have complained vociferously to their dealerships who often couldn't convince them that it was normal. So the dealers forced the factory to reprogram the oil pressure gauges to only show a mid-scale indication if the oil pressure was acceptable... that stopped the complaints from those who knew no better than to complain about a non-issue.
Is there a write up on how to change it back to a precise oil pressure gauge? I have a 2003 that has the dummy gauge and dislike not being able to know my actual oil pressure at a particular moment.
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Old 09-04-2013, 05:58 PM   #22
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I was just going to google search this. My new to me tj is showing the same. Thanks for the info!!!
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Old 09-05-2013, 08:13 AM   #23
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I was looking for the same as well, as I did it before but it wasn't the prettiest set up. I took out the sending unit at first, but that just made check gauges stay on all the time (obviously). With the sending unit in hand I was able to go to the parts store, find a few fittings and couplers, and pieced together a t fitting. Sending unit on one end so the ECU and dash were happy, and an analog gauge on the other side so I was happy.

Think of it as a field goal post, with the uprights being the gauges, and the center post to the motor.

It's on the "to do" list with the new one, along with temp and a few others too. I didn't trust the gauges before, and now more so after Jerry's insight to the readings.

When I do it, I'll add the write up if there isn't one, but at the moment I am on other projects.

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