Fun With Oil Pressure

[pjmoreland]

Last week I was out driving my truck. As I was accelerating to go around someone, I heard my engine start to make a faint noise it doesn't usually make. I looked down at my oil pressure gauge, and it was at zero! I immediately let off the gas, and the pressure came back up to normal. I headed straight home from there, staring at the gauge the entire time. I noticed that if I revved to 3500 RPM or higher under no load, the oil pressure would drop off. I checked my oil level when I got home, and it was right at the full mark. I had my oil pressure drop off one time several months ago, but that time my oil level was down two quarts. That was before I replaced my heads, and I had a huge oil consumption problem due to severely worn valve guides (smoke in the exhaust at startup).

I found a post on another website from a guy who had the same thing happen to him where his oil pressure would drop to zero at higher revs. The cause in that case was an oil pump pickup that was set too high. I pulled my oil pan and took some measurements. Sure enough, my oil pickup was about 1" off the bottom of the pan instead of the correct 3/8". The pickup was welded to the pump, so it hadn't come loose. It was just poorly installed by whoever built this engine 20+ years ago. This pump also produces excessively high pressure during warmup (gauge pegged out of view). I decided to install a new Melling 10553ST pump. I installed the lower pressure spring that came with it. I've got the pickup tube set to 3/8" off the oil pan floor (confirmed with clay), and now my oil pressure is right at 60psi during warm-up, and it hovers around 50-55psi when warmed up and driving. It's at 20psi at idle, which is a little higher than with the old pump, which was down around 15psi. I installed a Pioneer 839061 pickup tube clamp to keep it in place.

One thing I'm confused about is why I didn't see this problem earlier. Maybe the hot weather has something to do with it? I also noticed that the pressure relief valve in the old pump did not slide freely. It was scrapy feeling in the bore, and took a fair amount of pressure to dislodge it. Maybe it was getting stuck partially open. It could also be that this problem has been happening all along, but I just wasn't looking at my oil pressure gauge while accelerating aggressively. I hope my bearings are ok.