bought an'89 F350 4wd a few weeks back to fix up my '90 f250 with (bout time for tons of donated parts). the oil pressure gauge on the F350s 351w (both trucks have 351s) dropped down to nothing last weekend while I was putting in the back yard (pulling mulch around w/ the 4wd really f-ing helps!). anyhow at idle the pressure was at zero, and at about 2,000 rpms it was bouncing between zero and the normal, but not stable. when I took my foot off the gas, as the engine idled from 2k back to low idle the pressure was solid 'normal' (no wiggle or anything).

what could it be and what shoul i do to test it out. my main thoughts are sender (please...) or oil pump (ugh). and yeah, the oil level is good...

p.s. the engine is pretty tired... so I wouldn't be shocked if it was the pump...

 

that gauge is an idiot light at best. get a mechanical pressure gauge on there to truely know where its at. also if it was at zero the motor would be noisy. senders are probably bad, double check the wiring also.

make sure your not using a fram filter and run some 15w40 rotella or Delo motor oil.

 

If these are stock gauges with not modifications, the readings are "go or no go".
in other words, the OP needs to be above a specific pressure to activate the
Needle, like an on/off switch.
To test oil pressure, it is suggested that you test with a mechanical gauge.
Technically the gauge should not fluctuate.
How well were these trucks maintained?
What about the mileage?
I own trucks with high mileage and no signs of pump failure.
Then again I change oil frequently and have real OP gauges installed.
Like you suspect, the sending unit/wiring maybe failing.

There are people with more experience in this area than me, but I would
Check actual oil pressure and the sender/ wiring.

I think your post answered your own questions.

 

I just got the f350. she's got a lot of new parts (good) but she also really needed them (bad) and a few more. toughest thing- it was used as a snow plow for a number of years. outside of that I don't know anything about how good or bad the maintenance was.

all in all the truck is way better than I expected- I'm just pumped that I'm lookin at a functional truck at all...

I'll check out the wiring and sender this weekend. any chance I can swap out the working sender from the f250 w/o draining oil or a lot of time? looks pretty simple, but sometimes ya never know...

 

I finally got the sender replaced and it fixed the gauge issue perfectly. First time out the parts store gave me the wrong one and I had to buy a deep well socket for it (the wrong one).

anyhow, for anyone else changing the oil sender on a generation 8 truck ('88 to '91 for sure, probably most others too) w/ a 5.8, here's some info that you might find handy.

There are a few types of sender options that the parts store might ask you about: a) gauge or b) signal was first- if you have an oil pressure light on the dash you want the 'signal' type (it just sends a signal to the light if there is a problem). The gauge type works with the analog dashboard gauge. I wasn't ready for the question when he asked and was caught flat-footed for a sec. the next thing you'll need to know is long or short- apparently there are two different lengths of threaded sensor ends. I had no clue which one I needed (didn't have the right socket yet) so I rolled the dice on the short version and got lucky.

Lastly, you need a special 3/8" deep-well socket to get the sender on and off- there's another $12 for something I might use twice in my life! thanks for the suggestions- Besty's all better now that we know she's getting oil...

 

For thread content some take 1 " sockets..