View Full Version : electric vs mechanical oil press gauge

06-07-2012, 09:13 AM
hunting down the last of gremlins on the 302 , found they sent me the wrong sender well actually wasn't a sender was an oil pressure switch( so from 0 psi to pegged at the touch of the key). while testing I found the S/W electric gauge reads about 15 lbs lower than the mechanical test gauge is that normal?

mech 55lbs

S/W 40 lbs

06-07-2012, 10:52 AM
When I first changed to new gauges and senders I was getting odd oil pressure numbers, turned out the sender they gave me was for two gauges, as in a two station boat. There was one number different on the sender.

06-07-2012, 11:47 AM
did you get a new sender? they are generally matched to the gauge, the one you have could be a different range.

06-07-2012, 11:51 AM
Yup matched to the gauge

06-07-2012, 12:40 PM
I would think that most electric oil pressure gauges would only be a relatively close reading - even if the sender and gauge were matched properly. The Warlock I recently bought has both, and they are within 5 lbs of each other from the little time I had before developing engine problems. Bill

06-07-2012, 01:09 PM
Of course the mechanicss of electric vs. mechanical are completely different, and I would be surprised if they read w/in 10% of each other.
For instance, mechanical gauges almost all have Bourdon tube type mechanisms. Then just for fun, do a general search for common pressure gauges vs. certified gauges..........
The good certified gauges can be hundreds of $$$$$$$$.
The closest we come to good mechanical gauges in our boats are the better pitot driven, Bourdon tube type speedometers we buy.

joseph m. hahnl
06-07-2012, 09:51 PM
I suspect the mechanical gage is reading higher than it should. 40 sounds like the right number. Keep in mind that if an engine has to much oil pressure,the oil pump has a bypass that will open:yes:.

Research indicates that normal is 35 to 45 and the bypass opens at 55 to 70 lbs.

06-12-2012, 06:56 AM
Couple of things to consider. First, I assume that the 40 and 55 psi you mentioned were at idle? Depending on where you are getting your readings, the spread might tighten up at higher rpms. I have tested my pressure guages and found them to be within 5psi vs mechanical on the motor. Fuel pressure guages are notoriously fickel so I keep mechanical ones on the motors right on the fuel regulators. Were both your readings on the same side of your oil filter? If not, that could be a place to look. I had a brand new Fram filter collapse 2yrs ago which gave really funky pressure readings. If these things check out I would look again at the guage/sender setup.
- Marshall

06-12-2012, 07:24 AM
Marshall thanks for the info I used the same port as the sender, I did extend the tube about 2 inches and added a t fitting to add a pressure safety switch for the elec fuel pump. I needed to extend the tube so the sender and switch cleared the power steering pump. yes only at idle just setting the intial timing.

06-12-2012, 05:32 PM
Hmmm, not trying to spend your money but it sounds to me like either the guage is bad or the sender is wrong/bad somehow. I have had a pretty new ( 1 season) autometer guage crap out for no apparent reason. I would not attribute 15psi to a "normal" disparity. If the motor is new that 15psi makes a difference. 55 will drop down to 35 or so after some decent rpms/ warm up etc. 40 will prob go to 25 which is a little low imo. I would be curious what each guage reads at 3000 rpm.

06-12-2012, 09:48 PM
On the Minx engine, I had some small (very small liquid filled back port) gauges screwed into the engine for temp and oil press.
These were purchased from Great Lakes Skipper for about $65.00 each, they were good gauges ( Marshall all SST) and probably retailed for $100.00, or more, each. The really nice thing was, if you were wondering, you could take a look.