low oil pressure

[another escapade]

Maybe somebody can help...... I have the 7.4 mpi's in my 330 sundancer. In July I had the starboard motor replaced with a new mercruiser longblock. The pressure gauge on the dash reads only 20 lbs.
The mechanic who replaced the motor, claims he replaced the sending unit already , I replaced the gauge and still only reads 20 lbs. The mechanic said he put a mechanical gauge on it and it reads 50 lbs. , he says don't worry about it. Any thoughts on this would be greatly appreciated .

 

[fc3]

put a mechanical gauge on it and verify it yourself. If the mechanical is correct, don't worry about it, but it's probably the sender. You can check the sender against the mechanical with this chart.

Note that more resistance reads lower pressure, so you might also have a wiring fault. Check especially the engine disconnect harness.

 

[Larry]

put a mechanical gauge on it and verify it yourself. If the mechanical is correct, don't worry about it, but it's probably the sender. You can check the sender against the mechanical with this chart.

Note that more resistance reads lower pressure, so you might also have a wiring fault. Check especially the engine disconnect harness.

Like usual, great info Frank!

 

[another escapade]

exactly how do I check the sender. I'm not really sure where the sender is located. You say to check it against this chart, does the mechanical read in lbs. or ohms.

 

[fc3]

The senders read in ohms. You have a single control station, so you use the single gauge resistance values. Just pull the wire off the sender and connect your DVM between the screw stud and a good ground connection. Mechanical gauges read in PSI. Well, US standard pressure gauges read PSI. International might be calibrated in bar, or pascals, though pascals are usually kilopascals. I digress.

Run the engine with the mechanical at various RPMs, say idle, 1000 and 2000. Then shutdown and re-install the electric sender. Remember do not use pipe dope, either liquid or tape on the sender's threads! Using pipe dope will result in inaccurate readings. Verify with DVM that there is a good ground (near zero ohms) between the sender's body and the engine block. Remove, clean and reinstall if more than a few ohms of resistance. Run engine at same RPMs as above checking that the resistance values are in-range as per the chart.

The sender is a 1 1/4 inch diameter dome attached low on the engine block just under cylinders 5 & 7 if I remember correctly.