low oil pressure

[grumpy5]

I did my intake gaskets a couple of months ago(about 2000 miles). Not long after my oil pressure started dropping off once the engine got hot. I have good pressure on initial start up(over 60), but after the engine warms up it steadily drops off all the way to zero. If I shift out of gear it will come up to about 10psi, and while i am driving it is close to 40. I changed the oil. Put the high milage stuff in. The quantity is good. I don't think I let any debris fall into the lifter valley when I did the gaskets. The vehicle is a 99 2500 with the 7.4l. Any help would be most appreciated. I have 83000 miles on the engine

John

[OZBIRN454]

First when the oil press is zero or low does it knock or lifters tick? If not then I would check oil press with a manual guage. Might have a bad oil press sender or guage.

[Blue Bowtie]

Grump,

Disregard the printing on the oil filler cap, and read the Owner's Manual. Even though the filler cap probably clearly reads "5W30" you should find the place in your owner's manual which indicates that 10W30 is accepatable above 40ºF. Try that instead, then verify the gauge and sensder accuracy by attaching a mechanical gauge to the engine. Generally, 10 PSI per 1,000 RPM is considered adequate.

5W-anything is far too low a viscosity for my preferences. I've used 10W30 all the way down to cold starts at -42°F with no problems (other than the battery being barely able to light the dome light, and the key warning sounding like a sick eeel at that temperature).

[justin1978]

I saw your problem and just had to reply. I had the exact same problem with my 1996 suburban with the 7.4l v8 (454). My oil pressure was doing the exact same thing. I had it checked out by two mechanics on several occasions. Both said that it was fine because the pressure would come up when driving. My motor blew a few weeks later. Check out the Oil Pump and make sure that it is functioning properly, or better yet replace it to be safe. There is something wrong with it if it won't create pressure at idle. p.s. if you have this motor know that gm no longer sells it remanufactured, If you have to buy one it will be brand new and cost $4000.

[rhandwor]

With the cost of a motor coming out of your pocket I would go to napa and buy a pressure gage and check it yourself. If you don't have at least 10 or 15psi at idle I would pull the oil pan. A plugged strainer can cause this or you can have bad bearings which I would check while I was in the motor.
You can go to a machine shop and purchase slightly oversized bearings if everything else looks good I would mike a connecting rod bearing. Talk to the machinist as he can advise you what to do also put in a new oil pump.
Very seldom will just an oil pump work.
Could you have plugged any drain lines that drain oil back to the pan starving the pump. I always double check any work if it starts after working on something.
I put a 350 together after having it bored and the crank turned. It was turned 010 and he gave me 001 bearings. He had always given me quality work in the past and I didn't check the bearings. I used a tool I drive with a drill to pressure the system I couldn't get decent pressure so I dismantled the engine and found the problem.
A good machinist can save you lots of money most of them will talk to you.
He will be familiar where the drains are and tell you if this could be your problem.
I had a 97 ford that I had to go to a heavier oil because of lifter problems.
First oil change.

[grumpy5]

Thanks to everyone for their input. I changed the oil again and put in 10w-40. I also added some lucas oil additive. The pressure still starts out good, but now only drops down to 40psi at idle. I still get a rod knock at start up when it's cold so I don't think it is an indication problem. I have a feeling some junk got down into the crank case when I had the top end off for the intake gaskets. It is my own fault. I got sloppy and rushed the job. Now I'm going to have to milk her along and hope I can get at least a year more out of the motor before I have to rebuild it. If nothing else hopefully some one else will learn from this. Take your time, do it right the first time.
John

[Space1]

I just put a new GM motor into a 1995 Subueban and have the same problem. I don't think you did anything wrong.

[rhandwor]

New GM engine install. If you just installed a new engine with oil pressure only 10psi at idle I would try the 10W30 or 15w40 and if it isn't better. I would try to get another motor. I used a rebuild shop and if they gave you a defective motor they paid $25.00 an hour flat rate for replacement plus another motor,with up to a 2 year warranty. While this isn't shop rate it paid the mechanics rate at that time.
My silverado has 40psi at idle. All you probably did was install the intake and valve cover gaskets. All I said was I always double check my work.
Just make sure you check the pressure with a gage as a sending unit can be bad.

[LunatickFringe]

Oil pumps in the 7.4 litre engines are known for failing, idle pressure at temperature should be no less than 35 lbs, I live in So. Cal and any high mileage V8 gets 20/50 with zinc additive.currently have a 460 with 305,000 miles and a 454 with 270,00 both 35 and 40 psi at idle and 55 to 60 at 70 mph.

[maxwedge]

Good info, but this is a 10 year old thread, check that out before posting, thanks and welcome to AF.

[LunatickFringe]

yes, my bad, I am best with a wrench not a keyboard

[j cAT]

Originally Posted by LunatickFringe

yes, my bad, I am best with a wrench not a keyboard

I use zinc additive in my 1978 boat engine. the boat engine has no cat converters ,but the use of zinc with a converter could be not good . esp in cali. zinc does protect the engine cam wear and other parts. oil made today has little if any zinc. the other issue with oil pressures low is using cheap oil filters. with high power engines towing I use the wix XP series .

[LunatickFringe]

Originally Posted by j cAT

I use zinc additive in my 1978 boat engine. the boat engine has no cat converters ,but the use of zinc with a converter could be not good . esp in cali. zinc does protect the engine cam wear and other parts. oil made today has little if any zinc. the other issue with oil pressures low is using cheap oil filters. with high power engines towing I use the wix XP series .

Yes excellent point,.... and not recommended unless you are willing to get dirty and make certain adjustments at least once every two years..... nuf said.