Mercruiser 4.3 oil gauge mystery

[stresspoint]

first off my 4.3 is a bitsa so model number is irrelevant , however the 2011 engine block and the wiring is OEM to a 1999 Maxum SR 1800 both engine and hull.

yesterday on an outing the motor was running fine with all gauges working for the most part of the day till i shut the motor off for around half an hour , on restart i noticed " no oil pressure " on the gauge , no alarm warning , fuel pump is switching on , oil on the dip stick is a little low as per usual as , that comes up once the motor has been stopped and the stick pulled to equalize sump to the tube.
so i figure WTF plenty of oil motor seems fine no knocks , rattles or alarms , must be a gauge issue so we continue to boat for the next hour or so and it is running fine hit the 5500 rpm limit a couple of times.

moving forward : today i checked the gauge , wiring and sender , all checks out fine.
now i am concerned as this is a reasonably new rebuild with a lot of performance mods.

so , i top up the oil to recommended level on the stick (added about 500ml) and fire up the motor , @ 2500 RPM on the muffs the oil gauge moves slightly and returns to 0 when the RPMs come down to idle , noting out of the ordinary as far as motor noises go , very quiet and smooth .

so with info provided what do you guys suggest to do next next ,
I'm still on the fence regarding the gauge , but I'm not sure now after it moved that little bit @ 2500 rpm. but i am not going to risk the motor just in case.

 

[flashback]

Get a mechanical pressure gauge and plumb it in at the engine would be an option...

 

[Scott06]

Get a mechanical pressure gauge and plumb it in at the engine would be an option...

Exactly T it in by the sender and you can run it and see . Hot if you have 10 psi for every 1000 rpm you will be fine.

to the OP did the oil get fuel contamination from running rich?

 

[stresspoint]

i was running in a 8 knot zone @ 2500 rpm doing around 15 knot for a while then it had a good run for a bit , oil smelled good so no fuel in it.

that is a good recommendation to get a capillary gauge and plum it in , i will chase one down tomorrow , funny thing , not so long back i threw out a box of crap from an old ski boat i parted and there was a VDO gauge in there complete with lines and fittings.

 

[Scott06]

i was running in a 8 knot zone @ 2500 rpm doing around 15 knot for a while then it had a good run for a bit , oil smelled good so no fuel in it.

that is a good recommendation to get a capillary gauge and plum it in , i will chase one down tomorrow , funny thing , not so long back i threw out a box of crap from an old ski boat i parted and there was a VDO gauge in there complete with lines and fittings.

I think last time i bought a bosch gauge at the autoparts store it was like $25

Advance Auto Parts - Down for Maintenance

fittings on the block are 1/8” NPT…

 

[nola mike]

Is your alarm sounding on initial key on?

 

[Scott06]

Is your alarm sounding on initial key on?

I think the alarm switch (which sounds on start) and switch for fuel pump are two different switches.

 

[stresspoint]

for some reason my alarm does not sound key on . it never has since i have owned the boat , i know it is supposed to , i have confirmed it does work over heat and low drive oil , i don't know about low oil,.
correct me if i am wrong here , i believe the fuel pump ill not allow the motor to run or start if the oil pressure is low enough to sound the alarm.

i did not get a chance to get working on the boat today , hoping to get out there end of the week.
i am going to change the oil filter as i have read that it is possible it may be clogged up , its only done maybe 15 hours since last oil change but i will change it anyway . ??? .

 

[nola mike]

I think the alarm switch (which sounds on start) and switch for fuel pump are two different switches.

They are. If the alarm sounds at start and then goes out, but gauge is reading zero, likely fault is with the gauge/sender rather than actual low pressure

Edit: that low pressure alarm doesn't sound until <6psi, which is about where the oil pressure switch closes as well. So really not very helpful in either case. Op, did you check the readings directly off the sender with an ohmmeter?

 

[Scott Danforth]

FWIW, every engine I build has a mechanical gauge mounted to the intake manifold and plumbed to the oil galley in the block. that way if I question an electric gauge, I can quickly look to see if there is actual oil pressure.

 

[nola mike]

It would be interesting to build an engine kill switch for op. As frequently as I check gauges, I might not notice op quickly enough (I got lucky that I noticed when sender pipe snapped twice, dumping all of my oil). Though I guess that's what the alarm is for.

 

[Scott Danforth]

It would be interesting to build an engine kill switch for op. As frequently as I check gauges, I might not notice op quickly enough (I got lucky that I noticed when sender pipe snapped twice, dumping all of my oil). Though I guess that's what the alarm is for.

on the Rogue, I am doing just that

I am running the ignition and fuel pump relays thru the oil pressure switch NO contact with a bypass switch that also silences the alarms. the relays come on after cranking a bit when the pressure goes over 7 psi (which is why I use the bypass switch). if the oil pressure drops to about 10 psi, the switch opens the NO contact killing ignition and fuel and closes the NC contact firing the dash buzzer.

I cover the NAPA switch in the build in my sig.

the relays and relay sockets are standard VF4 ice cube relays. the relay power comes from the key switch. the ground comes from the bypass switch or the oil pressure switch.

I also have a little jumper wire in the glove box should the oil pressure switch die, I wont need to keep my finger on the momentary switch.

