Mercruiser 7.4 MPI Oil Pressure Issue

[BigDog98]

Hi all. Seeking for some advice in today's discussion and hopefully help others out using this post with a similar issue. So here is the core of what I think caused this issue or maybe its a freakish coincident. Anyhow this is how it all start to my oil pressure situation. I was in the process of hunting down an alarm that I had by replacing the oil pressure switch which on my engine is behind and under the distributor. I replaced the switch in hopes of resolving my alarm but was not the issue, instead I believe that I created a new issue. I replaced my switch and accidentally over tightened the switch into the brass fitting and cracked the fitting without realizing it. Moments later I fire up my engine and leave the boat for about 5 minutes to untie my boat and have a chat with my neighbor. All of the sudden I hear some weird noise like air escaping in the engine compartment and immediately lift my hatch to reveal that 90% of my oil was in the bilge and my oil psi gauge was showing 5-10 psi and I shut my engine down right then and there. In attempt to replace the brass fitting the darn thing broke off in my engine and I had to drill out and tap new threads for the new fitting ( I was careful to make sure no metal shaving got into the engine) Replaced all the brass fittings and everything is good. Now my oil pressure is not like it use to. I remember my oil pressure was at 40 psi at all time prior to this incident. The weird thing is that the oil psi is always showing something different everyday. Sometimes it will stay at 20-30 psi and other days it will be 40-45 psi and some days it will be 25 at 3000 rpm or higher and 15psi at idle which scares me. So to me it seems like my oil pressure has a mind of its own and behaves differently every day. I will be installing a mechanical oil pressure gauge directly to the engine in hopes of the digital gauge or sender being bad. Can the mechanical gauge be hooked up to the oil pressure switch or does it have to be hooked up to the oil pressure sender? The oil I am using is a quicksilver 25w-40 with a quicksilver oil filter that is recommended for my engine. I had also read that some people had oil pressure issues due to a bad oil filter, can this be a cause? my oil filter is HOT to the touch when it warm up to operating temperature which is about 160-170 F. What are your thoughts? I am also thinking about adding some marvel mystery oil into the crank case as I am a firm believer in it and add it to my fuel 4 times a season. And input will be greatly appreciated. Thank you

 

[Bt Doctur]

Usually the oil pressure alarm switch is just above the oil filter and maybe T`d to a electric fuel pump switch. The one behind the dist is usually a oil pressure sender for the oil gauge

 

[BigDog98]

Usually the oil pressure alarm switch is just above the oil filter and maybe T`d to a electric fuel pump switch. The one behind the dist is usually a oil pressure sender for the oil gauge

I know for a fact that my oil pressure sender ( the one that sends the signal to the gauge is under my engine next to the knock sensor on the port side of the motor and it is a bell shape. I know this because I had pulled that wire off by accident and the gauge shut off. The oil pressure switch is behind the motor near the flywheel under the distributor but my engine starts right up no issues even with that pressure switch unhooked.

 

[Scott Danforth]

I know for a fact that my oil pressure sender ( the one that sends the signal to the gauge is under my engine next to the knock sensor on the port side of the motor and it is a bell shape. I know this because I had pulled that wire off by accident and the gauge shut off. The oil pressure switch is behind the motor near the flywheel under the distributor but my engine starts right up no issues even with that pressure switch unhooked.

then someone wired your motor different than standard

regarding 15psi at idle, most big blocks have only 10 psi at idle. however it would be good to compare to a mechanical gauge

 

[alldodge]

On the MPI motors the sender is on the port side next to the filter (or remote adapter). The switch is below the distributor.

Put the mechanical gauge and see what it tells you, right now you have a gauge, connection or sender issue

 

[Scott Danforth]

learn something new.

 

[Oldnbold]

I had a similar experience where I lost into the bilge all but 1.5 qts of oil. Since I was still showing oil pressure, I thought I'd dodged a bullet. 25 hrs later my 7.4 started knocking and on tear down, all the crank journals were scored but 1 started knocking sooner than the others.
You may get luckier than I, but ....... expect the worse and hope for the best.

 

[BigDog98]

Would just like to update everyone on the situation. Looks like the valve/spring in the old oil filter was stuck and caused low oil pressure. I replaced the filter with a AC Delco brand and oil pressure went back to 40 as normal. I added a little marvel to the oil just in case and seems like all is running well. Only issue I see is when the engine is at operating temp I guess the oil thins out and when decreasing rpm from cruising speed to about idle the oil pressure drops down to 20 psi which I read in the manual is within spec. Anyhow thank you all for the help and advice.