Oil Pressure Issue

[StevNimrod]

The engine is a carbed Merc/Chevy 383 with Vortec heads. Had an issue with water in the oil last season (cracked block) and had the offending cylinder sleeved. No water in the oil this year.

Just got done breaking the engine in. Did not notice any oil pressure issues during break-in. Went on vacation for a couple weeks, came back and decided to go fishing. The only thing that changed over that vacation time was the outside temp (from ~70 deg to ~90 deg).

(1) At idle at the dock (not in gear) oil pressure is about 40 psi.
(2) If I bring the RPMs up to 2000 briefly (still not in gear) OP goes up slightly.
(3) Idling out of the marina (in gear) OP is about 40 psi.
(4) Fast idling before planing OP goes up a little over 40 psi.

Didn't notice anything strange with OP on the way out to fishing grounds. After fishing all day on the way back in:

Oil pressure would drop with increased RPM. It would go from a little less than 30 at hot idle down to 20 at 2500 RPM. Not wanting to break 10 psi per 1000 RPM, I brought it back down to idle and the OP went up a little. Shut it down for a few minutes, turned it back on and got up on plane. So far so good. After a few minutes the oil pressure gauge was fluttering all over the place between 0 and 40 psi. It seemed at one point like I could tweak the throttle a little to get to pressure to come up a bit, then it started fluttering between just above 0 and below 0 and the alarm came on. Brought throttle down and pressure came back up.

Managed to make it 8 miles back in without further issue at about 2500 - 3000 RPM.

There were no engine noises at any point, even the brief period of less than 0 psi at 3000 RPM. Initially thought it was the sender even though it's newer (replaced last season). But the alarm switch wasn't fooled (it was replaced same time as sender since I had the engine on the stand). Pulled the oil (Merc 25w40, trying to not start another oil thread) and filter last night and filtered through a cheesecloth and no metal particles were captured. Put oil in as specified by machine shop and started it up at the dock. Reading slightly higher than 40 psi idle, and the OP bumps up with throttle in neutral. Which doesn't tell me much since my problem seems to be losing OP with increasing RPM under load.

Does this sound like anything anyone has experienced before? Places to look next?

 

[HT32BSX115]

Re: Oil Pressure Issue

Howdy,


If your gage is an electric one, try putting a "T" in the oil pressure fitting and screwing in a mechanical gage.

In any case, get another known gage and check it that way before you go looking for mechanical problems.......

 

[carcraze]

Re: Oil Pressure Issue

Well you might want to grab a mechanical oil PSI gaiuge and validate the reading your getting.

I was getting some wild fluctuations in my gauge this year and seemed linked to engine RPM but turned out to be a break in the wire somewhere from the engine harness to the sender. I replaced that wire which solved the fluctuation . The break only showed up when running from engine vibration, when I grounded the wire to the sender when not running the gauge would pin as expected.

You also said the alarm switch was not fooled does that imply the low PSI alarm went off?

Also was this a built 383 and if so was a standard oil pump used, main bearing clearances plasti-gauged and verified?

 

[StevNimrod]

Re: Oil Pressure Issue

Howdy,


If your gage is an electric one, try putting a "T" in the oil pressure fitting and screwing in a mechanical gage.

In any case, get another known gage and check it that way before you go looking for mechanical problems.......

Hi Rick,

Mechanical gauge is my next step so i can run it with the engine hatch up and verify readings. I have another electrical gauge too, should i even bother hooking that up first?

 

[StevNimrod]

Re: Oil Pressure Issue

I was getting some wild fluctuations in my gauge this year and seemed linked to engine RPM but turned out to be a break in the wire somewhere from the engine harness to the sender. I replaced that wire which solved the fluctuation . The break only showed up when running from engine vibration, when I grounded the wire to the sender when not running the gauge would pin as expected.

You also said the alarm switch was not fooled does that imply the low PSI alarm went off?

Also was this a built 383 and if so was a standard oil pump used, main bearing clearances plasti-gauged and verified?

Working on putting something together on a mechanical gauge i can zip tie to something and watch from the helm.

This seems more load, RPM and temperature dependent than just RPM dependent. But I do want to go through the easy stuff first.

Yes, the low PSI alarm went off. Which means the wiring would have to be bad on both the alarm switch and the sender, right? I'll find the wiring diagram in my manual after a few hours.

It was a built 383, standard pump, bearing clearances done by the machine shop (I think they actually measure them rather than plasti-gauge), I did not verify any clearances though.

It could jive with a lot of things - but the mechanical ones all involve pulling the engine (oil pump problem, clearances too loose) so I wanted to both see if anyone else has ran into this before I do my yearly engine pull.

 

[HT32BSX115]

Re: Oil Pressure Issue

Hi Rick,

Mechanical gauge is my next step so i can run it with the engine hatch up and verify readings. I have another electrical gauge too, should i even bother hooking that up first?

