The original sending unit caused the oil pressure gauge to bounce erratically, so I went to O'Reilly's auto parts (mistake) and they had one in stock. Made or branded by MasterPro.

The sending unit was slightly larger than the old one and was a tighter fit than the old one. Plugged it in, started it up, and the pressure was reading normally for a while, although lower than the stock sending unit. For example, at idle, the stock sending unit had the pressure just above the red line, which I've read is normal. At highway speeds, stock would be between 30-45. The new unit would read just less than 30 at highway speeds, and into the red at idle. Maybe it was calibrated for an 80psi gauge instead of a 60psi gauge.

Then the problems started. I'd start the car, and the pressure would read 0. Plenty of oil, plenty of pressure, gauge just wasn't reading. It would start reading a couple minutes into driving after putting some throttle into it, and then it would work for the remainder of the drive.

It's been like this for ~6 months when I finally had enough and put the old one back in. It seemed more accurate despite its schizo-moments, and at least it would work ALL the time.

Except, when I put the old one back in, now it won't read until I put some throttle into it after driving for a few minutes. I've tried jiggling the wire....no luck.

What the hell? Has anyone had any luck tracing the circuit? Any hints on what to look for?

I've got yet another sending unit on order from Rock Auto. Supposedly made by Airtex/Wells part 1S6569, specifically for 60psi gauge, so I'll try that. But I think there's an issue with the circuit. Any ideas?

 

There could be crap in the brass 90 degree adapter that the sending unit screws into.

 

Ahhhhh...excellent hypothesis. It's not real dirty back there but both old and new had some kind of tape on the threads, so maybe some came off and fell down in there.

Now the question is...how do I clean out that hole? I don't want to start the engine with the sending unit removed...I'm guessing oil will shoot out everywhere.

 

Likely you just got a bad sending unit new out of the box. Not like that never happens. Try with a standard motor products unit, and see if you have any better luck.

 

Yes, oil would shoot out everywhere.
It is most likely a piece, or small pieces of silicone gasket maker (Permatex) that broke off from something like an oil pan gasket replacement.
Take the sending unit back off again, and then unscrew that brass 90. Clean it out. While it's out, check the passage that the 90 screws into for debris.

 

Well, I will say that mine reads just under 30 psi at hot idle. That's on a 160k mile LB9 - bone stock after sitting for 11 years in a field..... "In the red at idle" isn't normal in this man's Army. The stock gauges and sending units aren't to be completely trusted but they shouldn't be that far off.

Verify the oil pressure with a mechanical gauge of known accuracy.

Start there. Any other path is just speculation or troubleshooting with Visa.

Checking for a blockage at the sending unit/adaptor costs nothing and you did recently disturb this so I approve of that also. Especially if you plan to use this port for the mechanical gauge check (most scientific location).

GD

 

Yes, verify the oil pressure at that port with a KNOWN GOOD gauge.

 

No worries about the pressure. When the stock unit reads (and isn't freaking out, it's fine).

30-45psi at running speeds (2200rpm-ish) and also when started cold.

Just above red at hot idle (500-700rpm) -which is actually very close to 15psi. That should be about normal for a stock 305.

Here's a pic of the oil pressure gauge, same as mine...

 

Sounds like (a) you got the wrong OPSU to begin with, specifically, the one for a 6-cyl or 4-cyl not a V8, which the gauge is 80 psi full-scale instead of 60, so it makes your gauge read ¾ (60 ÷ 80) of what it should at all times; and (b) it went bad or is plugged up as suggested above.

Get the right SU and try again. Be careful, if there IS any foreign matter of any kind, not to let it go down into the brass adapter fitting and into the engine. If whatever it is can block the SU from working, just imagine what it can do if it gets into some passage feeding a bearing or something.

Typical normal OP you should expect for one of these engines is 40ish hot cruising and 15ish hot idling, so if that's what it was reading before when the gauge was working right, your oil system as a whole is probably doing like it's supposed to.

 

Thanks everyone.

Got the new sending unit. Plugged it in. KOEO, oil pressure gauge read zero. Started the engine, still read zero. Frowny face...

I got the bright idea that maybe it was the gauge, not the sending unit. KOEO, sending unit unplugged, the gauge should read as high as it can go. However, during this test, it was still pegged at zero. KOEO, sender plugged in, it should read zero. Cleary there's a short-to-ground issue with the gauge.

Then I remembered the dash electronics in these things are literally the cheapest junk GM has ever made (hard to forget, but, oh yeah--try hitting it! Duh!). A swift tap on the black plastic just below the gauge, and it started reading. Viola. Problem #2 found.

So at least I've traced the problem and I can reliably fix the zero reading with just a tap on plastic right now. The new sending unit (correct 60psi unit) sends a steady signal and the gauge also reads steady when it's reading. I need to pull apart the cluster someday to check out the ribbon cables, grounds, or whatever else affects the inputs. For now a tap will do.

That was a head-scratcher of a project...

 

Gotta love those intermittent connections.