oil pressure

Matthew Edler · Newbie Joined: 09 Sep 2005 Posts: 39 City: St. Peters

On initial acceleration my oil pressure is droping to about 15 psi giving me a warning, then once on plane and cruising oil pressure fluxuates between 40-70 psi. This is on a malibu 5.7 ltr Hammerhead. My oil level is checking full. Should I be runnig a heavyer wieght oil or try running a extra quart? Any other Ideas?

MrBlean · Posted: Sep 09, 2005 11:01 pm Post subject:

Low oil pressure can only be one of four things; a worn oil pump, worn bearings, a faulty gauge/sender (or poor earth) or a low oil level. The first two would give a much lower pressure at idle and low speeds than at cruising speed but in both cases the presure shouldn't really fluctuate at a constant speed.

It sounds like your oil pump is being starved when the bow rises and engine angle changes under acceleration. Are you sure your oil level is correct? Also, when you say the oil pressure varies between 40-70 psi, is this when at constant rpm or it varies across that range according to revs?

Putting in a heavier oil will not address any of the possible causes above, only mask the symptoms and potentially lead to a big repair bill if there is genuinely something wrong.

Of course, it could be a faulty guage or poor earth. How quickly does the pressure fluctuate? If it's very fast (almost instantaeous - i.e. the gauge "flicks") it could be the gauge/earth.

A more precise description of the symptoms would be useful.

P.S. a friend of mine recently had an intermittent warning from a faulty oil pressure sensor/switch on his Nautique. The gauge read OK but the sensor for the warning system was faulty. It was dirt cheap to replace. The gauge subsequently failed (coincidence I think) and that was $150 or so in the UK.

Matthew Edler · Newbie Joined: 09 Sep 2005 Posts: 39 City: St. Peters

When I go to pull up a boarder the oil pressure drops to about 15 psi right as I'm backing off of the throtle to steady the speed, this is from a dead stop to a hole shot more or less .I talked to some guys that are big on inboards and they recomended running a extra quart of oil in the engine. What do you think of thier recomendation? Not only that but once in a while I'll get a single back fire when I punch the throtle on take off, I believe this is octane related though due to the fact that I've been running 87 in it and should be running 91 due to the cost. The engine only has 270 hrs on it, and has been well maintained. I appreciate your input.

MrBlean · Posted: Sep 10, 2005 10:44 am Post subject:

I don't think the two issues are related. Backfiring (assuming you mean combustion of fuel in the exhaust) is usually caused by a timing fault or misfire that allows unburnt fuel to pass thro' to the exhaust where it ignites.

Just to make sure I understand the oil pressure phenomenon exactly, are you saying the pressure rises with revs as you pull the boarder up but when you back off on the throttle it drops? If so, what does it drop to and does it then spring back to some intermediate pressure?

In principle, oil pressure is rev related but there's a bypass valve that allows it to bleed off above a certain level. So you should expect oil pressure to be dependent on engine speed. If it's not, there is something amiss.

If the pump is being starved of oil on take-off because the bow lifts and all the oil drains to the back of the sump (assuming the pump is in the front of the engine) this could be the problem and additional oil may solve it. However, I would have expected the pressure to drop on take off when the bow was at it's highest not when you close the throttle and the bow drops again.

Sure, there may be a slight delay between hitting the throttle, the bow rising and the oil pressure falling but I would have thought this would be a second or two at most.

Provided the engine isn't already overfull with oil, try the extra quart and see if that solves the problem. However, don't overfill as that can cause problems too.

BTW, what boat/engine combo do you have?