You could if you know it's good.

It's pretty unlikely that both gages would be the same amount "bad"

Don't use the same sender though!!


I'll also add that electric gages are fairly susceptable to ground problems too. If you run a separate sender wire and ground wire, you might eliminate that problem.


Check all your electrical grounds anyway....including block/batt grounds etc....

I would also run a separate wire for the "new gage to eliminate any wiring problems....


Mechanical gage is probably the easiest.....

 

[John_S]

Re: Oil Pressure Issue

It was a built 383, standard pump,

OK, I was going to ask if it was a high pressure or high volume aftermarket pump.

Does it have a windage tray? Did you notice any oil foaming on the bottom of the oil fill ports, or on dipstick?

A tilted or oil pick-up fell off.

As you said do the easy checks, but given the gauge the seperate alarm sensor, I'm not thinking that will be fruitful. The common ground is the engine block back to the battery. I think you would see allot of other strange things happening. Are they on a T? can't remember. No teflon tape on the screw in threads, correct?

 

[StevNimrod]

Re: Oil Pressure Issue

OK, I was going to ask if it was a high pressure or high volume aftermarket pump.

Does it have a windage tray? Did you notice any oil foaming on the bottom of the oil fill ports, or on dipstick?

A tilted or oil pick-up fell off.

As you said do the easy checks, but given the gauge the seperate alarm sensor, I'm not thinking that will be fruitful. The common ground is the engine block back to the battery. I think you would see allot of other strange things happening. Are they on a T? can't remember. No teflon tape on the screw in threads, correct?

It's a standard pump, Melling M55 if memory serves.

It does have a windage tray, no oil foaming on the bottom of oil fill ports or dipstick.

I was thinking along the lines of something with the oil pump/pickup even though it was tack welded when I got it.

They are on a T. No teflon tape, I use conservative amounts of the thread sealant paste.

 

[John_S]

Re: Oil Pressure Issue

It's a standard pump, Melling M55 if memory serves.

It does have a windage tray, no oil foaming on the bottom of oil fill ports or dipstick.

I was thinking along the lines of something with the oil pump/pickup even though it was tack welded when I got it.

They are on a T. No teflon tape, I use conservative amounts of the thread sealant paste.

Lets hope for a good mech gauge readings. I am not seeing much outside of just a bad pump. The only other "wild" ideas I have left: Something blocking some of the oil return passages, such that the pump can not get all the oil it needs, and the more "wilder" one of the distibuter drive being messed up. Can't see that one at all.

Anyway, I will watch the progress and hope the motor doesn't have to come out. Good Luck.

 

[StevNimrod]

Re: Oil Pressure Issue

Lets hope for a good mech gauge readings. I am not seeing much outside of just a bad pump. The only other "wild" ideas I have left: Something blocking some of the oil return passages, such that the pump can not get all the oil it needs, and the more "wilder" one of the distibuter drive being messed up. Can't see that one at all.

Anyway, I will watch the progress and hope the motor doesn't have to come out. Good Luck.

I'm with you on hoping for good mechanical gauge readings. And I'm hearing a pump going out does what I'm seeing happen.

My "wild" idea is that running the water/oil mixture last season somehow messed up the sender and switch. Maybe not as much wild as that's what I want it to be.

Got the fittings and hose for the mech gauge hookup. Now tracking down a mechanical gauge.

 

[carcraze]

Re: Oil Pressure Issue

Yep I was going down the high volume pump road too but that is not the case. With both units going off maybe the senders are both screwy, let's hope so.

If you do have to pull that motor to replace the pump, have the clearances checked, I have always plasti-gauged my mains but maybe I am really OLD school (god I hated saying that). Not too sure how mong she ran with water in the oil or how much wear it would induce.

Keep us updated.
Good luck

 

[StevNimrod]

Re: Oil Pressure Issue

Just wanted to follow up on this so if someone else has my problem they aren't left completely hanging.

I hooked up a mechanical gauge and it read the same as the dash gauge. Of course, this time the oil pressure readings were fine everywhere - at idle tied off, at idle in gear, part throttle and full throttle. The oil pressure never dropped below 40 psi at any time.

The only thing that changed between when the pressure dropped and this test - the oil. I kept the oil removed from the engine and it measured out at 4.5 quarts (fwiw, within the acceptable range according to the dipstick).

Do you guys think it would be worthwhile to have the oil analyzed? If so, any company you recommend?

 

[StevNimrod]

Re: Oil Pressure Issue

In case anyone is having this problem I have a final solution:

The oil pressure issue came back and I was able to get it resolved. The wire that goes on the back of the sending unit wasn't making good electrical contact. Cleaned everything up and now everything is good.

 

[HT32BSX115]

Re: Oil Pressure Issue

Great!

Glad you got it fixed!

Oil pressure, eng temp, problems in marine engines are frequently indication problems related to wiring/grounds, etc...