I had a similar dreaded low oil pressure event consistent with the bad O ring syndrome. I replaced the oil pressure sensor. Problem returned 3K miles later. No change other than a slight improvement. Every time I did an oil change, the problem went away until the oil had about 3k miles on it. Then everything went to hell. Changed the oil, the problem went away and then returned 3k miles again. Repeat, rinse, repeat. Being the curious type, the next time it happened, I replaced the oil filter and that cured the problem, for a while. The Jack the arse end of the vehicle up and over fill the oil to see if it was the O ring showed no oil pressure change.
So, what I finally did was to do the old Sea Foam trick before the oil change and then added in 20% Marvel Mystery Oil at the oil change. That cured it, until the sludge moved around from the galleries and I had to change the filter to cure that.
Now, when I do a cold start, the oil pressure goes up and hangs out at about 60 psi for about 30 seconds, drops to about 20 psi for 30 seconds and then abruptly rises to 60 psi. Hot idle at 30 psi and 50 at about 1700 rpm. My conclusion is that the 'dog bone'/'dumbell' in the the oil pump was sticky and the Sea Foam/Marvel Mystery oil unstuck it. That, and the oil filter (a Wix) was just a sh*tty filter that was plugging up and they bypass mechanism was upgef*cked (which it was when I ripped open the filter to look at it).
Most oil filters out there are crap and plug up and the bypass mechanisms are sh*ty and don't open properly (or get stuck open) at the right pressure differential. A sure sign of a crappy oil filter or plugged filter is the oil pressure rising to normal, dropping, and then rising again. Most filter elements clog up after a couple thousand miles and the bypass mechanism fails. That, and the bypass for the oil cooler can stick open and drop your pressure.
*Also* if you want to reduce sludge in your oil and you are brave and tenacious enough to drop the pan on some of these vehicles, make sure you install a deflector on the oil return from the oil cooler/return lines. GMC failed to install the proper deflector and O ring as a general rule at the factory and they did it deliberately just to be cheap and sloppy. Without the deflector, the oil sprays up onto the bottom of the crank and into the jugs and sludges up, big time. The GM engines are great provided the people on the line assembling them do it right (which they generally do not). I've seen 6.2 L motors in Yukes run fine with no significant oil loss other than at the rear seal at 450K and 750K miles (and the transmissions didn't sh*t the bed) as long as they had oil changes at 3k miles. And they all had miserably oil pressure since day one.
GMC should have been sued in a class action suit over the O ring issue.
So, what I finally did was to do the old Sea Foam trick before the oil change and then added in 20% Marvel Mystery Oil at the oil change. That cured it, until the sludge moved around from the galleries and I had to change the filter to cure that.
Now, when I do a cold start, the oil pressure goes up and hangs out at about 60 psi for about 30 seconds, drops to about 20 psi for 30 seconds and then abruptly rises to 60 psi. Hot idle at 30 psi and 50 at about 1700 rpm. My conclusion is that the 'dog bone'/'dumbell' in the the oil pump was sticky and the Sea Foam/Marvel Mystery oil unstuck it. That, and the oil filter (a Wix) was just a sh*tty filter that was plugging up and they bypass mechanism was upgef*cked (which it was when I ripped open the filter to look at it).
Most oil filters out there are crap and plug up and the bypass mechanisms are sh*ty and don't open properly (or get stuck open) at the right pressure differential. A sure sign of a crappy oil filter or plugged filter is the oil pressure rising to normal, dropping, and then rising again. Most filter elements clog up after a couple thousand miles and the bypass mechanism fails. That, and the bypass for the oil cooler can stick open and drop your pressure.
*Also* if you want to reduce sludge in your oil and you are brave and tenacious enough to drop the pan on some of these vehicles, make sure you install a deflector on the oil return from the oil cooler/return lines. GMC failed to install the proper deflector and O ring as a general rule at the factory and they did it deliberately just to be cheap and sloppy. Without the deflector, the oil sprays up onto the bottom of the crank and into the jugs and sludges up, big time. The GM engines are great provided the people on the line assembling them do it right (which they generally do not). I've seen 6.2 L motors in Yukes run fine with no significant oil loss other than at the rear seal at 450K and 750K miles (and the transmissions didn't sh*t the bed) as long as they had oil changes at 3k miles. And they all had miserably oil pressure since day one.
GMC should have been sued in a class action suit over the O ring issue.
