Thread: Intermittent oil pressure problems with my SBC

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I am having what an odd oil pressure problem with my 350. It is a marine application, with vortech heads and a hydraulic cam. It’s the standard marine set up rated at 300 hp. I am not sure of the cam specs but do know the engine was dynoed with the cam and made 300 hp with no symptoms. I had the engine rebuilt two summers ago(rings, bearings, pistons, gaskets) and have about 80 hours on it since rebuild. At idle I have 40 lbs of pressure, under power at 4,000 rpm and at operating temperature I usually have 50 lbs – 60 lbs. At some point during operation I lose nearly all oil pressure, drops down to 2 -3 pounds. I have an electric oil pressure sender and have removed my electrical sender and used a mechanical gauge to confirm oil pressure. The mechanical and electric readings were within a couple of pounds of pressure. I initially thought I had a bad oil filter and changed the filter. After changing the filter the oil pressure returned. I again used the boat and had the same symptom reappear after only an hour or less of use. I limped back to the dock showing 2-3 lbs of oil pressure. The engine was running fine otherwise, no misses or backfiring, so I don’t think the problem was or is in the distributor gear. As soon as I tied up at the dock I changed the filter again and started the engine. With the new filter oil pressure returned. I used the boat again this weekend for about an hour and was under full power for quite a while and kept a close eye on the oil pressure. I was turning about 4,000 rpm and had about 50 lbs at operating temperature, 160 degrees. As soon as I powered down to cruise in the oil pressure dropped to nearly zero. I turned the boat off and let it sit for 10 minutes. On restart I had 25 -30 lbs. I don’t understand how I can have an intermittent oil pressure problem which seems to be centered around the oil filter. I have tried several different brands and each, Wix, AutoZone, NAPA have produced the same results. We cut open a couple of the old filters and saw some of the pleats leaning on one another but generally the filter media looked ok and not visually collapsed. I changed the oil and filter right before the most recent problem. The oil is still clear and looks visual fine.

Is it possible I have a problem with my oil pump or should I be looking elsewhere? If elsewhere, where? Is it possible I have a bearing problem? If a bearing problem how can that be exhibiting the symptoms I describe?

You may be having a problem with the filter bypass in the filter mount. Hot oil may be aggravating a weak spring.

Thanks. Lots of Questions: Could that bypass cause the almost total drop in pressure? Would I just replace the unit or replace the spring? Is there some way to check the spring?

Why would the filter change bring back the pressure?

Could it be the bypass in the pump?

Just winging it here, but as changing the filter makes a difference the problem is very close to it. Take the filter adapter off the block . Check the gasket for tears. Check both the block and adapter for cracks scores and flatness. Take a mirror and look in the passages to see if there is an obstuction in there. The adapter is not an expensive item, just replace it. Usually comes with a gasket. Myself, before I went to all that trouble, I would take the sender out, screw a pipe nipple in there witha piece of hose and a can to catch the oil. Start the motor , let it run till you have pumped a quart of oil out shut it off and put the sender back in. The purpose of this exercise is to flush out that piece of crap that is plugging the sender. Changing the filter or shutting the motor off would let the weight of the oil pull it back away from the sender. As there is practically no oil flow there, it would take a while for it to work its way back up to the sender.
Thats all I got.

Thanks. I think that those are things I will definately try.

Can anyone point me to a good discussion of the 350 oiling system?

The part I am having a hard time with is why the oiling system has a bypass and even if the filter was being bypassed why would that cause me to lose pressure?

The filter bypass is just to let the oil flow past a choked filter, it won't cost you all your pressure. I'd say your problem is a sticking relief valve in the pump. I had a 345 IHC that did much the same thing, a change to 0-40 synthetic oil fixed it. Might be worth a try before you pull the thing down.

Thanks. I am currently running 10w 30. Why would a change to 0-40 do the trick? Is there anyway short of a teardown to chekc hte relief valve?

The viscosity isn't what did it, it's the synthetic oil. My oil supplier told me he'd give me my money back if it didn't work, so I figured I'd show him! I put the 0-40 in and drove it 2 miles and the pressure went to normal, 25# idling, 40 revved up. Before, the pressure wouldn't even wiggle the gauge at idle, and there was only 25 or so at speed. Go figure!

Here is a simple question...you have hydraulic lifters in the motor. When you had to "limp" back to the dock, didnt the lifters rattle and clack quite a bit during that trip? If the oil pressure went to zip, the lifters bleed down fairly quickly and it starts to make noise....did it?

