Why does my oil pressure drop when I come to a stop and then go back to normal when I drive off?

Tiny
STEVEN W. CANNEDY
  • MEMBER
  • 1999 JEEP GRAND CHEROKEE
  • 160,000 MILES
When I come to a stop the oIl pressure goes down to almost nothing and the red check gauges light comes on. Once I drive off, the check gauges light goes off and the oil pressure goes back to normal. As I speed up and slow down, the oil pressure goes up and down accordingly. I had the oil changed immediately and it's still doing this. They used 5w 30 synthetic blend oil. Any thoughts or suggestions?
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Saturday, December 10th, 2011 AT 8:53 PM

7 Replies

Tiny
RIVERMIKERAT
  • MEMBER
Because the oil pump is engine driven. As the engine speed goes down, the oil pump output pressure decreases. The oil pressure should not drop below about 20PSI. You should hook up a dependable oil pressure gauge and see what pressure is developed at idle. It sounds like you have an oil pump pressure relief valve that is stuck or a sender that is out.
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Saturday, December 10th, 2011 AT 8:58 PM
Tiny
CJ MEDEVAC
  • EXPERT
Even one of those under dash units will work for testing, mount it permanently later, or keep using it as a tester

a mechanical gauge will give you a truer reading, as compared w/ an electric gauge

if the mechanical gauge is tellin' you everything is a-ok, at all rpms

then your oil sending unit on the engine is probably (most likely) the problem----but not completely ruling out a gauge problem (seldom)

keep us informed

the medic
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Sunday, December 11th, 2011 AT 3:42 AM
Tiny
RIVERMIKERAT
  • MEMBER
Something like the rattle or bounce that doesn't occur when the mechanic takes a look at it, I guess. My experience shows most stock gauges are off by as much as 20%.
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Sunday, December 11th, 2011 AT 10:39 PM
Tiny
JACOBANDNICKOLAS
  • EXPERT
Steven:
Every comment that has been made is accurate. Some OEM gauges can be off, and others are not. However, since the light is coming on, we recommend that you check the oil pressure with a mechanical gauge. Most parts stores will lend them to you. All you do is remove the original sending unit and then screw in the adapter for the oil gauge. When you start the engine, it will read an accurate pressure. If it showing good pressure at an idle, then you should replace the sending unit. They go bad often. If the pressure is still showing very low, you should consider removing the oil pan to check that the oil pick-up tube isn't partially plugged. IF it isn't, then the pump itself will need inspected.

In general, if the oil pressure is extremely low, usually you will start to hear a knocking or ticking from the engine. If it is okay and the sensor is bad, you won't hear anything. However, it is better to be safe than sorry. Someone posted a pic of an oil pressure gauge. They are very easy to use, and from what I can see, you will have about 5 or 6 techs waiting to help you if you have any problems or questions.

Let us know what you find.
Joe
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Monday, December 12th, 2011 AT 1:03 AM
Tiny
STEVEN W. CANNEDY
  • MEMBER
Thanks for the assistance. It was indeed the oil sending unit.
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Monday, December 12th, 2011 AT 10:35 PM
Tiny
RASMATAZ
  • MEMBER
This thing should have been fixed if rivermikerat 1st suggestion was followed to the T-lesson learned here
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Tuesday, December 13th, 2011 AT 1:43 AM
Tiny
JACOBANDNICKOLAS
  • EXPERT
Glad you got it fixed. Let us know if you have questions in the future.
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Tuesday, December 13th, 2011 AT 4:37 AM

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