low oil pressure

[Grim 82]

I'm hoping you guys can offer some input about my oil pressure readings as of late. I was driving the other day and with the engine fully warmed up, in gear, sitting at a red light the gauge flatlined to 0. It read 50-60 psi on cold startup, 30-40 cruising, and about 10 in park. As soon as you put it in gear it goes to 0.
So, I just installed a Sunpro electrical gauge and sending unit. I replaced the stock sender (by the dist.) with the Sunpro one. At all other conditions, it reads the same as the stock gauge did, except when stopped in gear at the red light it reads in the single digits. (see artistic rendering) At this time I also changed the oil, using Havoline 10w30 and a napa gold (Wix) filter.

So after asking around a bit locally I've narrowed it down to 3 possible things

1. It's normal and you worry too much
2. Everything in between
3. Spun a main bearing

[Lt.Del]

throwing things out here:
pick up screen on the pump may be partially clogged, but, you are getting 50+ at startup.
Do you have the 3 inch extension, near your distributor, that the sender screws onto?  Try removing that to see if that is partially clogged?
I'd be tempted to remove the oil pan and put a new heart in it.

[Grim 82]

I have the brass angled extension on there. It had some regular pipe fittings on it with the brass angled piece and a pipe nipple snapped off in the hole when I started removing the old sender, so I had to EZ out that piece from the block. Now it's just the new sender going to the brass angle fitting going to the block, and I cleaned that piece and sprayed it out with shop air.

I need to replace the rear main seal anyway and I picked up a one piece pan gasket like I asked you about a while back, so over lunch today I picked up a new pump also since I'll be in there. I hope that takes care of it.
Do you think it's low enough that I shouldn't be driving it? I won't get to work on it til the weekend.
Also, if it was something serious like a spun bearing wouldn't that have more obvious symptoms than just low oil pressure at a stop? That would affect pressure throughout the whole spectrum, wouldn't it? I'm trying really hard to rule out that diagnosis, just for some peace of mind...

[jaredts]

What's the rpm when you put it in gear?  Is it just below 15psi at that point like I think I see on your pic?  That's not good, but typical for a tired motor.  Pickup screen or new oil pump might help a little, but if its a high mile engine I think its the clearances between main bearings and crank that drop the oil pressure the most.  I don't think a spun bearing makes sense in this case, but could be wrong.

[Grim 82]

Idle is around 800, and in gear under load it drops to about 650-700 rpm. What bothers me is that the motor really only has about 15-20k miles on it since the build. My brother had it built about 12 years ago and it made good power for a couple thousand miles until the rear diff exploded, then he parked it and bought a new truck. So, it sat for 10 years until I bought it from him and got it going 2 years ago.

If the mark between 0 and 25 is 12.5, it looks to me like it fluctuates below that line, somewhere between what would maybe be 6-10, which doesn't seem like a lot. Before this started last week it never went under 15 psi.

While I'm in there replacing the rear main seal and oil pump, is there any way to visually inspect and rule out a spun bearing?
Also, do I need to prime the new pump or just bolt it up and go?

[VileZambonie]

If it spun a bearing you'd know it!

If you're going to drop the pan pull a few bearing caps and inspect (plastigauge would be ideal) If it looks like .0025-.003" clearance run a high volume oil pump. Make sure you're using a steel sleeve drive shaft and yes you have to prime the pump. At the very least pack it with vaseline and crank the engine with the spark plugs removed and the coil disabled.

[Grim 82]

Well since I posted this yesterday the psi at cruising is down to 20-25. I took the truck over to a friend's shop and hooked the big face mechanical gauge up to it and warm at idle (750 rpm) it reads 5 psi, and now it sounds like there are rats running around in the crankcase. He said to push/pull on the balancer and check for travel in the crank. I could move it 1/8" or so...

[jaredts]

Well since I posted this yesterday the psi at cruising is down to 20-25. I took the truck over to a friend's shop and hooked the big face mechanical gauge up to it and warm at idle (750 rpm) it reads 5 psi, and now it sounds like there are rats running around in the crankcase. He said to push/pull on the balancer and check for travel in the crank. I could move it 1/8" or so...

You should probably try to put a mag base indicator on it to be sure, but that sounds way, way beyond shot.  You could probably live with .02" or .03".  If you want to continue investigating, Vile's advice (of course) is a good next step.  If you have anywhere in the neighborhood of 1/8" endplay there is no way that motor was properly rebuilt.

[Grim 82]

put a mag base indicator on it

I'm not sure what that is, but I'll do some research on it. It's always been in the back of my mind that this motor couldn't really be dependable after sitting for so long. Not sure if that has anything to do with it or if it just never was right.
If it has lost the resistance that builds the oil pressure, then I don't suppose a new pump would really make any difference? Does it still pay to plastigage it with that much forward/back travel on the crank?
I'm wondering if I should cut my losses and find a new motor. It's not so much a money issue to me right now as much as it is an issue of carpooling with the wife is not very feasable due to work schedules and locations, and we only have the 2 vehicles. I have no desire at this moment to learn engine rebuilding, but I could swap in a long block in a weekend easily.

Where to from here?

Thanks guys

[VileZambonie]

In an hour you can have the pan down and check some of the bearings and the pump at the same time. I would say do it or just pop it out and start rebuilding.

[Grim 82]

pop it out and start rebuilding.

lol, I can throw it in a crate and have it to your shop by the weekend

I'll have my buddy help me check that stuff, but I need a pretty expedient solution to this, especially with the spring flooding they are forecasting for us. I think I'm past the denial and grief stages, and now I'm daydreaming about what kind of motor I could drop in it...

[TexasRed]

It's too bad that you can't find a decent running 305 to swap in there for the time being. . . .

[Grim 82]

Funny you should say that. I have a 305 with one weak cylinder but otherwise runs decent. If I go through the work of swapping it all I'd rather just do it once with a new motor and be done if you know what I mean. Maybe build the other one for a future project

[VileZambonie]

pop it out and start rebuilding.

lol, I can throw it in a crate and have it to your shop by the weekend

I'll have my buddy help me check that stuff, but I need a pretty expedient solution to this, especially with the spring flooding they are forecasting for us. I think I'm past the denial and grief stages, and now I'm daydreaming about what kind of motor I could drop in it...

Send case of beer too please 

[Grim 82]

 

So many choices.. I have a line on a couple of really good motors. I'm going to start a new thread.