Out of the several times that the oil pressure has bottomed out only one of those times as the engine got noisy, lifter noise and valve tapping. I think that when I lose pressure I have about 2-4 pounds of pressure.

So the clacking is a good indication that it is really loosing pressure...not just a gauge or sender fitting stopped up temporarily.

In the SBC, there is the pickup tube with a screen...have seen shop towels in the oil pan that randomly covered the pickup. The pickup tube connects to the oil pump, if the tube to pump fit is loose, the pump can suck air and lose pressure. Inside the pump are the two gears and the relief valve. The two gears get worn but the symptom of grossly worn gears is not intermittent pressure loss. The relief valve has a plunger and the spring. Sometimes the plunger can stick open if debris gets in the system...it doesnt take much of a spec of debris to make the plunger hang open (or closed). The spring sets the force (oil pressure) that makes the plunger open and bleed off the pressure...it is kind of like a regulator. If the bypass is the problem, the solution is a new oil pump....dropping the pan in a boat is a chore unless you sazall out the bottom of the boat first (after lifting it out of the water, of course).

After the pump, the oil travels to the oil passage in the block which takes the oil to the filter. There is the interface between the pump and the block...if the pump bolts are loose, there will be a major oil leak with oil squirting back into the pan.

At the filter, there is the filter adapter which is a small aluminum die casting that has the close nipple to screw the filter onto. Not much there affects the oil pressure except IF the adapter is loose or the gasket missing...there is a bypass in the filter but that only allows the oil to bypass the filter if the filter is plugged.

Hope this helps.

mike in tucson

Thanks for the reply. I am leaning towards a sticky relief valve being the culprit. What I dont understand is how/why the valve is being forced open and how, if it is sticky/stuck, it apparently closes when I change the oil filter.

I am going to have a closer look at my bilge to see if an in boat, no sawzall, droping of the oil pan is possible. If not I may borrow your sawzall to do the job!

good question about how the valve gets stuck in full open

I reread your explanations...it seems as if "sitting" with the motor off heals the problem...not the oil filter. Can you confirm this? You said that you turned the engine off for 10 minutes and upon restarting, you had good pressure. Usually, time is heat on a motor.

Taking another direction on the problem, how long does it normally take (after starting) for the pressure to go to low? Does it always happen OR are there instances where the pressure stayed up for the entire duration of your trip?

You would think that, if the attitude of the boat made the presure drop (nose up, oil moves to back of pan) then the process of slowing down would change the situation. Also, if the oil was being pumped to the top end and some obstruction was keeping the oil from returning to the pan, it would happen consistently.

Sitting does not always cure the problem. During one episode I had 2-3 lbs of pressure after returing to the dock. I let it sit about 10 minutes and restarted and still had only 2-3 lbs. I immediately changed the filter and restarted and had approx. 50 lbs.

Over the weekend when the pressure bottomed out I was initally running 40-50 lbs at full throttle. When we slowed down I noticed, but did not see when, that the pressure had droped. We did sit for 10 minutes and on restart had about 20 lbs of pressure. There does not seem to be a consistent sequence which causes the pressure drop.

Is there any kind of flushing chemicals, like gunk, that I can run through the engine to try and resolve a potential blockage? I dont think I have a blockage becuase I can idle what seems like all day with no appreance of the pressure drop.

OK, the cheapest and easiest thing to try is what 61bone suggested....remove the oil filter and then remove the filter adapter. It is a small die casting with two bolts holding it onto the block. The filter adapter costs something like $8 at the Chev dealer .. part number 3952301 ...do not buy a crappy imitation from China at the auto supply! ALso note that the adapter does not have a gasket between it and the block...if it does, there might be your problem. The oil filter has its own gasket. Keep aware of the adapter bolt torque...if the bolts are loose, this might be your problem. Also, check to see if the surface on the adapter that touches the block is warped.

If you cant find the torque specs on the adapter bolts, someone on this forum should have them. Also, check the bolt length...ask the chevy parts guy to look up the bolt length when he looks up the part number to check the one I gave you.

On the adapter is a phenolic looking disc and a spring. This is the check valve that bypasses the filter if the filter plugs up.

This is a cheap shot to try. IF the engine rebuilder had left the bolts loose OR there is a faulty gasket where there is not supposed to be a gasket, removing and tightening the oil filter would affect such. As another thought, you could (after the oil pressure fails) loosen and retighten the same filter to see if that brings the pressure back.