I have a 455 engine in my GMC motorhome. After having the engine rebuilt I drove it for about a year, less than 5000 miles. Last year I planned a trip from Florida to Arkansas. Did some prep work including changing the oil. The engine has always had good oil pressure. On my trip I started to get tappet noises and noticed the oil pressure dropped. Pulled over and let it sit. Re-started and back to normal. Back on the road, did it again. I believe the problem is the oil pan & oil pump is starving. Pumping too much oil and not returning enough. It only happens at highway RPM's. The question is: can this be fixed with the motor in place or does the motor need to be rebuilt? On this vehicle I cannot drop the oil pan. Any help would be appreciated. Thanks.

 

What oil (brand and weight) did you use ?

 

You can mask valve train noise with higher viscosity oil (20W50), but low oil pressure can only be fixed with an engine rebuild.

 

Is it overfilled foaming the oil? Does the oil smell like gasoline? X2, what weight oil?

 

10W30 Synthetic not sure of the brand but could have been Mobile 1. Changed the oil 3 times since. Currently has 20W50. Changing oil didn't seem to improve the problem.

 

Tried 20W50. Oil pressure buries the gage at start up. After running at highway speed, oil pressure will drop.

 

Originally changed to 10W30 synthetic. After problem occurred changed the oil & filter again, 10W30. Problem continued so changed to 20W50. No help. Same problem. Changed the oil three times in two days. Oil was generally clean, no funny smells and not foamy.

 

What actual pressures are you seeing that you consider good/bad??? We need data and RPMs
Next, hook up another known good mechanical oil pressure gauge to see if the current gauge is fooling you.

If you're loosing oil pressure at speed the sump is sucking air.
Did the pick up get tack welded to the pump? If not it may have moved off horizontal.
Is the bypass valve in the filter adapter intact/seated?
Did you install a high pressure vs high volume pump?
Are the returns in the heads open/clean.
Pull the valve covers and observe the oil coming up through the push rods. Excessive?

 

How much ZDDP in the oils you are using ?

 

I am absolutely not an expert on Olds oiling systems, but have read many times on this forum site that the oil drainback holes in the heads can get plugged up pretty easily. It would be the least invasive thing to check by pulling the valve covers.
Just a thought.

 

If the engine ran fine for 5000 miles then I doubt that the problem is the oil pump "starving" the pan. If that was the problem it would have done it from the beginning. Same with all of the other stuff that people are suggesting. Oil drain backs dont just get clogged up from and oil change lol. It sounds like something happened when you changed the oil and filter ? Did you use the same brand oil and filter that was on the engine from the rebuild ?

I think the first thing you need to do is pull the filter and cut it open to see if there is any metal in it from damaged bearings etc. That will tell you your next move.

The only other thing I can think of is what DrOlds said about the oil screen coming loose. I always weld pressed in screens. If you can borrow a bore scope you might be able to look through the drain plug and see.

 

Has the timing chain been replaced? I have heard of instances when the Nylon teeth of the timing gear would get sucked up into the oil pump pickup screen at higher RPM losing pressure. When the engine would drop back down to idle, the teeth would fall back down into the pan and the oil pressure would return to normal.

Oops, Ignore my answer, I just noticed you said the engine had been rebuilt.

 

The engine was rebuilt about 5,000 miles ago. Start there. Did the shop have a clue about Oldsmobile engines (which are NOT Chevy engines)? The non-adjustable valvetrain requires the valve heights to be set correctly when the heads are rebuilt, or the valvetrain geometry will be off. What was the quality of the part used? Did they properly break in the cam? What were the bearing clearances used? You will find that most shops are clueless and treat Olds motors just like Chevy motors. They are not.

As for oil, I run high zinc 10W-30, as recommended by the factory. If you drive in hot weather, use 10W-40. My preference is Amsoil Z-rod, but any of the high-zinc oils are equivalent - Driven, VR1, Penn, Castrol Classic - they are all pretty much the same and will work fine in a relatively stock motor. There is no need to use diesel oil.

 

Ordered a bore scope. Will try take this weekend and remove valve covers for look see. Change the oil again and check flow when running. The engine was rebuild by one of two companies specializing in this vehicle. I've tried different oil filters. Also has oil cooler. Disconnected and got the same problem. It is definitely an oil sump problem. Nothing else makes sense. The oil pressure is about 60 lbs at start. Goes down to 0 to 10 pound at highway speed after about 20 to 30 minutes will re-prime and return to normal pressure. I have driven it locally with no oil pressure issues. It is not the gage. The oil pressure drop comes with a lot tappet noise. It is a cab over design so I hear everything. Will keep you posted what I find out. Thanks for all the replies.

 

Does the oil pan look like it's dented upward at all?

 

If your sending unit is in the normal spot, in front (assuming electric gauge), it is reading pressure prior to the lifter galleys, Therefore, the pressure loss is likely occurring in the crankshaft area. Maybe the rod caps are getting soft with temperature, and bleeding off pressure? No matter how you slice it, I feel like it needs to come apart, after reading all this.
My aluminum rods behave the same way, but the engine is designed that way.

 

Stock steel rods don't get "soft" like aluminum. Steel doesn't expand at a rate like aluminum. It seems to me that "oil starvation" is the problem and a problem with "oil supply". I think oil pump, loose oil pick up, clogged oil pick up, oil pump damage is what I would check first.

 

If I had to hazard a guess. I'd say something was left in the engine when put back together, and while at rpm, the oil pickup sucks the floating piece of rag or whatever up to the oil pump pik up and starves it for oil.
This would make sense why you shut it off, and when restarted it now has oil pressure, as the trash dropped when the pump was not sucking.
Rinse and repeat. The trash be it piece or a rag, or rtv,whatever gets to the oil pump inlet and round and round you go.

 

Put an extra quart or quart and a half in to start with to see if the problem goes away . the drain back on the Toro pans is bad.

i would also check with a scope in through the drain plug to see if the pickup is where it should be..too high and or touching the pan will cause that problem

 

Years ago, a friend bought a 78ish Toronado with a 403. The engine had great oil pressure at idle, but dropped quickly when you revved it up or drove it. He swapped on another 403 that came out of a flawless running but wrecked trans am. Once installed, EXACT same thing! He did an autopsy on the original 403, every bearing looked perfect.

Turns out the bypass plug in the oil filter housing was gone. Since the 403 that was installed came from a standard RWD car, he had to reuse the Toronado oil filter housing, basically reinstalling g the problem part on the new engine.

I’m guessing the plug blew out and was thrown away during a previous oil change.

Replacing the oil filter housing solved the issue.