I added another litre which means I should have a total of around 7 quarts. Didn't make much difference.

 

Did we check the Camshaft retaining plate for leaks?

 

I replaced it with a new one when I rebuilt the engine. Also, when I was replacing the oil pump, I didn't notice any oil leaking from it.

 

When I added extra oil to my C5, I had at least 10, maybe 11 quarts, in it. 7 isnt enough to bury the O-ring and pickup tube. 10 is OK as long as you only idle the engine. I wouldn’t try driving it with 10-11qts. However, at this point, I believe you have a crack in the block, somewhere in the oil schematic. I know this sucks. But at this point, I think you have the answer, albeit not the one you're hoping for. IDK what else it could be. If the bearings were set up so loose that oil pressure dropped to the readings youre seeing, it would be knocking somewhere......

 

Sounds like you need to pull that motor back apart and send the block to the machine shop again and make sure it isn't cracked. Do you guy think if he maybe shimmed the oil pump he may pick up a few psi? Unless your pinching the o ring or something simple like that but it sounds to me like you have touched all the basics. I'm no expert but I'd definitely pull the motor and send it to someone who is. You could possible pull the heads and pt the block with dye or the spray paint they use for pt and see if you see any cracks

 

This is a little embarassing. I think I've found the cause of the low oil pressure. In fact it was not low at all it seems. I decided to replace the factory oil pressure sensor out of desperation. As I stated in a previous post, I had added a riser under the original oil pressure sender so I could screw in a second sender into the side of the riser which is connected to the dash gauge. The original oil pressure sender was screwed into the top of the riser and connected back to the PCM using the original plug on the engine loom.

I suspected the second sender was faulty, and I confirmed it by measuring the resistance which should be around 240 ohms at 0 psi. It was reading 1500 ohms with no pressure applied so I knew it was faulty. I was checking the value on the dash gauge against the reading from the PCM and there was always a discrepency of around 10 psi, sometimes greater especially when the engine was hot. This is explained by the faulty sender connected to the dash gauge.

So not being able to trust the reading on the dash gauge, I was going by the reading from the PCM. The oil pressure reading from the PCM seemed to be valid, as it started at 40psi when the engine was cold, and then started dropping as the engine (and the oil) warmed up. The reading seemed to be ok for about the first ten minutes of running (it went down to around 20 psi). But it kept dropping until it got down to around 3~4 psi. As I had no reason to suspect the sender was the problem, I assumed the oil pressure had actually dropped to that level.

After I replaced the factory oil pressure sender with a new one, I ran the engine again. It started at around 40 psi when cold, and after about ten minutes of running it got down to about 20 psi as before, but it dropped very little after that. After about another 5 minutes, it levelled off at 18.5 psi and stayed there. I ran the engine for another 20 minutes and it stayed steady at 18.5 psi. (of couse when I hit the accelerator it goes up to expected levels)

I had two faulty oil pressure senders! What are the odds? The fact that the original sender showed valid reading for the first 10~15 minutes of running and then decided to go faulty I can't explain. Unless it has some strange fault related to heat, or the fact that the hot oil inside it causes the fault.

The only thing left to figure out is a slight 'ticking' noise I get at the front of the engine. I suspect there is not enough oil getting up the pushrods at the front of the engine (I noticed this when priming the oil pump when I installed it). Maybe a differenct grade oil may help? (I've got 10W-40 in there at the moment).

I wasn't looking forward to pulling the engine out again. (it a real pain in the butt to get out and get back in again!). Hopefully, I won't have any more issues with lack of oil pressure. Anyway, I replaced the oil pump which probably needed doing, and installed the F-body pan which also needed doing as the truck pan I had on there didn't have a lot of ground clearance.

Thanks for everyone's comments. Helped me eliminate things which it wasn't.

 

Glad to hear you found the issue.

My 98 use 5W-30, not sure where the 10W-40 comes into play here...

 

My tuner actually just had a similar issue. He built a motor for a truck and thr truck was reading low oil pressure from the dash and a mechanical gauge. Long story short the oil pressure sensor and the mechanical gage were both bad. At least you got it running, and you can enjoy it now. Mine also gave me hell and I just recently got it running right so I know the feeling.

 

WOW! I'm glad you found the problem! Hey, it happens. You live and learn.

Had anyone mentioned verifying with a mechanical guage?

I once did a compression check on my truck, and somehow the little adapter was left in the last spark plug hole. I screwed the spark plug into the adapter without realizing it. It was months by time I realized it, only because I noticed the adapter missing from the kit!

 

I AM very happy for you that you finally figured it out! What are the odds of having 2 bad GM oil senders? I'd say fairly good. I usually like OEM electronics stuff, but the quality and reliability of the GM oil pressure senders for LS engines is on another planet. They're pure junk. I wouldn't take a case of them for free, because a case wouldnt be enough. I've had a 1 year guaranteed O'Reilly el cheapo in my car for at least 6 or 7 years. Always works, never leaks. Whats not to like??

 

I just read deeper into your post, and have to ask: who recommended 10w-40 oil? You'd be much better off running 0w-40, or 5w-30. Most everyone here runs one or the other. Even GM has finally started recommending 0w-40 in their newer engines.....

 

I've used 10W-40 in my other cars and had no issues with it. My thinking was that 10W-40 is a little thicker oil, and thus would help with my low pressure issue.

However, probably a thicker oil would be harder to 'push' up the pushrods, I'll switch to 5W-30 and see how that goes.

 

I run 5w30 and my oil pressure at idle is able 45-50 psi with a mellings pump in my 5.3.

 

Flow is also very important. The thicker the oil, the longer it takes to get to the bearings. This is very important in any engine. But in the LS engines, the mains and rods, inexplicably, are last in line to receive oil!! Probably the other cars you've run 10W-40 in werent LS engines. IDK what sender you ended up with, but 18lbs-20lbs@hot idle is still mighty low. It may be sufficient, but most LS engines I've ever seen, or heard about that had that low of hot idle pressure had an issue somewhere, especially considering the 10W-40 oil. My opinion, and Ive been wrong before. Have you checked hot pressure with a good mechanical gauge?

 

BTW, running 0w-40 oil will give you most every advantage of 10W-40, but with virtually none of the drawbacks.....

 

that's why I had asked could he shim the pump. Ive read a few articles on guys shimming the pumps and gaining 10-15 psi. Not sure if its possible on his application but if so maybe something he could look into. I'd panic if my oil pressure went down to 18.

 

Who installed the cam bearings?

 

Not sure what you mean by "shimming the pump"? I've installed a double row chain, so I needed to install spacers to ensure the chain doesn't hit the pump.

 

Cam and cam bearings were installed by the engine shop. The guy that did it is very experienced, so I'm confident he did it correctly.

 

I just purchased an oil pressure test kit which comes with a mechanical gauge and various adapters. Will let you know what the results are.