Possessed Oil Pressure Gauge.

[aeidian]

89 2.9 4x4 Ranger

When the truck has been run for awhile here and there, an hour or so I guess, the stock oil pressure gauge starts to freak out at around 700rpms. If I'm in traffic, stopping at a redlight or such, and the truck drops to idle around 500-700rpm the oil pressure gauge starts to flick back and forth very quickly. Giving it gas so the rpms are up to 1k, it's fine, gauge sits at the A on the NORMAL of the guage. Letting off the gas it does it again. It doesn't do this upon first starting, or even after shutting it off and letting it sit for 20 mins or so. It's a very fast back and forth, sometimes it goes to half, sometimes all the way, very rapid. I put a new O2 sensor on it, because it didn't have one at all before I put it on, could this be related? Is the gauge just screwy? I've checked the oil 3 days in a row and it hasn't dropped below the full mark, so I don't think it's got a leak.

 

[Big Jim M]

Most likely it is the sending unit that sends the reading to the gauge on the dash. Although it could be related to the amount of voltage the alternator is putting out at 500 rpm.
What you have is a pressure switch... It is supposed to either read zero or somewhere in the NORMAL on the gauge. If it reads more than zero that indicates you have more than 10 or 12 pounds of pressure. Where the needle is on the NORMAL is positioned by the present voltage in the system.. It is however no indication of how much oil pressure you actually have over the 12 pound minimum.
Big Jim

 

[aeidian]

I think I'm gonna go nab an aftermarket oil pressure gauge and see what it does. I already got one for the temp gauge because it wasn't moving at all, regardless of temperature, even after replacing the coolant temp switch thing. Think it's just a screwy gauge?

 

[aeidian]

Put in one of those Sunpro aftermarket oil psi gauges. Runs about 10 psi at idle during first start up, 15 psi at 1500rpm, 20psi at 2000rpm. After its been driven for about an hour the idle rpm drops to about 700-800 and the psi gauge drops to zero. It doesn't tap or knock at all, and hasn't. Where should I look first? New oil pump? I've replaced the rear main, oil pan gasket, and valve cover gaskets. I don't appear to have any leaks, oil level has stayed the same for the past week.