I also am using neutral safety switch in the controls to trigger the starter slave relay vs running current thru that little microswitch

 

[nola mike]

on the Rogue, I am doing just that

I am running the ignition and fuel pump relays thru the oil pressure switch NO contact with a bypass switch that also silences the alarms. the relays come on after cranking a bit when the pressure goes over 7 psi (which is why I use the bypass switch). if the oil pressure drops to about 10 psi, the switch opens the NO contact killing ignition and fuel and closes the NC contact firing the dash buzzer.

Got the basics but I think I'd need to see a diagram of your setup. I have no alarm either, seems one or the other would be a good idea. I could just plug the ignition into the same relay pin that powers my fuel pump. The 30 seconds or whatever that the boat would run with residual fuel from the carb after the pump cuts out would be enough to wreck the engine. Ignition would still fire when cranking or when the switch closes.

I cover the NAPA switch in the build in my sig.

Didn't see it in there

I also have a little jumper wire in the glove box should the oil pressure switch die, I wont need to keep my finger on the momentary switch.

I have one now too after discovering yet another corroded wire while on the water. Luckily had wire terminals and wire on board.

 

[stresspoint]

i am curios to know how to check the sender with a OHM meter , i just did the air on off test , its hard to tell with that test when the gauge shows that reading @2500 RPM , the air test just says "yes the sensor works or no its dead " , no in between.

Scott your set up regarding the engine cut off sounds interesting , although i like to keep OEM wiring where possible on boats , that system could very well save a motor , can you please elaborate , or maybe start a thread with a how too for us.

 

[dubs283]

Your setup, if indeed oem wiring should have an oil pressure switch for the fuel pump circuit, a separate oil pressure switch for the audio warning system and an oil pressure sender for the gauge.

Based on provided info my guess is there is an issue with the wiring for the audio warning system at/near the switch and possibly some foreign material blocking the pressure reading hole on the sender

Oem setup would have the oil pressure switch for the fuel pump and sender plumbed near the oil filter port side of block aft down low. Pressure switch for the audio warning should be plumbed in the valley edge, aft of intake manifold near the distributor

 

[stresspoint]

thanks for all the reply's , i am free on the weekend to get out and do more investigation.
based on the reply's i have more to work on , it bothers me not having a reliable low oil warning or cut out system in place so using the boat as is is not an option for me till its conclusive what the issue is,
I'm am hoping its not an actual low oil pressure issue and the motor needs to be rebuilt again, some how i doubt it but stranger things have been known to happen.

 

[QBhoy]

I think your limiter is broken too. If this is a 4.3 GM in later form. Shouldn’t get anywhere over 4900 or just a touch over at best. Nothing over 5000 certainly

 

[stresspoint]

I think your limiter is broken too. If this is a 4.3 GM in later form. Shouldn’t get anywhere over 4900 or just a touch over at best. Nothing over 5000 certainly

i understand your concerns , my 4.3 motor is no where near stock , as i posted in the first paragraph in the thread , the motor is a bitsa (bits of this and bits of that) , OEM RPM limiter ???? what's that ,
this motor will pull 6000 rpm if i let it , RPM limiter is me pulling back on the throttle so it wont break.
with that said , i was advised not to allow that little V6 split journal crank to rev past 5500 so i avoid going much over 5000 however i let it go to 5800 on occasions but only short bursts, @5500 it seems happy to run there but i consider the advice given by those that know to keep it reliable.

 

[Lou C]

That split journal crank is the big difference between the V6 & V8 which are otherwise quite similar. It is possible that later units with the balance shaft are better in this respect.
Back to the oil pressure issue do you have any other issues with the other gauges? All the gauges get 12v power from the ignition circuit but if there is a bad connection on either the + or - side of things that gauge won’t work. One year I had an oxidized fuse (old glass fuses in the old style fuse box) and that cut power to all the gauges. It tricked me because the fuse had continuity between its terminals but oxidation prevented current flow between the tabs in the fuse box & the fuse. Same thing happened this year with the 12V power outlet. Oxidized fuse.


I know on mine running 25/40 oil the oil pressure is usually from 40-60 psi, the lowest it ever gets is 20-25 psi at slow idle after coming off plane.

 

[stresspoint]

That split journal crank is the big difference between the V6 & V8 which are otherwise quite similar. It is possible that later units with the balance shaft are better in this respect.
Back to the oil pressure issue do you have any other issues with the other gauges? I know on mine running 25/40 oil it’s usually from 40-60 psi, the lowest it ever gets is 20-25 psi at slow idle after coming off plane.

how's it going Lou ? ,thanks for chiming in .

my 4.3 motor is working well and keeping in line with your advice i am looking after it and have not managed to break it yet , i probably put 35 to 40 hours on it under all sorts of conditions this summer , baring a few minor hiccups it did not miss a beat ,i have found the 4.3 to be powerful enough for the Maxum , very underrated IMHO.

to answer your question ,i did have a discrepancy with the tach where it would show some weird numbers IE; it would show 1200 rpm @idle and the gauge read between 2000 and 7000 rpm at cruise a couple of outings ago, that was found to be caused from the tach wire on the AM distributor having a bad connection , some dielectric grease + some crimping fixed that, it was working fine last outing and verified with the digital shop tach.

ATM i am trying to get some direction on this oil pressure issue so as when i can get to work on the boat at the weekend i can look at what's been said.

i will be sure to post results as soon as i have something .