New Guy With Oil Pressure From Bad To Worse!

[JB05Yukon]

Well now it’s back... in fairness I hadn’t driven the car much since I thought I’d fix it but i took it for some stop and go driving on a fast food run tonight and oil pressure dropped low enough to trigger the ‘low oil pressure’ warning whilst sitting in the drive through and at red lights on the journey home. At initial startup it sits at around 20psi. Is it possible I’ve still not seated the pickup tube properly or is this pointing to the more malicious causes?

 

[SnowDrifter]

I think what would be appropriate, at this point, is to borrow a mechanical gauge from the parts store and go from there. Rule out stepper motor errors, sensor, wiring, or anything else that could give an erroneous reading from the dash. From there we can chase mechanical vs. Electrical.

Some notes about oil Pressure:

If its low and gets proportionally worse with RPM. That is to say, oil Pressure increases but not as much as it should, that signifies worn rod bearings. Crankshaft spinning will suck oil out, hence the odd behavior

Low oil Pressure that remains low in accordance with RPMs can be indicative if either crank bearings or cam bearings.

Good oil Pressure when cold, but worsens as the oil temperature rises, independent of the oils natural change in viscosity could be a worn oil pump. Clearances out of spec here will let much more oil past when Thin and warm rather than thick and cold.

Low oil Pressure that seems to top off of refuse to rise would be the oil pump bypass spring worn or sticking

And low oil Pressure independent of temperature or rpm, particularly if fluctuating would be a pickup tube leak of some sort.


Note that all of these rely on 3 things
1. Mechanical gauge. No room for electrical error
2. That the oil pickup screen isn't clogged. E. G. Forgot a rag in the oil pan during assembly
3. That the engine is in somewhat serviceable condition. Like you've not been driving in collapsed lifters and rod knock for the last 10k miles to where anything and everything is ******.

 

[swathdiver]

Hello from Texas! I just picked up a really tidy 2005 Yukon XL with about 135k miles on a 5.3. The previous owner seems to have taken great care of it, with a log book full of regular oil changes. Paint and interior are immaculate for the age.

When I got the car, oil pressure was showing zero at all times. I figured it was the sender, and sure enough, I replaced this and started getting a reading. I then got low (10 psi maybe) at startup, but when driving I got just under 40. All along the car has had a faint but noticable knocking from the lifters.

I decided to go ahead with the pickup tube o-ring replacement to see if I could get it more consistently higher. When I pulled the pan, I recognized that somebody had likely done this fairly recently. The pan gasket looks fresh and the o-ring was red and flexible, not brittle or mis-shapen. Coincidentally the cross brace with the 4 big bolts that would normally block removal of the pan in a 2wd model is completely missing!

Anyway, I went ahead and replaced the o-ring and put everything back together, including a new filter and 6 quarts new 5w-30.

I now get a low oil pressure message and zero oil pressure on startup. This is accompanied by more noise from the engine. Naturally I haven't run it, just started twice and come straight to the forums.

Does this sound like the pump needs priming after removing the pan? I haven't messed with the pump. I'm reluctant to start the car or rev it until I have rectified this.

Did you replace the oil filter? We've seen more than a few louse up the pressure inside a motor. Ask the previous owner if they replaced that o-ring but in such a way that they won't feel threatened. They may have sold the car because of it and passed the problem on to you.

If the new filter and mechanical gauge still show low pressure, it's time for bearings and the likely culprits are the cam bearings.

Someone mentioned over-filling the motor with 2 extra quarts to submerge the o-ring to rule it out. Sounds good but would want more input before trying it myself. You might get by with running a thicker oil, say 10W-40 to keep the pressure up until you're ready to tear into the motor or replace it.

 

[JB05Yukon]

Thanks again for the good advice gents. As my wife just had our baby last week, I've quit messing with it and taken it to a local mechanic - not my usual approach but I can't be out in the driveway at the moment! I've asked them to stick it on a mechanical gauge but I imagine it will only confirm what I know. I am hoping to get away with a new pump but we'll see!

On the subject of thicker oils.... what are other high mileage folks using? When I changed it last week I stuck to the GM recommendations; but I'm thinking a heavier full synthetic might be good as I have 135k miles and I'm down in the Texas heat. Thoughts on oil type and weight anyone?