I did a lot of searching on the net and found few answers to the common question.

Last week a truck came into the shop. 1999 with a 350 Vortec. When slowing down or at an idle, the oil pressure would drop to zero. I did the typical first diagnostics. Checked the oil level, good. Installed a mechanical gauge and confirmed it had zero pressure at idle which eliminates the sender and gauge. Reving the engine up increased the pressure to about 15 psi. Still too low. No noises except some minor lifter ticking at idle.

The intake manifold gasket was leaking coolant and the truck was due for an oil change so I pulled the intake and changed the gasket. Since the oil was going to be changed anyway, I dropped the pan and installed a new oil pump since it wasn't hard to do. Cut open the filter and there was no metal in the element so no bad bearings. None of the mains had any discoloration to indicate a spun bearings and the crank spun over with little effort.

Bolted everything back up. New oil pump, new filter, clean 10W30 oil, no leaking intake gasket. Fired the engine up and it had pressure but after about 10 minutes of running, pressure dropped back down to zero. Actually, using a mechanical gauge, it was closer to 1 PSI. Just enough that the engine was still getting oil and at an idle, you could just barely hear the lifters clattering. No other engine noises and it ran fine.

Today I pulled the pan back off to do a better inspection. I wish I had a bore scope because it probably would have made a diagnosis much easier. I pulled the oil pump back off and used a rubber tip air nozzle to pressurize the oil system. Excessive air seemed to be coming from the #3 and #4 cam bearing area. My boss told me to pull the cam. Off comes the intake again and by mid day I was pulling the camshaft out.

When the cam came out. number 2, 3, and 4 cam bearings fell out of the block. Mystery solved. The cam is fine but the low oil pressure at idle after the engine had warmed up was caused by loose cam bearings which is just about impossible to see past the crankshaft after pulling the pan. A bore scope probably could have seen the bearings coming out of the cam bores.

Now I get the fun job of putting in 3 cam bearings with the engine still in the truck and the crankshaft still in the block and hopefully the bores are not worn out enough for the new bearings.

I have never seen this happen before but was told it's common in the Vortec blocks.

 

Update

Changed all but #5 cam bearings this morning with the engine still in the truck. To change #5, you need to remove the transmission. By mid afternoon I fired up the engine and had lots of oil pressure even after the engine warmed up.

 

ive been experiencing a similar problem with my rs. except the oil pressure floats around 30 psi. what is the normal pressure at idle? any throttle and the pressure goes up to 45+

 

Rule of thumb is that an engine only requires 10 psi per 1000 rpm. At an idle, I don't like seeing the pressure drop below 30 psi but as low as 15 is still acceptable and safe.

Fluctuating oil pressure is normal. Even in my race car the pressure fluctuates from 30-70 psi as rpm changes. My oil pressure warning light comes on at 25 psi and I never see it come on unless my oil accumulator is turned off as I'm slowing down after a run.

If you're experiencing very low pressure at all rpms, all the other possible problems need to be eliminated first. Confirm with a mechanical gauge the actual oil pressure. Change the oil and filter in case of possible contamination. Cut open the filter to look for possible bearing material which would indicate bad main bearings. If bad rod bearings, you'll hear them when the engine is running. Use a WIX filter if possible, Fram is garbage. I put the filter in a vise and use a putty knife and a hammer to cut it open. After that, you need to get more in depth such as dropping the pan to make sure the oil pump pickup tube screen is plugged or the pickup tube isn't loose or fallen off. When all else fails, you have more serious issues such as what I found with loose cam bearings.

 

oil pressure is only low at idle(low as in 30ish psi). when on the throttle the pressure goes up to 60 and anywhere in between. no strange noises from engine. just posted as a precaution. thanks for you help man, i appreciate it.

 

At warm idle 30psi is just fine.

 

Glad I did a search!!! I became terrified when my oil gauge dropped to zero today. It has the same symptoms as the truck above but I KNOW that it is due for an oil change so I will start there.

 

The first thing you should do with the oil change is to cut the filter open to inspect for any metal.

Take a section of the filter material and squish it in a vise to get all the oil out of it. Open up the pleats and look inside them. Small metal flakes may not show up in all the oil so that's why you squish it out.