Low Oil Pressure

[plowboy76]

I have a 99 GMC 2500 6.5TD. About 5 months ago I noticed white smoke and found out the block was cracked. I found a new bare block and had a mechanic put the motor back together. I have good oil pressure at cold start 70psi, while it warms up the pressure drops. 35psi at 2000 rpm warm and 7 at warm idle. The new motor has much less oil pressure than the old one did. I had new cam bearings, rods and mains. New oil pump was also installed. I haven't been doing the work on it but the mechanic has a good track record. Not sure what to do short of tearing the motor down.
Thanks for any help

[trukdoc]

...put in a new oil pump? did you use your "old" crank? did you have the crank turned? did you check oil clearance on crank with plastigage? did you use the correct bearings on mains? and rods? are the cam bearings installed properly?did 1 turn when being installed? I've seen old pros make all these mistakes{the turned cam brg.on install was mine}...did he take his time or was it a "flat"rate job?...did you rush him to finish? ,,I can do this all day guys...jd

[plowboy76]

I did put a new oil pump in and all new bearings. the old crank was reused and cam too. The only thing I can think of is a cam bearing is out of place somehow. I assume plastiguage was used. How much oil pressure should it have? I put in a high volume oil pump. Thanks for the replies.

[grancito]

70 psi is not good, pressure should be regulated to around 40 psi. You should get 20 psi hot idle. Your gauge or sender sounds suspect, or something is wrong with the oil pressure control valve.

[M123KT-450]

Really 40PSI ?

Then my truck is wacked out, cause with my water temp running 187*F I have 38PSI at 750rpm and 60PSI at 1800rpm and above.

My oil sending unit is new, it was replaced due to it was reading 80PSI at start the falling to 0PSI shortly thee after...

Mike

[grancito]

It probably means that the electrical system, not reliable like a pressure gauge, is still giving a false reading.

[plowboy76]

The oil pressure was good before I had the motor rebuilt. I put in a new sending unit as well as a mechanical guage. I am not familiar with the oil pressure control valve. How to I get to that/location?

[grancito]

They are usually in the oil pump, don't know on these engines, try a seach or pull the old pump apart.

[trukdoc]

,,,what oil are you using? did they put in a "break in" oil? as opposed to the 15w40 you normally run?

[plowboy76]

I wasn't sure on the 6.5's if the pressure control valve had an adjustable spring or not. On some diesels there are adjusting screws. I run Chevron 15w40 and that's what I started with. This is the second oil pump we've tried and no change same thing, starts up with good pressure and drops while warming up.

[M123KT-450]

Some thing isn't happy then that is for sure.


Mike

[chessie6.5]

adjusting the pressure regulating spring will change max pressure, not minimum. Check a sample of oil for metallic look. Look at sample in direct sunlight; drag your finger through it. Does it look metallic? Did the mechanic properly pre oil the engine? They are sensitive to this, as the mains and rods are the last things to see lubrication. Oil goes from pump to oil cooler first, then to oil galleys, then to bearings. Maybe he didn't use appropriate bearing sizes for a used crank. If oil looks ok, change to your usual oil and see if it improves.

[M123KT-450]

I can't imagine the oil itself would cause the issue can it ?


Mike

[JD_countryboy]

Did the person who assembled the engine use a good assembly lube? I have seen where people dont use a good assembly lube, and or dont prime oil pump before starting and end up wiping out cam bearings on startup. Same symptom every time, oil presure bottoms as it warms up. Hope this isnt your case, but its a thought.

[rooferwill]

Hey just a thought i have seen where a oil filter has caused similar symptoms. A buddy built a 351w pretty well back stock but he plasti gauged everything and had all the necessary machine work done. the oil pressure would almost drop to zero when it heated up so he checked a few things, well he changed the filter and oil and it straightened out. Another reason i don't like fram:p but it is just a thought

[M123KT-450]

You may have some thing there, I change my oil filter 3 times per oil change cause a plugged filter causes the oil to get dirtier faster and lowers oil pressure as well so this was my reasoning behind multiple filter changes during the oil interval :D

I run AMSOIL brand oil in my truck and i change my oil every 9,000 miles and my oil filter every 3,000 miles ;)

Now i have noticed in my 6.5 when it was on the road right around 2,900 miles the oil pressure would drop off noticeably once i changed out the filter and topped the oil off it went right back to where it was :cool:

I have done 4 of the 9K mile intervals with filter change every 3K miles and my 9K mile oil sample tested "good continued use" all 4 times So i quit testing my oil now i know its still ok for use at 9,000 miles i change it any way i don't mind pushing it but at 3x the recommended drain interval is plenty for me !

Some people actually run it the full 25,000 miles or 12 months :eek:

Mike

[rooferwill]

just remember thats exactly what the military used to do. they would change the filter and test the oil. If the oil passed they would top it off and go.

[plowboy76]

thanks for all the help and ideas. I think we have tried everything we can think of, short of tearing down ther motor little by little and see what we find. I will let you know what we find when or if we find anything.