I've got the 400's notorious oil pressure problem, here's my symptoms...

When I start the car the oil pressure's great, but after about ten miles the oil light starts flickering at stoplights. Tap the gas barely and it goes away. The longer the car's been running, the higher the RPM must be to make it go away. I've never seen it blink when I'm going at a steady cruise. Light goes away when you put it in park, unless I've been driving it for a long time. I swapped out the sending unit, and it seems like the problem is worse as the new unit would be more sensitive.

I'm guessing its either the oil pump or too much clearance in the main bearings. I've been told that if your mains are wearing out, you can hear it around the oil pan. So, I drove til the light came on, left it in drive so it'd say on, set the brake and blocked the wheels. I listened all around the motor, by the pan, and heard no clanks or knocks. The only suspicious sound was the #1 or #2 cylinder valves. But my #1 cyl puts out zero compression; I think its a bent valve thats stuck and the noise is just the rocker arm tapping on the stuck valve.

What do you all think? I don't have the means to replace my bearings right now. If you all think its the bearings, are there any quick remedies. I'm running 10W30; I've been told that's too thin for an old motor in the southern missouri summer. Previous owner said he ran 20W50 before.

Apparently this problem has been here a while. The previous owner had a repair receipt saying the sending unit was changed a few years ago, making me think maybe the problem went back that far. You'd think if the mains were ready to go, it would have happened by now.

Thanks in advance for any input.

 

It's usually worn camshaft bearings that cause the problem you describe, since the oil goes to the cam first. Be advised that the others are not far behind.

Stop gap - run a thicker oil at least 10-40. Increase the idle speed.

You also may want to install a mechanical gauge to see what the oil pressure really is.

 

20 - 50 oil in the warm weather

 

If the 50 doesn't get it back up, start adding STP oil treatment.

 

Would replacing the oil pump with a high volume pump do any good?

 

You are getting enough oil. If you weren't, your valves would be tapping. Heavier weight oil and a high volume pump will help put the oil light off, but since you are already getting oil that is all that will change.

Evidently it has been like this for a while and it is still running. It will probably still be running for quite a while yet. Just keep an eye on the oil level and ignore the light, unless it changes drastically.

 

I had the same problem with mine, I live in florida so I used 20-50 it did a pretty good job at keeping the oil pressure up. I just ripped the motor apart a few days ago and man those bearings where very "copper" warm really bad so just keep an eye on that oil and run the thick stuff.

 

I have the same problem with my 351m. Oil pressure is fine at startup and then drops at idle when warm. I'm running 15W40 with a bottle of the STP oil treatment and so far that has brought the pressure up into the "normal" range at idle.