Was asked to look at a mild 396 powered car yesterday. Supposedly rebuilt several years ago, according to owner. Said the car sits a lot, sometimes for months without being started. Said he thought the oil pump was bad, just changed the oil, and now the pressure seems normal up to around 4,000 RPM, then it drops. I checked the dipstick, level was full, oil clean. Rode along and had him drive it around easy while I monitored the pressure gauge. It did seem fairly normal, assuming standard volume oil pump - he said he used 10W30 oil, and the pressure at hot idle was around 30-35 PSI, and was about 50 at 2,000 RPM. I didn't note any smoke from exhaust, no unusual noises, bearing knocks, no lifter tapping, etc. He turned onto an open road and stepped into it maybe 1/2 throttle in 2nd gear, and as the RPM climbed up close to 4K, the pressure did drop off slowly, to about 40 pounds, but as soon as he lifted, it rose back to 50. My initial thought was the pressure relief valve was possibly opening up, OR that it may be sucking the pan dry, although that seemed unlikely with a standard volume pump and thin oil. I asked him to repeat the test, but with a bit more throttle, slightly harder acceleration. The pressure dropped off more quickly, down to about 30, and again went back up fairly quickly as he let off the gas.
I had him do one more test, using light throttle, high vacuum, and ease the RPM up slowly. He took it to 45 or 4600, and the pressure stayed high, about 55 PSI. So logic tells me it is not strictly RPM related, and not likely to be the pump relief valve.
I told him it might be sucking the oil out of the pan, but he says it never did that before. I told him one or more of the oil drain-back holes in the heads could be blocked or restricted, keeping a lot of oil in the top end at RPM. He said it has always been well maintained, and I saw no indication of any sludge looking in through the oil fill cap with a flashlight, or on the dipstick. I said if it was my car, I would either pull the valve covers off, or try adding another quart of oil (temporarily) and repeat the test, to see if the results changed any. He did not want to have me do either, said he would talk to some other "mechanics" first.

As I thought about it this morning, I wondered if severe blowby could possibly be keeping the oil from draining into the bottom end? (If the volume of gases traveling up through those oil drain ports was high enough to interfere with the oil return)? What do you fellas think?
It made me wish I had pulled his PCV valve or hose while it was running, which I did not.
Thinking maybe a broken ring, or just bad ring seal, (possibly even from sitting/varnish on the cylinder bores) could be the problem?
What else would make a difference in oil pressure at various throttle settings, at the same RPM? Certainly the ring loading would be higher, as would the bearings, but what (if anything else) might I want to consider, in addition to recommending that he allow me (or whomever) to perform a leakdown test?

Thanks guys. -Jim.

that is odd for sure.i wonder if the oil pan has had a jack placed on it and pushed towards the pickup..thinking about the other normal things just doesnt make sense to me.i think and this is only a guess,the clearance or tube crack

Brand of oil filter? Seen some bizarre issues in years past.

It sounds to me like the pick-up may have dropped off the oil pump.

I would drain the oil, and look inside the oil pan with a bore-scope camera. I would be looking at the pick up to oil pan clearance, loose pick up, cracked pick up, loose pump cover, loose pump to block, etc....if anything looks out of place, the oil pan comes off.

quote:

Was asked to look at a mild 396 powered car yesterday. Supposedly rebuilt several years ago, according to owner. Said the car sits a lot, sometimes for months without being started. Said he thought the oil pump was bad, just changed the oil, and now the pressure seems normal up to around 4,000 RPM, then it drops

Let's start here. The problem didn't start until he changed the oil?
I've seen cars that sit a lot with owners that change the oil cold build up incredible goo at the bottom of the pan. I'd also check what brand oil filter.

I'm betting oil filter.

quote:

I'm betting oil filter.

I agree, easy thing to check too, then I'm going with pickup issues.

I've seen the filter due that and when the pickup falls off also.
Drain the oil and smack the pan with a dead blow hammer.
Listen for the pickup bouncing around.

My BBC runs at 40psi for last 5 years no issues. Changed bearings as maintenance items stays the same. Verified on mechanical and Racepak.

Bearing look like new when replaced every two years. Standard pump. 150 runs this season according to Racepak logs.

5.7s eight Mile, 9.0s quarter.

The main question is what was it when the motor was fresh. You will likely never know. Therein lies the issue...

The guy finally called me back this morning, and said he would bring the car back over the weekend. I asked what oil filter he used, and he said when he bought it he asked for a Wix with no anti-drainback valve. (IIRC, thinking that would probably be a 51061)?

Lots of good ideas fellas, thank you all for the input.
My thought about blow-by really doesn't make sense, BBC's have plenty of passage volume for gasses to escape the bottom end and still allow the oil to drain back down.

TopFuel, the borescope is an excellent idea, I have one and rarely use it, but that sounds like a good way to check the pump pickup and fasteners.

Another thought I had - Could the guys dipstick be incorrectly marked, (or the tube be wrong length), so it indicates full sump, but maybe it is low enough to suck the pan dry? Although I assumed he put the standard 5 quarts in it. (But we really shouldn't assume anything, right).

Thanks again, will post up the findings...

My moroso pan takes 6qts

With accumulator it takes 9.

Adding a quart to see if it helps if all else checks out would be an easy thing and not hurt anything.

Excessive main bearing clearance will do what you are describing. At light or no load, pressure is good. When under load, the crank is pushed "down" and would cause the pressure drop. It a problem I had many years ago. Pressure was 35-40 thru the traps at 6500 and NEVER had a problem.

Well, I finally got back to this guy's car.
I attached his oil pressure line to an old known-good gauge I have, and drove it. Same issue. Drained the oil, (which was very clean, no signs of sludge). Filter was a Wix, nothing looked unusual inside. Had to wait a long while in order to get my borescope in the drain plug hole without the lens getting covered in oil. The oil pump pickup was in place, and I could not see any cracks, but couldn't see all the way around the tube. Could barely see the pump bolt, but it appeared to be tight, so I reached a piece of welding rod into the hole and tried to wiggle the pump/pickup, it seems very tight.
I am puzzled, don't think it is the pressure relief valve/spring. So I plan to go with a new filter and oil for him, and see if maybe that was the problem. If not, I am back to square one...
Then I got to thinking about what 62 Galaxie said:

quote:

Originally posted by 62Galaxie:
Excessive main bearing clearance will do what you are describing. At light or no load, pressure is good. When under load, the crank is pushed "down" and would cause the pressure drop. It a problem I had many years ago. Pressure was 35-40 thru the traps at 6500 and NEVER had a problem.

This kind of makes sense to me, one of my old big-block trucks had fairly loose main bearing clearances, and whenever you lifted off the gas at speed, the pressure would jump up a few pounds, I figured the high vacuum/low cylinder pressure when decelerating was "sucking" the crank up against the main saddles, reducing the clearance and boosting the oil pressure. That was just my theory anyway.
So this made me think I could put 20W50 oil in it, and see if that makes any difference? (Or should I first try the 10/30 he prefers, and hope the filter was the problem)?
62Galaxy, do you recall what weight oil you were you using?
Anybody got any more ideas? (If it was mine, and the new filter didn't fix it, motor would be coming out).
Thanks.