1970 Duster 340 Oil Pressure Question
 

First time owner of a classic car. Just bought a rebuilt 1970 Duster 340

Previous owner told me the oil pressure should be 60psi when the engine is warm.

When I drive the vehicle the oil pressure stays between 50-60psi, but when I'm at a warm idle my oil pressure drops and stays at about 20-30psi. If I give the engine gas in park the oil pressure goes back up to 50-60.
Is this normal? Any questions or suggestions?

The pressure drops enough that my oil pressure light will flicker when I slow down to idle but then goes out once I'm fully stopped and pressure stabalizes around 20-30.
Also, when I first fire up the car the oil pressure stays at 60psi during idle

It's a 340 that has been rebuilt 9500 miles ago (roughly 10 years ago)
-High lift solid lifter crane cam
-12:1 Keith Black Pistons
-4 barrel 750 Holley Double Pumper Carb

1.........Never believe factory oil senders and gauges

2........What kind of gauge are you using to check?

3.......IF this pressure is correct, you have nothing to worry about so far as oil pressure

Put some 30W Engine oil in it and see it that helps.....
If it dont,,,, Then it just has some loose clearance in the lower bearing area..

Why? Doesn't sound to me as if there's a thing wrong.....................

I agree, but I think he should check the oil level. I had a 318 that would light the warning lamp when slowing to a stop, it meant I needed to add 2-3 quarts of oil, it was a nice system we'd worked out.

Thanks guys.
The oil level was perfect but I changed it yesterday anyway because I just bought the car. I don't think the guy before me changed it in a while because there was some sludge in it.

The oil pressure now stays at 65psi while driving and only goes down to about 30-35 when I'm slowing down from 1st gear to a stop

I'm running autometer, pro comp gauges. They're liquid filled. Pretty sure it's a mechanical gauge but I haven't actually checked.

Viscosity break down.. Could be one cause....
I'm not say 30psi is bad....
High Pressure pumps can cause long sweeps..
Loose Mains can cause long sweeps in pressures also..
But when you hear someone say, it dipped low enough to set the light flicker, 10-15psi...
What Am I telling you for you old goat... You know this......lol
I go for the easy stuff... Filter or oil's... But there is always the assumption on the location... If he said Alaskan area... I would have been in trouble with the 30W... :)

Glad you got it to your liking Chris.

Well I ended up checking compression today to get to know my engine and this is what I discovered. I have bad rings in one cylinder. I posted a new thread about my problem but I'll copy and paste it here. If you guys have any advice/tips would be great as I'm new to the classic car and mopar world.

Just bought a 1970 Duster 340. Driven it a few times but figured I'd check everything over so I know she's good to go.

Checked compression today
Cylinders 2-8 all had between 170-185psi
Cylinder 1 has a reading of 16-18

Decided to do a wet compression test and the reading went up to 45psi

Any idea why my piston rings would fail?


The rebuild only has 9500 miles (10 years ago.) It was built to run on a strip but the previous owner only ran it a few times when he first rebuilt the engine.
I don't have all the information on the rebuild but what I do know is this:
-12:1 Keith Black Pistons
-Hi Lift Solid Lifter Crane Cam
-4 barrel holley 750 double pumper carb
-Griffin Rad

There was some sludge when I changed the oil yesterday. Not much but it was there. It had 20w50 Synthetic in it.
The oil level was low the last time I drove it. I had to add 2 quarts. Didn't realize it was missing that much oil. The oil pan holds about 8.5 quarts though so I never thought much of it.
Also one of the first times I drove the car it was backfiring about every 2-20 seconds. Got worse near the end of the drive and backfired every second or two. I drove for about 10 minutes until I was home, checked the plugs, wires, and octane level. Everything was good and it never backfired again.

Whether it was my fault or whether I bought it with a bad cylinder is my problem and I know that.
Any tips on rebuilding, a quick/easy fix (unlikely), or any help would be appreciated.

Thanks

Check you check the valve train yet? Might have just a stuck valve or a flat cam....

For that low of a reading ? It is something in the valve train Stuck or burnt valve Or worst case a holed piston.
Pull the intake and head from the bad side

IMHO. from idle as the RPM comes up a few 100 RPM if the pressure goes up with the RPM you are OK.

You may have had gasoline in the oil. Either due to bad rings, or hole in piston. A stuck valve would not cause that.


In order for the oil to lose its viscosity, it would have to have may be 10,000 miles on it.