Oil Pressure Guage Mystery

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g164600

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Keep in mind that GM built 100s of thousands of these trucks from 1973 through 1991 and the quality of the gauges used may very well have met a level of quality acceptance at the time of installation. However, we are now talking some 30 to 40 years in service for some of these trucks. If an engine as an internal problem that results in other than normal oil pressure indications it most likely will have other symptoms as well. Anytime I see a questionable oil pressure reading on a gauge from an engine my first reaction is to shut it down and investigate the source. Install a known good mechanical gauge at the block and start the engine and confirm yea or nay on pressure. Typically it will be in the system either mechanical or electrical and they can then be investigated accordingly. 1973 through 1977 used a mechanical gauge meaning it had a solid metal line from the engine block to the gauge, 1978 and on was electrical with a sending unit at the engine block with a single wire to the gauge cluster.
 

Redfish

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Agreed. If the gage moves at any time it's probably okay. Senders on the other hand live in a pretty harsh environment and have lots of fiddly little moving parts that can impact calibration. If memory serves, circa '87 senders are a pressure sensitive, variable resistor that controls current to the gauge. With a multimeter, it should read about 200 Ohms with zero pressure; less than 60 ohms if you press your blow-off nozzle to the fitting and hit it with shop air.

If the gage goes to zero and the engine is still running dosn't mean you've lost oil pressure. There is a oil pressure switch in the fuel pump power path that will shut down the pump if the pressure goes to zero.
No it will not. That switch is a backup to the rest of the fuel pump circuit. The only way it can shut off power to the pump is if the primary circuit is compromised in some way.

I initially thought exactly what you posted but the wiring diagrams and some very helpful folks on this forum taught me otherwise.
 

bucket

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No it will not. That switch is a backup to the rest of the fuel pump circuit. The only way it can shut off power to the pump is if the primary circuit is compromised in some way.

I initially thought exactly what you posted but the wiring diagrams and some very helpful folks on this forum taught me otherwise.

Yep. That "switch shuts off the pump" way of thinking has been an epidemic for many years. Covid is going away faster, lol.
 

Urbanburban22

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I had an issue similar to this on my 91 350 tbi suburban. Put a new melling oil pump in (changed out the spring for higher pressure) since it was handy in the shop, installed a manual gauge I got from the parts store and was surprised with 80psi cold idle and 40-60 warmed up and driving. gauge on the dash will never move off of 30 psi EVER. I used to run fram 20w-50, but I'm using delvac 15w-40. glad to have a manual gauge though. I would install a manual one and disregard the electric dummy gauge in the dash. But that's just me, I have a gauge pillar with rpm, trans temp, and the manual oil pressure gauge.
 

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