oil pressure drops at 3500rpm only under load.

[470mercruiser]

got a rebuilt marine chevy 350 because i spun a rod bearing on the original motor. It is a long block so i reused most of the parts from my old motor. I used the original pan but i didnt use the original oil pump and pickup tube. I couldnt use the windage tray though because the new block didnt have the bolts for it so i figured it wasnt needed. Did the break in procedure after priming the oil pump. everything was ok. decided to take it out on the water and noticed the oil pressure drop to about 20 psi above 3500 rpm. didnt run it like that for long as soon as i drop below 3500 and rev it a little it would come back up to about 40-50psi. i changed the oil filter because of a post on the internet of someone having a bad one. it made sense because that was the same part number oil filter my other motor had and i figured thats what blew the other motor. I put a fram on it that i had and the pressure at idle was up to a little over 40 psi. Figured i was good. Ran it in the water again and the oil pressure still goes down. A little higher in the rpm range but its still there. I did the best i could on the pickup height. I put the pan on with no gasket and pushed the pickup down till the pan hit the block. I took off the pan and pushed the pickup another 1/4'' and i brazed the pickup on after pulling the spring and removing the cover from the pump. Im wondering if i screwed something up. Either by the pickup height or the windage tray not installed. It will rev up just fine out of gear and not lose pressure. Im worried about blowing this engine. Anyone have the same problem?

 

[Don S]

Re: oil pressure drops at 3500rpm only under load.

Hook up a mechanical oil pressure gauge and see if the problem is mechanical or electrical.

 

[470mercruiser]

Re: oil pressure drops at 3500rpm only under load.

I will try that just short on funds right now to get a guage. Ive talked to a few mechanics and they say its normal. I dont believe it. Does anyone agree its normal?

 

[Don S]

Re: oil pressure drops at 3500rpm only under load.

No it's not normal, oil pressure increases with rpm, typically 10 psi per 1000 rpm, it doesn't suddenly drop at 3500.

 

[470mercruiser]

Re: oil pressure drops at 3500rpm only under load.

Thats what i figured. Ill get a mechanical guage on it.

 

[470mercruiser]

Re: oil pressure drops at 3500rpm only under load.

So what gets me is why does it only do it in the water and moving. I can do it sitting still in the water and its fine. Im hoping for a guage problem and not a clearance problem. If it was clearance wouldnt it do it all the time? I did notice the main fuse holder under the dash gets real hot while the engine is on. Maybe a voltage loss to the guage making it unacurate? but then again why only under load? im gonna have to fix that too. The sender and guage is most likely from 83. Also could be something got into the sender when the other motor blew up? Just thinking out loud.

oh and someone recommended i use 20w50 oil instead of the sae 30 thats in it and thats recomended on the valve cover. would that help?

 

[Speakrdude]

Re: oil pressure drops at 3500rpm only under load.

If you have too much oil in the engine, it will cause low to no oil psi when under load and stern low, (like when trrying to come out of the hole) The high oil level hits the crank shaft and causes foam. The oil pump cannot build pressure with foam. Had this happen to me. I disconnected the oil hose from my mechanical gauge and filled up a quart bottle (let the engine pump it out instead of trying to drain it)
Problem solved!
Good Luck

 

[470mercruiser]

Re: oil pressure drops at 3500rpm only under load.

I have perfect amount of oil in it. Right on the full line cold.

 

[fishrdan]

Re: oil pressure drops at 3500rpm only under load.

I have perfect amount of oil in it. Right on the full line cold.

Is that with the boat sitting in the water? How many quarts of oil are in the engine now?

 

[470mercruiser]

Re: oil pressure drops at 3500rpm only under load.

Im pretty sure ive checked it in the water. It has about 5 quarts. But i think i put 6 because it wasnt full after i put the 5 in. Cant be sure though im gonna change it for the 20-50 that was recommended and be sure of the amount.

 

[470mercruiser]

Re: oil pressure drops at 3500rpm only under load.

So im curious. I thought at first i blew my motor from lack of oil but now im wondering. I just got this boat the tach didnt work. He had a 17 pitch prop on it. Sounded like it was running high in the rpm but i figured he said he had it out a couple weeks before i got it. So i figured the prop was right. the engine seized after running full trottle for a little while. Tore motor apart and the rod cap broke off one rod and other rod is all twisted up. anyway heard no warning of the motor just stalled one time wouldnt turn over till i let it cool for a while. i got is started again and wanted to go to the dock got halfway at full throttle and stalled again would not turn over again. Also, notice oil pressure loss at one point but thought guage or sender because it came back. still no noise maybe a little rattle at the last engine run before it seized. ok anyway im thinking i was running at about 5800-6000 rpm with that 17 pitch prop. Im runing a 23 pitch now and im still at 4800 rpm. Had a 21 pitch and i was running 5000 rpm. wouldnt running these marine engines at 5800 rpm be like running them at redline? Kep in mind also its a 2 bolt main block with about 1200 hours from 1983.

 

[fishrdan]

Re: oil pressure drops at 3500rpm only under load.

wouldnt running these marine engines at 5800 rpm be like running them at redline?

Ummm, no...

It would be like running it 1000RPM over redline, maybe 1200RPM over redline

Actually, it's not reline in a boat, it's wide open throttle RPM

WOT RPM is something like 4200-4600 or 4400-4800 RPM

 

[thumpar]

Re: oil pressure drops at 3500rpm only under load.

If you are turning a prop that big at those RPMs I wonder if you have the correct drive ratio.

 

[470mercruiser]

Re: oil pressure drops at 3500rpm only under load.

If you are turning a prop that big at those RPMs I wonder if you have the correct drive ratio.

I was wondering the same thing. I dont know if the previous owner changed the outdrive or the gears. There is no ratio number stamped to the outdrive. Is there a way to tell from the serial number? I had the upper apart to change the yoke seal i should have checked. I counted the teeth on the one gear but i couldnt get to the one in the drive itself the one on the yoke was 20 teeth.

Ok scratch that i think the outdrive was changed. The stickers on the outdrive and bellhousing are differant and there is no serial number on the drive. What happens if im running the wrong ratio? and what ratio should i have?

So running it at that rpm is probably what blew my motor?

 

[series60]

Re: oil pressure drops at 3500rpm only under load.

Generally you can find the gear ratio on the port side of the upper half of the outdrive. Check the picture.

​

 

[470mercruiser]

Re: oil pressure drops at 3500rpm only under load.

Yep generally. But there is nothing on this one.

 

[Don S]

Re: oil pressure drops at 3500rpm only under load.

What year? What drive?

 

[470mercruiser]

Re: oil pressure drops at 3500rpm only under load.

Its a 83 formula 20 foot open bow deep vee with a 350 mercruiser alpha one outdrive.

 

[Don S]

Re: oil pressure drops at 3500rpm only under load.

Is it the original engine and drive?

 

[Don S]

Re: oil pressure drops at 3500rpm only under load.

What you can do to find out what the ratio is, is to remove the spark plugs so you can easily turn the engine. Put the drive in gear, then turn the engine in the normal direction of rotation until the timing mark comes up. This will remove the free play in the shifting and start moving the prop.
Mark the prop so you know when the prop makes one complete revolution. Then turn the engine by hand and count how many turns it takes to turn the prop one revolution. It should take 1 and one half turns of the crankshaft to turn the prop one full revolution.
1.50 gear ratio.

It might even be stamped into the casting, under the decal shown above.