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Old 05-22-2010, 11:06 PM   #1
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Dropping Oil Pressure

So today my check engine light came on and now the oil pressure gauge is dropping from normal 40 to about 20. Its only dropping to 20 at idle and fluctuates higher during acceleration.

Now I know that the oil gauge isn't really a real mechanical gauge so I'd have to install a mechanical one for real readings, but I also experienced a few possibly unrelated problems

I haven't pulled the code yet. I'm on an internship in philly till friday and can't do much until then.

For a little while now I've been having a problem very rarely where my idle sits at 2k rpm. Ill hit the gas and it'll drop back down to 1k no problem.

My engine has also been starting in the cold with some knocking, a possible sign to the lifters and oilk pump about to fail.

My oil is at a safe level and other then the random high idle that did occur about ten minutes before the cel, there are no other problems.

One apparent sourxe I know is a faulty sending unit. How do I change this and how do I go about installing a mechical guage. I'm hoping to not have to do an oil pump pull, but I'm hoping that these three problems fix the situation. Any suggestions or help?

1999 i6 125xxx miles manual

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Old 05-22-2010, 11:40 PM   #2
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Replace the oil sending unit and call it good. As per my 2003 Factory Service Manual.

The 4 cylinder oil pressure switch is located below the A/C compressor. It is a single wire connection with a gray/yellow striped wire.

The 6 cylinder oil pressure switch is located on the passenger side rear of the block. It is a single wire connection with a gray/yellow striped wire.

I am thinking it must be close to the same place for a 1999

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Old 05-23-2010, 12:14 AM   #3
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It seems that your guage is working properly. My oil reads around 40 psi while driving and at 20 psi during idle. This is of course after the enging has gotten to normal operating temps. While cold, it will read 40 psi even during idle.

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Old 05-23-2010, 09:34 AM   #4
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If it falls below 10 psi then you need to start to worry. Other than that you are good to go.
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Old 05-23-2010, 10:25 AM   #5
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There is nothing wrong with your oil pressure and it isn't what caused your Check Engine light to come on. The Check Engine light is primarily for smog related issues anyway, not serious engine problems.

20 psi at idle is normal and is actually very good oil pressure. That the oil pressure rises and falls with engine rpms is normal and just how it works since the oil pump is engine driven. So long as you have at least 10 psi for every 1K engine rpms, your engine is fine. With 20 psi at idle, your engine has double that and a perfectly good oil pressure.
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Old 05-23-2010, 12:41 PM   #6
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Okay, thanks for everyones responses. I definitely feel a little better, but I find it odd that it's just started to vary with me pressing the accelerator. It's basically acting like a tachometer.
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Old 05-23-2010, 04:34 PM   #7
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Quote:
Originally Posted by burton160w View Post
Okay, thanks for everyones responses. I definitely feel a little better, but I find it odd that it's just started to vary with me pressing the accelerator. It's basically acting like a tachometer.
You just never noticed it before. Oil pressure has always gone up and down with engine rpms.

Only on newer TJs does it not because the factory reprogrammed the oil pressure gauge so it does not vary so it does not unnecessarily alarm those who don't know oil pressure is supposed to go up and down.
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Old 10-31-2010, 08:32 PM   #8
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I have a 97 Gr Cherokee (4.0 IL6) that did the same thing. Yesterday the Check Engine light came on - checked the code and it wasd an O2 Sensor code - figured it would wait until this week - then today, while idling the oil pressure guage dropped to "nothing", bottomed out. I apply the gas and back up it goes. Stop it drops

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