1989 GMC Sierra....oil pressure

[vzfox]

Still more questions....haha.

The oil pump and both front and rear seals have been replaced as well as the oil switch (gauge switch). The spark plugs burn clean with absolutely NO signs of oil on them and there is no oil leak anywhere.

Question.....My oil pressure starts out at max (60psi) when the truck is started for the day. After it gets warm, the oil pressure will drop dramatically to just slightly above 0psi, causing the "check gauges" light to flicker. Once you press the gas pedal and/or your driving, the oil pressure will rise to around 30psi and stay until you stop again. What would cause such a fluctuation in oil pressure?

P.S. The truck does NOT smoke a lick from the tail pipe.

350.....5.7L

[Marsal420]

I had that very same problem with my 94 sierra but the engine was going, i highly dout that is your issue, i am going to look up a few things and ask around 2 see if i can find any information about it, im currently clueless at the moment.

-Will keep you updated.

[2000CAYukon]

In cases like this, a mechanical gauge should be used to verify the correct oil pressure. If the pressure is really going to zero, there is probably a bearing clearance issue. If the pressure is good, the problem could be the sender, gauge or wiring.

//2000CAYukon

[vzfox]

Well, when the oil pressure does get close to zero at a stop light...etc. I can hear a slight knocking noise coming from the motor. So that kinda tells me that the gauge is right and the oil pressure is sinking too low.....my opinion only. I thought about adjusting the idle speed up a tad, but on a TBI, I really dont know if you can with the computer, sensors and all.

Thank you Marsal420, all help is greatly appriciated.......you too 2000CAYukon.

[2000CAYukon]

Originally Posted by vzfox

Well, when the oil pressure does get close to zero at a stop light...etc. I can hear a slight knocking noise coming from the motor. So that kinda tells me that the gauge is right and the oil pressure is sinking too low.....my opinion only. I thought about adjusting the idle speed up a tad, but on a TBI, I really dont know if you can with the computer, sensors and all.

Thank you Marsal420, all help is greatly appriciated.......you too 2000CAYukon.

You can try to run a thicker oil but that is really just a band aid. Idle speed is controlled by the ECM.

There is an adjustment for the butterfly on the TBI but you have to drill out the idle cap screw. See http://s-series.org/component/option...,63/task,item/

//2000CAYukon

[vzfox]

I may try to adjust the idle speed....just unsure if that will help much. As for the thicker oil....the oil pressure starts out at 60psi when you first start the truck for the day and after it gets warm; while driving down the road, it will stay right around 30psi. The oil I run in it now is Havoline 10W-30. If I were to thicken the oil, what thickness would you recommend? Remember, I live in TX where the temp is always kinda high.

[2000CAYukon]

Originally Posted by vzfox

I may try to adjust the idle speed....just unsure if that will help much. As for the thicker oil....the oil pressure starts out at 60psi when you first start the truck for the day and after it gets warm; while driving down the road, it will stay right around 30psi. The oil I run in it now is Havoline 10W-30. If I were to thicken the oil, what thickness would you recommend? Remember, I live in TX where the temp is always kinda high.

You could use a 10W-40 but it will probably hurt gas mileage a little bit.

On my 90 GMC Sierra, I had an oil leak near the oil filter housing. There is a gasket between the block and the oil filter adapter. There is also an O-ring and in my case that was what was leaking; however, the gasket between the block and adapter was hard as a rock. I replaced the gasket and O-ring and my oil pressure went up! Clearly, I was losing pressure between the block and adapter.

On my truck, the pressure is around 20-23 hot at idle. It is around 35 - 40 while moving. Before I fixed the leak, it was around 15 hot at idle and 30 while moving.

//2000CAYukon

[vzfox]

I will check and see if there is any oil leakage from around the oil filter, but I dont think there is. Your last reply poses yet another question I have been thinking about.....

If I remember correctly, a few years back I had a 1988 Chev stepside with the black to white gauges and tried putting the newer needle style cluster panel in its place. It fit right in, but I believe the plug that goes in the rear of the cluster panel was not correct and after "trying" to wire the newer dashboard......well, it never worked. I guess what I am trying to ask is if anyone has done this replacement and what does it involve? Or would it even work at all?

I am sure it would give much better/accurate readings.

[CreditKing]

The oil sending unit or sensor/guage could be put in backwards.. thats why it goes down not up..I had the same problem but its fixed now.. My oil guage still goes up when im excelerating. its supossed to.

[Regalwhiskey]

Originally Posted by vzfox

Well, when the oil pressure does get close to zero at a stop light...etc. I can hear a slight knocking noise coming from the motor. So that kinda tells me that the gauge is right and the oil pressure is sinking too low.....my opinion only. I thought about adjusting the idle speed up a tad, but on a TBI, I really dont know if you can with the computer, sensors and all.

Thank you Marsal420, all help is greatly appriciated.......you too 2000CAYukon.

When all else fails,you can drop the oil pan and install a high volume oil pump to compensate for leaky bearing clearances. That will also raise the oil pressure some.

[vzfox]

Just thought I would let you guys know that I sold my truck. My new toy is a 1983 Camaro Berlinetta. It's a 5spd with the V6 motor....ughhhhh, I know, but anyway.....I am sure I will be posting many a many questions about it to come, but obviously in a different room/forum. thanx for all your help guys.