Oil pressure fluctuates

[Doug&Deb]

Okay this is a weird problem. My oil pressure fluctuates between 10 and 40 lbs until I get a few miles on the road then it settles at around 40 and stays there. This is a rebuilt engine. It didn’t do this on the test stand. I’m going to hook up an aftermarket gauge to see if I get the same results. What else should I check? The engine doesn’t make any noise when the pressure drops and there’s no power loss so I’m stumped.

[Sniper]

I'm wondering if the bypass valve is cycling?

[Los_Control]

Hearing rebuilt & engine run stand .... is the motor fresh rebuild fresh install? Almost no miles?

 

Just wondering if you have changed the oil since the first start?

What @sniper suggest sounds very plausible but why? .... does the engine still have assembly lube & leftover gunk in the oil from the rebuild?

 

Just a question, I do not know the history of what you have been doing.

[kencombs]

Is it fluctuating with speed?  Temp,  other changes in operation?  Quick moves or slow cycling?   

 

Electric gauge?   Maybe sending unit changing value as temp builds?   Sealer on the threads messing with the connection?   Wild guesses.

Edited May 25, 2023 by kencombs

[Doug&Deb]

Fresh rebuid with few miles. No sending unit just a connection to the oil galley. I’m suspecting the relief valve. I’m running 10w30 oil for break in. I’m about due to change oil. I’ll go back to shell Rotella T 15w40. 

[kencombs]

Was the relief valve removed and cleaned, valve and bore, during the rebuild?  If so shouldn't be sticking unless there is a burr or some fault on the bore or valve.   Or maybe a faulty spring, broken and coils binding.

[Los_Control]

Jmho, I'm not a mechanic ..... what I would do if I had a fresh engine, I would start it & run it maybe a hour or so ..... On modern engines you need a good 20 min to break in the new cam to the lifters. ..... No idea how that plays into our engines.

 

A fresh cross hatch cylinder hone going against new cast iron rings, there will be microscopic metal shavings as the rings seat into the cylinders.

Same thing with all the moving parts, timing gears/chain, bearings ..... not chunks of metal but microscopic flakes as the parts wear into position.

I figure any good mechanic will want to add adequate amounts of assembly lube ..... meaning too much.

 

So in my mind, we have start up oil .... run it for a hour then dump it.

Then you are using break in oil, run it for 200-500 miles then change it & run your normal oil.

So there is 2 oil changes before you get to normal oil ..... Just my opinion & I do not claim to be a mechanic.

 

 

1 hour ago, Doug&Deb said:

I’m running 10w30 oil for break in. I’m about due to change oil. I’ll go back to shell Rotella T 15w40. 

So if it makes any sense, are you on startup oil or break in oil ..... others have their own ways, I'm not saying my way is the only way.

 

The only rebuilt engines I've owned, I took the vehicle to the shop, they did R&R sent it to a engine builder then installed it & started it. Changed the startup oil & sent it too me with instructions to change the oil in a few hundred miles.

[keithb7]

I seem to recall some confusion about reassembling the oil pressure relief in the block. It was likely because a year after I tore the engine apart, I was finally reassembling it. The parts diagrams weren't real clear on which way the plunger had to go in. Is it possible you put yours in, incorrectly?  

 

Another side note: I noticed my oil pressure was a bit lower with 10W30, versus 20W40 or so.

 

 

[Dodgeb4ya]

Possibly as is some what common the oil pick up float is rising too high and sucking some air because  no one checks the adjustment tabs when rebuilding or R&R'ing the oil pan.

There are limiter drop and rise metal tabs that get bent because of rough removal and installation of the oil pick up or oil pan with baffles.

To do a quick diagnosis of this problem  make sure the engine is full of oil then add one extra quart of oil  to see if the erractic oil pressue issue is solved.

Edited May 26, 2023 by Dodgeb4ya

[D35 Torpedo]

My engine was fresh when I got it. I changed the oil every few days until I was happy. My engine didnt have a filter so I really didnt want to take a chance. Oil is cheap compared to an engine.

[DJK]

15 hours ago, Doug&Deb said:

Fresh rebuid with few miles. No sending unit just a connection to the oil galley. I’m suspecting the relief valve. I’m running 10w30 oil for break in. I’m about due to change oil. I’ll go back to shell Rotella T 15w40. 

Check out Penn Grade oil, when I emailed them, I got a phone call back explaining their products.

[Doug&Deb]

I’m going to start with the basics. Oil change. I’ve put a few miles on driving to work etc. varying the speed and load on the engine. I’ll see if that has any effect . If not you’ve all given me good options to try.