Help Oil Pressure Problems

jn452 · September 20, 2009

I have a 2004 2500 with a LQ4 6.0 and a NV 4500 five speed. My truck has 200000 miles on it and i am just now having my first engine problems. My oil pressure is too low at start up. In the mornings when I first fire my tuck up I will only have about 5-10 psi witch is ok. But the problem is that when i rev the engine the oil pressure will not incress. All it will produce at any rpm is 5-10 psi. Now after my truck has idled 10-15 minutes the idle pressure will increase to 20 psi and increase with rpm like it is supposed to. I have insalled a manual oil pressure gauge to help me diagnose this problem as my stock one reads 30-80 at any time. Any ideas

Spurshot · September 20, 2009

I'd want to look at the oil pump and pickup. I'm thinking the engine computer is in protection mode at that low a pressure and then lets it out of that mode when it warms.

How have you been maintaining it? Oil change intervals? Filter? Type of oil?

jn452 · September 20, 2009

I have been using royal purple since about 20000 miles change the oil every 10000 miles recently changed from 5w 30 to 10w 30 and have been adding Lucas full syntetic stabalizer to my oil for about the last 50000 miles. I change my filter with my oil every time and always use a Wix filter.

Some one else mentioned the computer I do not have any expenence with computer controled engines but this does not make any sense to me. the oil pump is mechanically driven on the snout of the crank shaft. I am not saying that is not or can not be computer related I just do not understand how. Also it is running now with the oil pressure sending unit out of the truck and wire unhooked from it. The factory gauge reads 0 like this obviously. I have had to replace the oil pressure sending unit before because it failed and read 0 in the truck hooked up. The only thing that happened as far as the computer was a warning came on the message center saying oil pressure dangerouly low, but it kept on running like a champ. As soon as i put in a new sending unit the message went away.

Pia · September 21, 2009

JN,

I am not certain if the 6.0 uses the same style oil pump as the 5.3, but I can let you in on what happened to my 04 5.3L. The oil pump has a pressure relief circuit which is nothing more than a check ball backed by a spring. Such that at high revs the excess flow is diverted back into the pan. My pump, in my opinion, has a manufacturing flaw (a burr) such that one day the ball decided get stuck in the open position.

The behavior I saw was very similar to yours; low/no oil pressure at idle and 'normal' oil pressure at high RMPs.

If the oil pump configuration is the same, it is not fun to fix. Remove the entire front end and drop the oil pan to get at the pump.

Good Luck.

ChrisNJ · September 21, 2009

I'm on the same lines as Alex. My guess is at start-up the oil is bypassing, leading to the decrease in psi. If this has been happening for a long time, then my next guess is that the bypass circuit (ball and spring) may be compromised or worn out. Then the next question is, where are you reading the pressure? Because if it is in bypass, how much pressure is acutally getting to the furthest crank/cam bearing. I don't know, but I would want to know what the pressure is this furthest point. If it is lower than 10psi at startup, I would change the pump/bypass system asap.

jn452 · September 21, 2009

Thanks for the help. Yes the 6.0 and the 5.3 are very close in design and build. I was thinking o-ring on the pick up tube or the pressure relief value in the oil pump. Looks like if I pull off the old oil pump and replace it and check the pick up in the pan at the same time i should be able to fix my problem. Thanks. I plan on replacing the timing chain and sprockets, oil pump and water pump as long as i have every thing pulled apart. Thanks again for the help I will let you know how it turns out.

Spurshot · September 21, 2009

Alex's comments on the pressure relief valve made me recall a similar scenario on my Pitts (airplane in the picture below) from many yrs ago. I had a low oil pressure on the gage. I eventually pulled the pressure relief valve and found a small fleck of aluminum stuck to the aluminum seat. The other part of the valve is a 1/2" steel ball with a spring behind it (Lycoming engine).

The pressure relief valve could be a cause, but there could be other causes as well. If the pressure relief valve is accessable, check it out. I'm not sure where it is in these engines.