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Old 10-24-2009, 06:52 PM   #1
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Question how fast does oil pressure come up

I just put a new oil pump in my 99 TJ with a 4.0L. I was just wondering if there are some TJ owners out there with older Jeeps like mine that can tell me how fast their oil pressure gauge starts to rise.

Mine jumps a little as soon as I start to turn the key (before the Jeep even turns over). I let it fall back to zero and then start the Jeep. It's like "one-thousand-one, one-thousand-two" and then the gauge goes up to a normal pressure. This is on a cold start-up (at least an hour not running). I guess I never paid attention to what it did before. Just wondering if this is a normal response time for the pump. The newer Jeeps have pressure gauges that don't fluctuate like the older ones so I have nothing to compare it with.

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Old 10-24-2009, 07:43 PM   #2
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WhenI startt the engine the pressure is there at the same moment.

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Old 10-24-2009, 08:03 PM   #3
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Quote:
Originally Posted by BlackJeepoftheFamily View Post
I just put a new oil pump in my 99 TJ with a 4.0L. I was just wondering if there are some TJ owners out there with older Jeeps like mine that can tell me how fast their oil pressure gauge starts to rise.

Mine jumps a little as soon as I start to turn the key (before the Jeep even turns over). I let it fall back to zero and then start the Jeep. It's like "one-thousand-one, one-thousand-two" and then the gauge goes up to a normal pressure. This is on a cold start-up (at least an hour not running). I guess I never paid attention to what it did before. Just wondering if this is a normal response time for the pump. The newer Jeeps have pressure gauges that don't fluctuate like the older ones so I have nothing to compare it with.
Mine sounds like that as well...when I start it, I can count to about 2 before the needle slowly rises.
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Old 10-24-2009, 08:40 PM   #4
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I allow all the electrics to start and settle (seat belt bell, etc...)

then turn the ignition to engage the starter.

I haven't notice much of a delay in the the oil pressure indicator but
will examine it closer tommorrow morning if there is any delay occurring.
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Old 10-26-2009, 07:45 PM   #5
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I turn the key so only
all the electrics turn on as well as the dash board indicator lights and gauges
wait a bit...
..then turn the key to ignition start
as I observe the oil pressure
I do a slow count from 1 to 4/5 when the needle stops.
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Old 10-26-2009, 07:53 PM   #6
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Newer Jeeps like probably 2001 and newer don't directly indicate oil pressure on the gauge... the computer just gives a mid-scale reading for all pressures except when it's way too high or way too low when it will give a "Check Gauges" indication and give that extreme high or low reading. When the computer is happy with the true oil pressure, the oil pressure signal it sends to the gauge won't vary in step with engine rpms and it doesn't need to actually get any oil pressure at first to show a mid-scale "ok" oil pressure reading before the engine starts. That is to keep the less astute people from unnecessarily worrying and pestering their Jeep dealers when they wonder why their oil pressure "goes up and down" (as it should).

Older TJs like mine acually give a true oil pressure reading that rises and falls as it should in step with engine rpms. Mine sits at zero for a second or two after starting the engine and then it jumps up to where it should be. This is probably true for '97 to perhaps the 2000 or 2001 model year, it's after that where the gauge was reprogrammed to not show a varying oil pressure if everything is within normal parameters.
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Old 10-26-2009, 09:19 PM   #7
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Snap your fingers. There's your answer.
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Old 10-26-2009, 10:20 PM   #8
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Quote:
Originally Posted by Jerry Bransford View Post
Newer Jeeps like probably 2001 and newer don't directly indicate oil pressure on the gauge... the computer just gives a mid-scale reading for all pressures except when it's way too high or way too low when it will give a "Check Gauges" indication and give that extreme high or low reading. When the computer is happy with the true oil pressure, the oil pressure signal it sends to the gauge won't vary in step with engine rpms and it doesn't need to actually get any oil pressure at first to show a mid-scale "ok" oil pressure reading before the engine starts. That is to keep the less astute people from unnecessarily worrying and pestering their Jeep dealers when they wonder why their oil pressure "goes up and down" (as it should).

Older TJs like mine acually give a true oil pressure reading that rises and falls as it should in step with engine rpms. Mine sits at zero for a second or two after starting the engine and then it jumps up to where it should be. This is probably true for '97 to perhaps the 2000 or 2001 model year, it's after that where the gauge was reprogrammed to not show a varying oil pressure if everything is within normal parameters.
Mine's an 02 and on startup it shows 0 and slowly rises over a period of a couple seconds to where it should be...I will have to check on my drive to work tomorrow if it fluctuates though. Never paid attention to that part.
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Old 10-27-2009, 01:50 PM   #9
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Thanks for the input. Sounds like mine is normal. It's a 1999 so the gauge does fluctuate from around 20 psi at warm idle to around 45-50 at 2000 rpm at normal operating temp.

On a warm start the oil pressure goes up immediately. On a cold start it takes about 2 seconds before it starts to rise. Just wanted to hear that other older TJ's are similar so I know everythings back to normal after replacing the pump.
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Old 10-27-2009, 05:18 PM   #10
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I took a look at my gauge while driving today...I wouldn't say it's accurate, but it does fluctuate some. At idle it was showing around 40 psi...whereas while driving it was closer to 50-55ish.
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Old 10-27-2009, 08:15 PM   #11
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At warm idle mine fluctuates around 20 PSI and then while driving it will range from 40-60 depending on the RPM's, gear, etc. I'm wondering if my oil pump will be going soon though because on cold startup, it goes up to 45 at idle.
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Old 10-30-2009, 05:50 PM   #12
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gaddman,

I have a 99 Sahara and on cold startup my pressure goes to 45-50 also. I replaced my pump but only because on cold mornings in Minnesota (sub-zero) my oil pressure would shoot up to 80+ psi. The pressure release valve was bad, not the pump itself. Cold idle pressure of 45 is not a problem.

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