|09-10-2012 07:47 PM|
One thing that hasnt been mentioned, especially for a marine motor, what are you running for an oil pickup? Is it a flex hose? I have seen even braided ones collapse under load. The early Chris Crafts were notorious for that.
Ditch the high volume pump, get a Melling Select Series, with the larger base, made specifically for racing and off road use. I think it comes with an intermediate shaft. Use a high pressure spring for 70 psi or so. If your bearing clearnces are right you ll have 20 psi when hot and 10 lbs per 1000 rpm.
Check your pickup and oil level is critical. If motor is at an angle, too much oil could whip that oil to a froth.
|09-10-2012 07:31 PM|
|gongchuan||Thanks a bunch everyone! Lots of good things to look at. Ill let you know what i find in coming days/weeks. Thanks alot again!|
|09-10-2012 03:33 PM|
As I was saying, the oil might be climbing up the rear of the pan and getting whipped-up by the crank assy., especially if the boat is tilting back. The air in the oil will lower the pressure as it will displace some of the oil that should be entering the pump and compress in the output side, where oil won't compress. They make baffles just for this problem. Not saying thats your problem, just a possibility, that you can test by draining some oil.
There are two bypasses that I know of on a sbc. One is in the oil pump cover itself that can sometimes stick if the hole its in has a machining burr, or other irregularity. Its a valve that dictates the max oil pressure by swapping springs. Sometimes they can get weak if the spring wasn't removed before the pick-up is welded in, but that doesn't sound like whats happening.
If it sticks open it will lower your pressure.
The other bypass is the one on the fitting adapter that your oil filter screws onto. Its there to bypass the filter somewhat when the oil is too cold or thick, or the filter is clogged. If it opens you should still have close to if not better than full pressure from the pump to your gauge, only the oil won't be filtered.
|09-10-2012 03:27 PM|
One more thing, You didn't say where the oil pressure was at cruise or speed after it had dropped to 5 PSI at idle. If it's 20 to 30 pounds or so at 3000 RPM that should be enough.
In 1967 I worked in the Roadway Express shop and it was common and acceptable to live with almost any oil pressure on the gage at idle as long as it would pick up with RPM.
What I'm saying is that if it were my truck engine, I have lots of time, money and skills so I can afford to be a perfectionist. My old boss would have accepted the risk and run it since repairing a major failure is nearly the same cost and effort as making it perfect as a preventative measure.
|09-10-2012 03:15 PM|
Lotsa good ideas. Check the easiest first.
1. Foamy, air entrained oil.
2. Check the gage and the location of the connection.
3. Replace the oil filter.
3. Check the oil bypass valve.
If this doesn't work (and it likely won't), Pull the engine, remove and replace the main bearing caps. Plastigage the mains and then the rods, Clearances should be per the range in your engine manual.
Check the installation of the bearings and location of the holes. If this is the problem, have the crank turned and get some correct bearings.
A new standard oil pump should be fine. A high volume and high pressure pump can empty the pan during extended high rpm running. This is why NASCAR and other endurance race cars run enlarged pans with kickouts. A pan like this might be something for you to consider.
Let us know what you find.
|09-10-2012 02:15 PM|
#1 I was running 10w30 at original startup and break in. Had the same problem as with the 20w50. I went with the valvoline 20w50 for the zddp for the flat tappets.
#2 even if we stop and swim for 15 min the psi still acts funny (which would make me think its not the oil not returning to the pan). Only after sitting overnight does it run in normal range again. Also, its a ski boat and there is a pretty big angle at which the engine sits. Dont think oil can sit up in the heads so much, it all runs to the back of the head pretty quickly (found this out while adjusting the valves with cut off V.C.'s). Also after returning to over 2000 rpm's the pressure is right back up to 40 psi which would indicate there is plenty of oil in the pan, only after dropping to idle does it drop out to nearly nothing.
I am in no way "trying" to discredit what you are saying and I appreciate the feedback, but these are the reasons Im not certain if what you are thinking may be exactly right.
|09-10-2012 11:49 AM|
I can't believe nobody has mentioned the HV pump. You are pumping all that oil right up to the top of the engine and it probably has had no drainback work done in the heads or the lifter valley. The fact that it's 20w50 only makes it worse-just returns to the pan that much slower. Under 3500 rpms may be the sweet spot where it just returns fast enough to be alright. Over that, and it doesn't come back to the pan fast enough.
As far as any pressure issues, they're fine up to when the problems start. If your clearances were fine on a stock motor, I would run 5w30 or 10w30. Just a rering and good clearances wouldn't dictate thicker oil. But verify that you have a good gauge as mentioned and a good filter (like Wix)-not Fram. Not sure about the delco, but the smaller one is probably hurting you here.
Did you verify the pickup to the pan clearance? If it's too low or too tight to the sloped up part of the pan sump, the pump will cavitate and not be able to pump oil at a certain rpm. In that case, though, the pressure usually comes back as soon as you let off the throttle.
|09-10-2012 11:10 AM|
|09-10-2012 10:57 AM|
Id like to know a little more about this oil bypass block off. This is something that can be blocked from an external point or are we talking directly on the pump itself?
Im trying to avoid pulling the engine. As described this motor is just to get me bye until I have one that is on the stand finished.
|09-10-2012 10:32 AM|
Ill be honest, I was thinking oil pump release the entire time. There are a few other good points and I do appreciate the feedback. Not really looking forward to pulling the motor to change an oil pump thats one of the big reasons I threw this out there.
Thanks again all, and if anyone else has anything feel free to throw it out there.
|09-10-2012 10:18 AM|
You may have too much oil in the pan. It may be getting churned up by the crank at higher rpms greating a lot of air bubbles in the oil in the pan which will reduce the oil pressure.
You might want to look at the oil on the dipstick after running at a low rpm and check for bubbles. Then do it again after running at 3500rpm and see if there is a difference? Might try dropping the oil level by a quart temporarily and see if it makes a difference.
Just a thought FWIW
|09-10-2012 10:09 AM|
|09-10-2012 10:00 AM|
an oil temp gauge would be a good idea too.
|09-10-2012 09:45 AM|
Posible causes of varying oil pressure
1. Loose clearance on bearings. This description of good oil pressure cold low pressure hot is a common problem in high mileage/ worn engines. Try 20W50 oil if it comes up you can go to heavier straight oil.
2. Pressure spring in oil pump poorly heat treated. looses tension as it heats up.
3. Where is your oil pressure gage attached? Try the attachment hole beside the distributor in the rear of the block. Your bypass spring in the oil filter mount may be partially bypassing the oil to the filter and thus to the lower oil port on the block. try high performance filter like a Wix or K&N and block off oil bypass for full oil flow through the filter.
|09-10-2012 09:27 AM|
|gongchuan||Oil temp no, but the pump is what im thinking. Head temp is 140 (regular marine temp)|
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