Thats possible I guess scooter, not sure if the oil filter bypass would affect actual engine oil pressure though. The factory oil pump has a spring and piston type valve, debris is about the only thing that will cause them to stick and when that happens it's usually always high or always low pressure. The pressure range sounds pretty consistant here. Eighty psi is not excessive on startup and when hot the oil pressure can drop to 12 psi or lower at idle depending on oil temp. Not sure what type or quality of gauge he bought either.

 

Yep did that and gauge shows pressure drop with light and new sending unit

 

Ok so we are certain that you are actually getting a pressure drop not just a faulty gauge or sendor. When you ran a gun brush through the oil passages did you remove both of the small cup plugs that cap the ends of those passages? Did you install a factory oil pump or after market?
The only time I can recall an issue like the one you have here, is when someone at the factory left a shipping tag in the oilpan during assembly. The engine came back from dealership for the exact problem you have. When we removed the oilpan we found the shipping tag in the pan. When the engine was started the pan was filled with oil and the tag floated around. When the oil level in the pan dropped the tag would get sucked up to the pickup screen and stop most of the oil flow dropping oil pressure. This kept repeating itself over and over. Anyway is it possible that you left a papertowel or rag in the oilpan when you reinstalled it? I know its a long shot but just trying to cover all the bases.lol

 

Ok so we are certain that you are actually getting a pressure drop not just a faulty gauge or sendor. When you ran a gun brush through the oil passages did you remove both of the small cup plugs that cap the ends of those passages? Did you install a factory oil pump or after market?
The only time I can recall an issue like the one you have here, is when someone at the factory left a shipping tag in the oilpan during assembly. The engine came back from dealership for the exact problem you have. When we removed the oilpan we found the shipping tag in the pan. When the engine was started the pan was filled with oil and the tag floated around. When the oil level in the pan dropped the tag would get sucked up to the pickup screen and stop most of the oil flow dropping oil pressure. This kept repeating itself over and over. Anyway is it possible that you left a papertowel or rag in the oilpan when you reinstalled it? I know its a long shot but just trying to cover all the bases.lol You are positive that the cyl head oil drainbacks are not plugged as well even in the block?

 

I have pulled pan and all looks well. As far as heads I had them off and pressured washed them they were pretty gunked up and i saw no other way than to just clean and replace stems.lifters ect. If all the wear surfaces in this engine had not looked so good i would have replaced it. I have decided to replace mains this weekend and i am thinking i will find out that this has been going on for a while I just didnt know with the factory idiot light. Oh and thank you for the tip on pulling the oil plugs and cleaning I will do that too.

 

It's difficult to get a tru size clearence with the crank hanging down on the mains. Plastigage will register the weight of the crank. You will need to support it up in the journals to plastigage clearences. Also not all mains are the same size from the factory. Look closely. One loose main ( over .0025" ), will wreck all your hard work in short order. I've witnessed a lot of worn engines loose oil PSI as they warmup, rule of thumb though is they don't have that awesome oil psi cold unless the oil is very thick/cold climate starts. Reading back in the posts, I'm leaning toward a PSI drop due to relief valve failure, foreign object or pump having particles in it cloging the relief open some. Strange it always closes backup when shut down though. A weird one, but a quick check of the mains are in order. Relief valves are a mechanical devices and some fail in strange ways. When checking your mains look at your thrust, see if it is broken/piece lodged. Many times the failure of a build deals with not recognizing the proper fit of the bearings with the crankshaft. That's made many engine mechanics sucessful careerswith the oil pump the life of the bearings. With the oil PSI situation, I am totally supprised you havn't already lost a rod bearing. They usually don't do well new, loosing oil PSI on initial warm up. I'm still leaning at the oil pump/relief valve being wrong for your application. One final thought, these oil pumps don't do well on initial start up if they are dry. It will damage the outer ring bore and cause low oil PSI. You remove the pump and mic the bore, if you are seeing clearence increase over the pump gear diameter this is what has happened. Although it should not go back to a normal PSI with engine shut off and restart. It would have to be back to room temps. These types of pumps take time to pull the oil up to them/dry= heat friction/distruction/in a matter of seconds. Going from .002" to .007" + bore. This will bring about cavitation and a very strange pump. Later after the motor is destroyed the pump will be trashed.

 

I found a used motor at bone yard with 60 k out of wreck. That should fix all of this and now I have lots of extra parts too. I will get crank sent out and rebuilt heads and pack it up for spare. I still have no clue as to why oil pressure drops like it does will be looking at mains tonight.

 

When you pull your motor down check the motor oil pump bypass valve, see if it has been put in upside down. Check your VCT chain tensioners. If your chains or chain is slack?

 

Put old motor back in a truck with new mains and rod bearings and after a hundred miles or so all seems well. It is a lot hotter now so I will run a little heaver oil. Oil pressure has stayed above 30 lbs even when idling at light in 90 plus degrees. So the good side of this is I have a spare 4.6 to stash ( need to look that up on how to do it) The wife has an expedition it could go into in another hundred thousand. Thank you for all the help.

 

I would not run anything heavier than 5W30.