Projects HELP! SBC Oil Pressure is going HAYWIRE!

My oil pressure in my 54 has always held steady somewhere in the 40 psi range while cruising, and jumps between 60 - 70 psi when you tromp it. After warming up, it idles somewhere in the 15 - 20 psi range.

Just the other day while on my way to work, the pressure began to bob up and down slowly, and drop to less than 5 psi when I stomped it. Let out of it and go back into a 2500 rpm cruise, and the pressure goes back to 40. Stand on it, and it'll drop back to nearly nothing.

-When the pressure drops, I have not heard the lifters. This would be at about 3500 rpm, and about 3-5 psi per my gauge.

-I replaced the gauge with a new one as a test, and the results were the same.

-The symptoms only occur after teh engine has warmed up to about 160 degrees.

The engine is a 1990 GM 350 truck engine. RV cam, .030 over. Mid 60's cast iron GM intake with oil filler tube and breather cap, early aluminum Corvette valvecovers with PCV fitted into the RH one. GM points dizzy, not that that really matters.

The engine has around 40,000 miles, and was bought from Jasper in 2004. I have never had any trouble whatsoever up until now, and this is the second vehicle I've had it in.

What could cause the pressure to drop when I goose it? It almost acts as if there is a relation between the vaccuum and oil pressure, which doesn't make sense to me.

Thanks for any advice or direction you can send my way...


Evan

 

Low oil level....? Sucking pan dry...? Pickup fell off pump?

 

Id sure have a look inside the pan and at the pump and the pump drive components.
Not tied to vacuum, but to Rpms going up.
Just to confirm things, verify with a mechanical gauge. Things like wires, grounds, senders before going nuts.

 

oil pickup problem
over fill with oil reck.....................

 

If your using an electric oil pressure gauge, It might be the sending unit going bad that screws into the block... I always use a mechanical oil pressure gauge in my car...

 

I had a 94 350 that did the same thing, it had the factory electrical sending unit and swapping gauges repeated the results. I put a mechanical gauge on it and viola, problem fixed. I could have replaced the sending unit but I prefer mechanical. All the bases were covered above, I would pull the pan and check the pickup for sure.

 

I once drilled a hole in the oil pump cover and the pickup tube for a .125" dia. dowel pin... That idea works also.. Sorry! I don't know how to weld...

 

Cam bearings or electric gauge.

 

There is only 2 reasons why it would do that:
1st one is the oil level is low resulting in the oil running away towards the back of the pan when you step on the gas resulting in sucking air.

2nd the oil pump pick up has changed positions or possibly fallen out of the oil pump.

If you check the oil and its low, youre good. Add oil.
If the oil level is OK, you better pull the pan and check out the scene just to be safe.

Good luck. PM me if you have any questions

 

Thanks everybody!

Oil is not low, and it's a mechanical gauge Looks like time to drop the pan!

 

pickup loose or fell off

 

You could try over filling the pan to get the oil level up to the pump to see if the problem is in your pick-up.

 

Had this happen with a 1987 model engine. My problem was the pressure relief valve spring in the oil pump. The damned thing broke up, one long coil became 50 wire circles. I replaced the spring and cup, reinstalled the same oil pump and never had another problem with it.
Please let us know what you find.

 

Tried the overfilling thing, 2 qts. didn't help. Pan droppin time.

 

Hate to be the bearer of possible bad news, but oil pr. going down in tune with engine vacuum (load going up) can also mean worn main bearings. A sensitive mech gauge with a 1/4" line will pick up the loss of pr. when the pistons are getting a heavier load of fuel/air mix and actually pushing the crank down and away from their oil feeds holes, allowing more oil to flow out and pr. to drop. In messing with worn out old cars for 60 years I've seen it more than once.
Dave

 

Well would this apply to a situation like his as in, "fine yesterday but not today".
The worn bearing theory would be gradual & over time no? A catastrophic failure in bearing would be sudden but have many other symptoms.

 

We're not talking "catastrophic failure" just yet, just an early warning to check the main bearings while the pan is off. Apparently not knocking (yet), not on the verge of a blowup if driven carefully.
Dave

 

At first I was leaning toward a bad sender or wire connection but you ruled that out with having a mechanical gauge and then trying a second gauge.

I'm betting oil pump or pickup. I had a GTO that the oil pressure light would flicker on as I came to a stop. Turned out the pickup tube had fallen off the pump and the oil was washing away from the hole in the pump when braking.

Put a preluber on a drill and spin it to see what happens. also pull a valve cover and see if oil is coming out of the pushrods. Then pull the pan as a last resort.

Oh, and look at the oil pump driveshaft and the hex hole in the bottom of the distributor, as well as the gear on the distributor. I rounded off a driveshaft one time and it would catch at low rpms and skip at higher rpms.

Don

 

Missed my point entirety Dirty.

 

Maybe a kink in the line???.. Those cheap nylon oil pressure lines sometimes get bent out of shape..

 

So far, looks like the motor is fixin to have to come out. The aftermarket crossmember I'm using won't allow the pan to drop far enough to clear the crank. Hoping I can just support the engine with a hoist and pull the crossmember out from under it.

Pulled the dizzy, everything looked fine. I did have more crud than normal on my drain plug's magnet. Something has come apart...

 

got a 200 stat, problems start at 160.

 

Clogged or imploded oil filter ?

 

Hope it isn't too serious BSC....Probably oil pump parts on your oil plug...

 

Fram filter ?

 

Change out the gage and see what it reads. JC

 

Nope :

 

I agree w/Unkl Ian-fram filter?

 

Check behind timing gear and there are three gally plugs,on rebuilt engines they can come out causing low oil pressure,they need to be staked in place,not a guarantee just had it happen before.

 

Ohhh them nasty fram filters. I had forgotten people might still be using those.