Strange oil issue...
I have a recently re-rebuilt Ford 460. It's not stock and it has been rebuilt by 2 different party's. I bought a '71 Century boat that had been ground up restored. The engine, I am told, was original but was modified and bored out during the rebuild. Here are some of the highlights:
roller cam and rockers
Melling hv oil pump
Milodon oil pan
holley 650 dp
You get the jist of it...
Anyway, I bought this boat, which is by the way, a Berkley jet. Everything is right. New or restored as new. Engine starts right up, runs fine, sounds phenomenal. This owner says to me that, as a winter project he was going to put in Smith Bros. .020 oil restricting push rods because he didn't like the oil pressure at certain engine loads, but he was not too worried about it since he and I would not be using the boat for racing or anything. Just a family fun boat.
I took the boat home and then out a week later. Put it in the water, started it right up and idled out to the lake via a long channel. Took about 15 min to get out. I brought the boat up to plane, not hard mind you just easy until I got the feel for it. I gave it the beans for a few seconds at a time, everything sounded great, nothing unusual. I was going about 30 mph (out of 60) Then was suddenly I heard the engine banging and I brought it down and shut it off. I waited then restarted and there it was, dreaded piston rod clunking. Got towed in.
Yanked the engine tore it down and the machinist told me that it was probably due to crap in the passageways that caused an oiling failure to the crank in certain areas. The engine had maybe 15 hours on it.
He rebuilt the motor replacing the oil pump, bearings, magnafluxed it, the whole nine yards. Anything he didn't like he redid.
Put the engine back in. Everything is as it was, starts right up, sounds great, BUT this is where the curve comes in...
The engine sounds and responds as if there is nothing wrong. No hesitations or anything. At exactly 3100 rpm the oil pressure drops like a rock to 20 psi or sometimes a little lower (15). The normal idle pressure is 80 cold, 70 hot. We are monitoring via electric pressure gauge in the dash AND have switched to a screw in mechanical to confirm the reading, same. The sender is at the rear of the engine. As soon as you bring the throttle to 3000 or below, the oil pressure returns to 70-80 psi instantly. We have tried overfilling the oil to see if somehow we are sucking the pan dry or sucking air, nothing changes.
Now to see if it may be angle of the boat causing dry pan we have ramped up the throttle fast, then very slow, with the bow up (coming up to plane). With the boat level and on plane. Slow or fast throttle ramp up, the result is the same every time.
Now, since the original owner had suggested the oil restricting rods may help the situation I got a set and installed them. Took it out and got some interesting results. When the engine is cold it was holding, that is, until the engine warmed up a little, then down it went! But this time it doesn't recover quickly as it did before. It takes a few seconds (3-5) and sometimes you have to bring it all the way down to idle for it to recover. Also, the replacement rods made a very predictable problem, unpredictable! Once it went up to almost 4000 rpm before it dropped. Other times it was dropping at 2800 rpm. Also the pressure recovery was now flaky as well.
I, and my mechanic, have been scouring the web trying to find a solution to this issue. While a stock 460, when having oiling issues always points to oil pump or tube, a modified engine can have all sorts of issues. Most say when you change from flat solid cam, lifters, to roller, you have too much lift and it lets oil run unrestricted briefly in some positions. Almost everyone agrees that you need some type of restriction up top to solve low oil pressure problems. At this point I am out of ideas. If we have to pull the engine again to recheck the pump/pickup situation some have suggested to dump the Milodan pan and use an Armando's pan. I have tried many things chasing this problem and I truly think what ever is the problem now was the ORIGINAL reason for the engine puke the first time!
I'll be happy to purchase one of these pans if I know it will solve the problem, but I don't wanna throw away 500 bucks to find out it didn't.
And yes the oil pump was changed from the first rebuild.
Your opinions would be appreciated.