Rebuilt 383 Low Oil Pressure

[kp1241]

Greetings all. Need some advice. Bought an unassembled 383 that had all machine work done. Had to have crank polished and heads gone through. Block looked really good. Upon assembling all rotating assembly parts I followed the Mopar rebuild big block book step by step using all proper torquing specs. I plastigauged the crank journals and all were withing spec.
Finally, after months and hours of prepping, put the motor in my 69 runner 4 speed and fired her up. Ran at 1800 for a half an hour and noted that the idiot light on the dash for the oil was on. The car runs awesome, very smooth and has a stock exhaust that sounds good. Changed the oil (straight 30 and oil filter) and same thing, stupid idiot light on dash stays on. Put in electric cheapo sending unit to see gauge. At cold I have 40psi (tops) oil pressure, when it warms up it drops to 25 psi, even if you are revving. I am puzzled. I am thinking something in the oil pump which was a used unit but appeared to be in really good shape.
Any thoughts or advice would be greatly appreciated. I don't want to drop the pan as I triple checked the mains before buttoning her up. Doing the windage tray and pan upside down on my back would suck.

Thanks for any help.

 

[BeastlyBirdie]

Well, a bit of info would help...does it smoke? Noise? Can you put a mechanical gauge on to truly check the pressure? I would guess (if I had to with only what you have posted), its the pump if all the other things are fine besides pressure. And a pump is easy on a 383.
What it could be, there's a lot. But start with the basics.

 

[70rr-brian]

$70-90 bucks for high pressure pump and a few bloody knuckles later (also on your back) just to see. Most places will take it back even after it has been used if you clean it up OK.

 

[kp1241]

Well, a bit of info would help...does it smoke? Noise? Can you put a mechanical gauge on to truly check the pressure? I would guess (if I had to with only what you have posted), its the pump if all the other things are fine besides pressure. And a pump is easy on a 383.
What it could be, there's a lot. But start with the basics.

HI, thanks for the reply. No smoke, no noise, super smooth. I am going to go through the pump this afternoon. I have two spares. I am thinking of a high volume pump and found an adjustable regulator. What do you think of those?

 

[coloradodave]

$70-90 bucks for high pressure pump and a few bloody knuckles later (also on your back) just to see. Most places will take it back even after it has been used if you clean it up OK.

Brian,
What does the parts store do with the now 'used' pump you return? Can't sell it to somebody else as new once it's been installed. I used to try and be courteous to customers who were using my inventory to diagnose their problems, but brother, sometimes it was REALLY hard...

And, yeah, hook up a mechanical gauge before you mess with the oil pump. Might save you some work...

 

[Funship32]

Well, a bit of info would help...does it smoke? Noise? Can you put a mechanical gauge on to truly check the pressure? I would guess (if I had to with only what you have posted), its the pump if all the other things are fine besides pressure. And a pump is easy on a 383.
What it could be, there's a lot. But start with the basics.

X2. As long as you are sure your mains were in spec, I would suspect the oil pump.

 

[70rr-brian]

Oh I hear ya... I got a good place down here near me and I always let them know I am going to do something like that and get their blessing before doing it.

 

[Histoy]

You didn't mention whether new cam bearings had been installed in the 383. If the actual pressure checks low with a mechanical gauge, and a different pump doesn't improve the pressure, worn cam bearings might be the problem.

 

[BeastlyBirdie]

Try any pump you know will work....but stick a mechanical guage on first

 

[Meep-Meep]

Assuming you verified you have low oil PSI with a good gauge, my first guess is you have one or both rocker shafts on up side down. Pull the valve covers and if you see oil holes right over the rockers they are wrong. Look in the FSM for proper oil hole orientation and rocker alignment.

 

[kiwigtx]

If everything mechanically has been assembled correctly, and that can be proven, I would suspect a blockage or partial blockage in the oil galleries. Sometimes machine shops and re-builders get a little lazy when it comes to prepping the block after all the machine work. I have always spent a few hours scrubbing a block even if it has been 'hot-tanked' and pressure washed. Those oil galleries need to be spotless - any small lump of sludge could be sitting there waiting to cause problems. There could also be some build-up of sludge around the oil pressure sensor area causing your gauge to read low.

Obviously this is not an easy thing now that you are in the car and running......just putting the suggestion out there.

 

[Meep-Meep]

Actually a blockage will tend to increase pressure because flow is reduced. Low flow = high pressure and vice versa. But you can never be too clean during block prep and I do exactly what you say after a block has been "cleaned" at the shop.

 

[thomas93254]

The mechanical gauge is the best bet and easier than laying on your back. Had an idiot light come on in a car once and that was the first thing I did. Sending unit was bad, had plenty of pressure.

Thomas

 

[RustyRatRod]

First off, 25 PSI is not low enough to cut the idiot light on. Did you hook the cheapo electric gauge up to the sending unit? You should put a GOOD manual gauge on it first and go from there.

 

[KB9GIB]

Hi kp124 i have had this same deal on 2 new 440 with the low oil press i came across the hp blue mopar book and it said if you use a full grooved bearings you will have lower oil press i use to worry on mine cold woud be 40psi to 50 psi cold and hot 25psi to 5psi been driving them for years hope this is your problem hope this helps mike

 

[Lowhound]

Try a new oil pump. I once built a 383 & it held 80 psi at idle. When I revved it the oil sending unit blew oil all over the place. Even though I had a new Melling pump on it I had to replace it with another. The new one held 45-50 psi.

 

[kp1241]

Try a new oil pump. I once built a 383 & it held 80 psi at idle. When I revved it the oil sending unit blew oil all over the place. Even though I had a new Melling pump on it I had to replace it with another. The new one held 45-50 psi.

Thank you all for the advice. I am going to put on a mechanical gauge and see what it reads. The bearings were grooved and it has all new cam bearings. I also did put the rocker shafts on correctly. All great advice and thank you for this great forum and information. I will keep you all posted!

 

[99ss]

First off, 25 PSI is not low enough to cut the idiot light on. Did you hook the cheapo electric gauge up to the sending unit? You should put a GOOD manual gauge on it first and go from there.

good place to start

 

[kp1241]

good place to start

Thanks boys!! Put on a nice mechanical oil pressure gauge. 55lbs when cold, drops to 40lbs warmed up. Good to go. Put in a new sending unit, no idiot light. Car runs GREAT! Thank you for all of the advice. This site rocks. Mopar or no car!!
Ken