Crazy oil pressure

Tiny
DONTHEMANF
  • MEMBER
  • 1995 CHEVROLET S-10
  • 2.2L
  • 4 CYL
  • 2WD
  • MANUAL
  • 125,000 MILES
Friend's truck, I'm stuck (why oh why do I do this to me) working on. At a cold startup, the oil pressure gauge hits 75-80 pound. Me thinks that ain't right or the valve cover would be through the hood. And, after around five minutes running, it reads 0-5 PSI at idle and 15-20 at speed (62 in fifth, sorry no tachometer). But there is no lifter rattle, no knock. It just sits there an purrs. I think the sending unit, or wiring, or under dash wiring, or the gauge is toast. Had to do a head gasket on it (again, why me) and I always adjust rocker arms hot. At idle with 'no' oil pressure to speak of, the oil was spiting out of the push rods and rockers like crazy. One site showed a diagram with the fuel pressure switch and the oil pressure switch together behind and slightly above the filter. Like they were the same thing. That ain't good for me. But on another a mechanic said the oil sending unit is behind and below the filter by itself. That one I like. So I'm sitting in the AC trying my darnedest not to go outside until I know what and where to look. Going to put a aftermarket mechanical gauge on the %#*& so I actually know what's happening because I cannot believe super high and then super low oil pressure. Just doesn't make sense. Oh, by the way my truck is a 1991 1/2 ton with a 350/350 combo, hot cam, and a Quadrajet. Think it gets strange looks when the hood is open? But it sounds nasty with dual glass packs and runs like a grape ape. Oh, and it can melt the 31x12.50x15's I have on the rear with a 3.91 posi. Fuel mileage. What the heck is that? When the needle on the gauge drops, you just stop and put some more go-go juice in her. Me 66 years old and living the dreams man! Had to do everything over again. Got to the next page and couldn't find shade tree mechanic for a choice. :)
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Friday, April 19th, 2019 AT 7:30 AM

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Tiny
STEVE W.
  • EXPERT
Hello and welcome to 2CarPros.

Those pressures could be correct if it has a thicker conventional oil in it. First thing I would do is stick a mechanical gauge on it to see what is really going on.

The fuel pump and oil pressure switch are the same part. The way it works is that you turn on the key, the fuel pump relay kicks on for three seconds to pressurize the system and then shuts off. As soon as the engine starts turning and starts running the oil pressure turns on the fuel pump through the switch. This also functions as a roll over system as the oil pressure will go to zero if the oil moves away from the pump pick-up in the pan.
It also is a way to tell if the relay is bad, you will need to crank the engine a while until the oil pressure trips the pump switch on. It is mounted to the front of the ignition module under the manifold inline with the alternator.
On the diagram the orange wire is battery voltage, The gray wire is power to the pump.
The green wire is the prime signal from the ECM. To run your oil pressure test you need to power the pump as removing the sensor will also stop the pump. This is easy to do, just jumper battery voltage to the Red prime wire or in the pressure switch just jump the orange wire to the gray wire.

Then there is also the oil pressure sending unit that is the other part of the switch. It tells the ECM what the pressure is and the ECM tells the dash what it should display. It uses the Tan wire in the connector combined with the ground it gets through the housing and engine block. There is a variable resistor inside that reacts to oil pressure.

Oil pressure on that engine should be around 55psi at 3000 rpm.

Hope that helps.
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Friday, April 19th, 2019 AT 1:21 PM

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