Oil pressure slowly goes to zero

[ku3kyc]

Hi. I have a 1989 Supra Sunsport with a 351. Something very strange started today. The oil pressure slowly drops to zero. From a coldish start it is at 60 psi. Over the next 3-5 minutes it will drop until the red light comes on and the guage shows zero. After letting it sit for a minute off, I can get 20 pounds back, enough to get me another mile.

Temp is fine and does not rise.

The oil level is fine and the oil looks new.

Any suggestions what the issue may be?

 

[Reinell-BRXL-191]

Re: Oil pressure slowly goes to zero

Well, you have 1 of 3 issues

1) Gauge or sender is going bad
2) Bearings are getting worn out
3) Oil Pump is failing

of course there are a few other things that could cause this.. like a leaking lifter, plugged pump pick-up, bad oil viscosity, oil floating on water etc etc... but the normal cause of oil pressure that is good at cold start and gets worse and worse is normally bad tolerances in bearing to crank/rod/cam shafts. Unfortunately, the only way you will find out is to wait until a failure because you continue to use it, or you pull the motor

I myself would do the latter...- but of course, chase the little items first like changing out the oil for the highest viscosity recommended for you engine with a new filter

 

[Don S]

Re: Oil pressure slowly goes to zero

Since you do have two supposedly separate systems showing low pressure, I would suspect you actually have an oil pressure problem. If you would like to do one additional check, install a mechanical gauge to eliminate any kind of an electrical problem of some kind.
If you still show low oil pressure, it's time to pull the engine, disassemble and inspect.

 

[Alan_Scott]

Re: Oil pressure slowly goes to zero

The oil pressure sending unit has a diaphragm in it and if it develops a hole, pressure will equalize behind the diaphragm and eventually it'll read zero pressure. Once you shut the engine off, the trapped oil leaks out and then it will start read pressure again. As the hole gets bigger, it'll take less time to equalize and show zero pressure. Yours probably looks like the one if the pic although it may be painted to match the engine.

If that boat was mine, and the sending unit does look like the one in the pic, I'd replace it (Ford E4ZZ-9278A) with a new one. You'll be able to pick one up at about any parts store for around 10-12 bucks. Teflon tape is a good idea to keep oil from weeping out from around the threads.

 

[ku3kyc]

Re: Oil pressure slowly goes to zero

thanks for the help. I will replace the sending unit tonight since it is a quick fix. But if I have two sending units then I suspect this is a real big blow to my summer vacation.

 

[ccsears1981]

Re: Oil pressure slowly goes to zero

you should only have one sending unit that is electronic the light is just wired in with the same sending unit to catch your attention if your not watching the guage. My personal preference is the manual guage that actually sends the oil straight to the guage for a more acurate reading, only problem is the kits you buy the hose isnt long enough to go from engine compartment to the dash of the boat but you can get hard plastic tubing from lows that fits and long enough to make the journey. The electronic components tend to fail on the engine due to heat as long as the orface doesn't clog up on the manual guage your always getting the acurate reading.

 

[Don S]

Re: Oil pressure slowly goes to zero

Over the next 3-5 minutes it will drop until the red light comes on and the guage shows zero.

Assuming the "Red Light" is a low oil pressure warning light, if it's coming on at the same time as the gauge goes to 0, then it's not the sending unit.
The gauge sending unit, and the oil pressure switch are two completely different senders. One unit can not do both.

 

[Reinell-BRXL-191]

Re: Oil pressure slowly goes to zero

as Don said, there are normally 2 systems in place for a gauge and a warning light- but, if the light is part of the gauge ( and not on the dash) it could come on when the sender shows low pressure. Most people will agree, that a mechanical gauge ran from the engine compartment to the dash is bad news on a boat. The acrylic tubing used for these gauges does not stand up to the heat, UV and vibration that occurs on a watercraft, especially a piece that is up to 15 feet long. More often than not, it will end in a failure. Then your engine will start pumping oil out that little tube all over the inside of your engine compartment, or worse... on your deck and upholstery in the boat. That's why all the gauges in modern boats are electrical.

Also, on this sender, if you do use teflon tape to seal the threads, make sure to use very little, the threads on the gauge need to pass the negative signal from the block to the gauge, and heavy use of teflon tape can degrade that signal. I recommend Loctite thread sealant instead.

 

[ku3kyc]

Re: Oil pressure slowly goes to zero

I could only find one sending unit. The light appears to be wired into the guage. I replaced the sender with part number as shown above. It looked right. The old part was painted over eliminating part numbers if there were any. Now on initial start it pegs the guage at max, obviously the wrong voltage. Advance Auto Parts gave me BWD part M12073, or S334, I do not know which is the part number. Is this the correct part?

I really hope the sensor is the problem.

 

[Reinell-BRXL-191]

Re: Oil pressure slowly goes to zero

Here are the interchange part numbers for the sender

700425, RO20001,90806, 982650,185899, 5899,700425, 815425T 982650, 3853800, 3857532, 65899

if any of these correspond to a auto application, I have no idea.. normally a gauge pegging would indicate a straight thru short to ground in the sender or wire... try touching that wire to the block, you will see what I mean.. I would put a multimeter on the sender and see if it pegs the ohms reading, it could be bad right out of the box

 

[ku3kyc]

Re: Oil pressure slowly goes to zero

The single sender was the problem. The correct part solved the issue.