No oil pressure at idle

[Gryphon]

I searched the forum and found that yes, there is something wrong, but I don't know what or where...

'72 multifuel idles fine, runs fine, oil pressure hits in range at 35-40 psi while underway, but at idle, it reads 0. If I feather the throttle up to about 1000 rpms, it jumps pressure to about 20 and goes up fine from there.

As I posted in my intro in the Testing area, we know nothing about this truck. Our Cheif says she has done that as long as the department has had her.

I will be relocating the mushroom and a few other things I read about in the forums.

Where do I look here for more info on ID'ing the truck (engine and such)?
(I did find the VIN thread and will be getting that later today or tomorrow.)

 

[Westech]

first of all check the pressure with a mechanical gauge. The military sending units are not all that great. If you do that and still have 0 oil at idle the best cause of that will be cam bearings.

 

[jimk]

Many years ago I read it takes 2 psi to float a bearing. Sure you'll need more at RPM, and under load, but idle is not too demanding. My Ranger P/U has almost no oil press, verified, at idle, when hot. Adding a few 100 rpm it rises off the peg quickly. I run SAE 30, sometimes a mix of old Deuce oil(15w-40 w /relatively low mileage). It has been this way for years, prob loose bearings because of anti-freeze as it has a cracked head plugged now with ceramic seal.Maybe some worn pump gears/housing too. . I'll bet everythings a bit loose inside.Never knocks, but lifters rattle when hot. It tolerated towing another Ranger half way cross the country in Feb. That was a real work out for the 300K 2.9L. No problems running WOT for 12 hours. As westech points out- get a mechanical gauge- even just a simple bourdon tube item, screw it into a gallery/sender port. Make sure oil is correct weight, 15w40 or SAE 30 (except arctic conds.) and not full of fuel. Old oil does have some viscosity loss as some of the molecules get sheared(chopped up).Make sure idle is 650(LD) or 850(LDT), It's ok to use the latter for all as it makes H bulbs last longer.JimK

 

[DDoyle]

Be aware that sending units and gauges must be matched. Two ranges were used on these trucks, if missmatched you can be reading half the actual oil pressure (or twice the actual oil pressure). The range is stamped on the sending unit, and of course is legible on the face of the gauge.

Hope this helps,
David Doyle