Mercruiser 7.4 sudden oil pressure loss

[ludovic815]

Hi all,
I have oil pressure issues on my 1996 Mercruiser 7.4 (Thunderbolt ignition).
At idle/low revs the oil pressure fluctuates a lot between 1 and 3 bars (10 - 40 psi).
When I throttle up to 3000 revs the oil pressure fluctuates between 3 and 5 bars (40 - 70 psi) and after a few seconds/minutes drops suddenly down to 0. Then when I idle the engine for a few minutes the oil pressure comes suddenly back to 3 bars (not progressively, very sudden). I can hear no alarm when this happens and no weird sound from the engine yet.
I tried to make a small video to show the issue:
https://www.youtube.com/watch?v=Ezhr7TNjKXU

What I did so far:

• Changed the oil. I used Quicksilver 25w40 Synthetic oil (between 6 and 7 liters)
• Replaced the oil filter.
• Changed the oil again with Quicksilver 25w40 oil (not synthetic, the bottles with the blue stickers), again between 6 and 7 liters
• Changed the oil pressure sending unit. Bought a "noname" sending unit, had no pressure anymore on the gauge
• Changed the oil pressure sending unit again, this time with the genuine part (it's a lot better )
• Changed the oil gauge (Faria oil gauge)

The oil level in the engine is exactly in the middle between "min" and "max"
I have no idea anymore what to look for to solve this issue. Do you have any hints?

 

[alldodge]

I think you have a bad connection, either 12V or ground. The gauge and the sender must have a good grounded. Run a jumper from the ground lug on the tank to the negative battery terminal

If no change, then check connections on the gauge and the pink wire from the sender to the gauge

 

[GasCan II]

Take the wire off the sending unit and ground it to a good ground , the oil press. gauge should peg out if not it’s your gauge.
Hope this helps.

 

[ludovic815]

Thanks for the replies! I will try that, a bad connection could be possible, I will check.
I replaced the gauge last week with a new one, so I don't think it is a gauge issue.

 

[Scott Danforth]

also, plumb in a mechanical oil pressure gauge to verify the real oil pressure.

 

[ludovic815]

Yes I could try that, but here in France it is not easy to find mechanical oil pressure gauges with US threads. Does anybody know which thread has the oil pressure sending unit?

 

[alldodge]

It should be 1/8 NPT

 

[ludovic815]

I finally managed to find a mechanical Gauge. Plugged it in and cruised for about 1 hour at 3200 rpm. The reading was always the same : between 4 and 5 bar (60 psi)
So I assume this is an electrical issue...

 

[alldodge]

Good to know your motor is fine.

 

[scoflaw]

Faria gauges are cheap crap, I got a set on my boat.

 

[wingless]

Mine had that problem from the right-angle push-on barrel-shaped sender connector. I snipped off that part, replacing it w/ a properly-sized, properly-crimped ring tongue connector, retained to the existing threads by washers, lock washers and a nut.

Problem solved.

The oil pressure was also first verified using tee plumbing and a mechanical gauge.

 

[ludovic815]

Replaced the connector with this one and a nut... No change...

 

[Scott Danforth]

use a multi-meter. troubleshoot where the problem is...... then attempt a fix.

your gauge sender should have a variable resistance to ground. reading at zero will be 240 ohms. reading at half gauge will be 103 ohms, and reading at full gauge will be 33 ohms.

 

[wingless]

The engine has two sensors connected to the oil galley. One is the oil pressure sensor, the other is the low pressure switch for the alarm.

Which connector was replaced?

 

[ludovic815]

The one from the oil pressure sensor

 

[ludovic815]

I cleaned the harness connector. Now the pressure gauge works like a charm!
Thank you for your help! Problem solved