1994 K2500 454 7.4L Proper Oil Pressure When Hot?

I'm curious because when I first start the engine, the oil pressure gauge reads just over 40, but as the engine warms up, it slowly drops until it sort of just sits just above 0. It fluctuates a bit with engine RPMs, which I think is normal.

I don't think that dropping to near zero when hot is normal though, is it? Maybe if it's idling after a long drive on a hot day I guess; the owner's guide (as I recall) does mention that as a situation where the oil pressure gauge might briefly drop that low, but this is consistent regardless of outdoor temps.

A mechanic (who I no longer do business with for various unsavory reasons) said it was the sender unit failing, but is that true? If so, is it a relatively easy and inexpensive fix?

c

First thing to do is hook up a mechanical guage. That will reveal actual pressure and will be easy to determine if sender is at fault ( probably not if it reads higher when you increase rpm.)

I had a 4.3 Vortec that did what describe...guage on instrument cluster (not too accurate) would dip below zero and the "check guages" light illuminated.

On the mechanical guage under same conditions I had 5 psi. The spec at 1000, 2000, 3000 rpm still net spec, but idle pressure was right at the mark.

I changed to the heaviest weight engine they sell to get me through the summer. I figured worn bearings was the cause. I eventually pulled the engine, but it still ran fine.

Good luck.

Hmm, OK.

Please forgive my ignorance, but where do I connect the mechanical gauge? In place of the sensor?

By the way, I forgot to add that I recently replaced the gauge cluster, and the oil gauge on the replacement reads the same as the original, so I'm inclined to believe the accuracy of the gauge itself isn't the problem.

Also, the Check Gauges only came on twice, but in both cases the engine VERY briefly overheated while idling due to a radiator and water pump failure about three summers ago (both have since been replaced with no apparent harm done to the engine, though now that I think of it, it may have weakened the transmission, setting it up for last summer's failure).

c

With a couple of brass fittings and a mechanical oil pressure gauge kit you can piggy-back it to the sensor that's on the engine....I did.

The sending unit is most likely failing. As others have said though, you can verify that by removing the sending unit and putting a mechanical gauge in place. But, if you're still driving it, the next time it reads zero just listen for valvetrain noise. If you don't hear that, it's probably ok.

The sending unit is way back near the distributor. Remove the cap to get better access. Even then you'll remove the sender almost just by feel.

You need a special socket to remove it. A little 1/4" flex head ratchet is how I removed nine.

The 454 in my truck is around 10 at idle when completely warm, but 30+ when driving at a steady speed. Also on the 454 the sender is on the front of the engine and easy to change so you might as well just replace if its original.

I have a '97 Z-71 Sierra, 5.7 with the drop in oil pressure. I firewall mounted a Stewart Warner oil pressure gauge connected to a tee on the sending unit with a store-bought 16" grease gun hose. This gauge proved the dash gauge to be accurate.
Research has some suggesting the drop in pressure due to bearing clearances opening up with temperature; the cam bearings were mentioned. Surely crank or rod bearings would knock.
I recall a '67 283 Impala and a '68 425 Olds Delta 88 with the same behavior. They didn't fail and so far my vortec small block is ok.
I'm surprised to see 'cc333' having this issue with his 454. The oil pump in a big block is stout with 7 cover bolts and bigger gears than a small block or the fellow on here with the v6.
I have a '67 327 that has great oil pressure; 60 to 80 psi cold, 40 psi hot. When I rebuilt it years ago, the trick was to install a Z-28 oil pressure relief valve spring in the oil pump. It was available from any GM dealer and cheaper than buying the whole Z-28 pump.
Finally, if I had to bet, I'd say our current crop of oil pump pressure relief valve springs are weak and releaving once up to operating temperature.

My 92 454 with 140k is 50ish or so cold

A hot idle is 30-35, cruise warm is mid 40s