No oil pressure, new build
 

The fresh 388 we just built for my sis in laws 86 GMC has a oil pressure problem. Motor had 70 lbs when cam break in was started. During the break in pressure slowly dropped to 40 lbs. After break in motor had about 20-25 at idle out of gear. In gear it has none.

Changed gauges, no change.

Verified that the plug is under the rear main cap, so I pulled motor out and tore it down.

Expected to see bad bearings....not the case. All bearings look good. Clearances are all fine. 1.5 on rods and mains. Plugs in front side of block are all seated. Cam bearings, journals look fine. Bypass in filter housing appears to be good.

Oil pump seems to be OK. Rotor clearance looks (havent measured, but probably .002 or so) good. Pulled relief spring out and it seems fine. Havent verified that the plunger is moving freely. Probably will let the machine shop look at that.

Checked block thoroughly for cracks in valley/galley area. Cant see any.


Ideas on where to look next?

Re: No oil pressure, new build
 

Are you using an electric guage or mechanical?

Re: No oil pressure, new build
 

Mechanical. Gauge works fine.

Re: No oil pressure, new build
 

Have you run the pump with a drill while watching the lifter valley and bottom end for unusually large amounts of oil?

hard to say I would expect with no oil pressure you would see some sort of damaged even if the pressure relief valve was stuck either way you would have oil pressure. there is nothing that I am aware of that would make it change that much going for n into gear. let us know when you find out


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Re: No oil pressure, new build
 

having same issue! waiting on your findings!

Re: No oil pressure, new build
 

My truck has the same issue. Pressure drops slowly until the gauge barly registers any at all. I tried 3 different gauges, all behave the same way.
My 305 is stock, and has never been opend as far as i know, so I figured it was just starting to show it´s age and was ready to be pulled. Since Im going to replace it I decided to see how long it would run before something happend.
Well It has lasted the whole summer, and is still running good, no excessive oil consumption and not blowing smoke eighter.
I never had an engine that behaved like this before

Re: No oil pressure, new build
 

how does the pick up tube look?....any holes in it....stopped up?

Re: No oil pressure, new build
 

Plastigauge alone will not tell you your clearances. It's more of a guideline gauge if a anything.

Use a telescoping gauge and a vernier micrometer and check your journals and the inside diameter of your bearings installed. If you have standard bearings on a 10 under crank then yes you will have these issues once the engine heats up the clearances change. Right now what's saving your butt is oil flow. But this motor is short lived and will spin a bearing down the road in the future
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Re: No oil pressure, new build
 

I did not mention plastigauge anywhere. The crank, block, rods, etc all mic fine. Clearance is fine. Clearance on the rods/crank, bearings, etc is not the problem......FWIW, I did plastigauge the motor when I put it together and then it was mic'd. In this case, the plastigauge was actually very close to the actual measurements. To be blunt, this isnt my first, second, or third, etc rodeo. This issue has 3 machinists baffled. Just looking for something oddball.


In fact, I have used the crank (Scat 3.75 stroke) and rods in another block (it was a dirt motor) and it had fine pressure. I am using a different block and different oil pump...sure hope the problem is not the block.

Re: No oil pressure, new build
 

Just throwing some thoughts out...
Pickup to oil pump leaking and sucking air rather than oil?
Distributor lifter oil galley slot not seated allowing oil to bypass directly to the lifter valley?
Oil galley to sender hole blocked with crud?
Diagrams of Smallblock oil galley flow. http://www.chevytalk.org/fusionbb/sh...hp?tid/192284/

Re: No oil pressure, new build
 

none as in 0 or none as in 3-5? how much does the rpm drop when you put it in gear? did it happen once? how hot is it when it drops?


i dunno i can't see how an engine would lose that much pressure by dropping it into gear.. it's like the extra load from the trans is causing the driveshaft to stop spinning? which is like impossible in its own right lol

Re: No oil pressure, new build
 

Pickup shouldnt be an issue. I forced the pickup in as far as I could get it. Buddy tacked it at the correct angle.

Distributor lifter oil galley slot?? Not familiar with that. I do have a vertical slot in the dist housing. I cant remember why I did it, but I used this dist in 4 dirt motors and never an oil pressure issue.

Sender hole is clear.

Re: No oil pressure, new build
 

None, zero, zilch. Out of gear idle speed was high, id say 1200 or so, had 20ish at that speed. In gear idle speed 600-700. Carb/timing not even close. Reason (maybe even a vauum leak) for such a huge drop in RPM from in gear to out of gear. BTW, first BB Mopar I had suffered from the same issue. 20 lbs at idle in gear, 5 or less in gear. All of the bearings were shot, cam bearing was spun.

Driveshaft stop spinning?? What are you talking about?? You mean converter??

Re: No oil pressure, new build
 

Just making a generic statement to the prob. I actually meant to post that reply in another post about low oil psi
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Re: No oil pressure, new build
 

oil pump d/s... like i said also impossible lol

Re: No oil pressure, new build
 

Distributor lifter oil galley slot?? Not familiar with that. I do have a vertical slot in the dist housing
is this refering to the bottom of the dizzy shaft that drives the pump,how much play/wear was in the shaft

Re: No oil pressure, new build
 

At the base of the distributor body casting is a groove or slot between two machined surfaces just above the drive gear. This slot directs oil to the lifter galleys.

Did you prime the engine when it was still on the stand before you bolted the tin on?

Re: No oil pressure, new build
 

Years back I filed a small vertical slot in the bottom most machine surface. The slot runs from just above the dist gear to the groove between the two machines surfaces. Cant remember why I did it??? I never had any pressure problems in the dirt motors, but being stick cars they never idle below 1k anyway. The slot I filed is very small and not very deep. I wouldnt think it would be the cause of this issue?

Motor was primed on the stand, but tin was already on.

Re: No oil pressure, new build
 

The "groove between the two machined surfaces" is the slot I was referring to. If it's not seated all the way down you can starve the lifters. Doubt that's the problem though.

I try to prime newly rebuilt engines with all the tin off using a clear plastic tub instead of an oil pan. It shows any flow problems and leaks so I can 86 them before I get it in the engine compartment with all the accessories installed.

It sounds like you have an internal leak. The only way to find one of those is to use a priming tool without the valve covers, intake, timing cover and pan so you can see what's going on.
Harbor Freight has a couple inspection cameras that could save you the headache of removing anything you don't have to.

Re: No oil pressure, new build
 

When you rebuilt the engine , what was the measurement for the crank shaft end play ?

Re: No oil pressure, new build
 

End play was very tight, on the bottom of the scale.

Re: No oil pressure, new build
 

What brand of filter where you running? I've seen fram filters cause this

Re: No oil pressure, new build
 

Napa gold. Did it with 2 different filters.

Re: No oil pressure, new build
 

Only suggestion I have is to pull off the oil gauge connection at the block and replace with appropriate plumbing to run a tube to a bottle to verify you get oil flow out of the sending hole.