hogwild4

I just completed a valve stem oil seal replacement job on my 1977 350 SBC. I was very careful during the job to guard against letting any debris from getting into the motor. I had all the oil drain holes plugged and completely cleaned the top of the heads. I also performed the rework according to the “book”, and the motor sounds and runs very nice following the work. However, during the initial test drive I found that the oil pressure gauge holds steady at 40psi (which was the norm before the work) until the motor reaches 1500rpm, then it immediately pegs out at or beyond 80 psi. When I back the rpm’s down to 1500rpm or below the gauge comes back to 40 psi. I also see that when the car is turned off the gauge stays at 40psi (does not return to "0" as it should) until the ignition switch is put into the on position (not running just switch on) at which time the gauge returns to normal “0”. This leads me to think it’s an electrical issue rather than a true oil pressure issue? Possibly I hit a wire or connector during the work that has led to a bad ground or something?

MelWff

have you considered removing the sender and checking if there is any debris in the sender?

7T1vette

Most likely a bad by-pass valve in the oil filter. There are other possibilities, but that would be my guess. Remove & swap filter and see if that fixes your problem.

hogwild4

So far I have changed the oil filter with a new one but the issue remained unchanged. I looked around for loose or broken wiring both under the hood and behind the gauges but found nothing. I've also removed the oil pressure sending unit and inspected for debris but found none. For the little cost I've ordered a new sending unit and will install that tomorrow or Monday. I'll post an update following the new sending unit install. Thanks to everyone for your thoughts on this!

7T1vette

If electric oil pressure sensor, the signal wire could be fractured or about to fail. If that wire goes to 'open circuit', I think the gauge will go full scale 'high'. If you short that wire to ground, the gauge should go to zero. You could have something that is heat-sensitive: when cold, the wire has enough contact to get accurate reading; when hot, insulation softens, wire sags, and connection might be broken. With engine OFF and ignition to ACCY ON, wiggle the sending wire and see if the gauge goes from zero to full scale erratically.

0Willcox Corvette

Simple test... pull the wire off the sender, with the key on the gauge should peg.... Then ground out the wire with the key on, the gauge should go to zero.... If it works this way then replace the sender, if it doesn't the gauge has an issue.

hogwild4

Problem solved! Turned out to be a faulty oil pressure sending unit. Just still very coincidental that it failed at the same time as I did the valve seal rebuild but none the less the gauge is responding properly now. You can believe I'll be keeping a close eye on that gauge for a while though. Thanks again to all those who offered their thoughts on this!

cor66vette

Thanks for posting the outcome. Many ask questions but don't follow up.

cu79vette

I am diagnosing my oil pressure gauge and noticed a black wire (circled) cut on my connector. I'm pretty sure the tan/brown wire is the main connection. Does anyone know what this black wire is for? Add this to the list of cut wires I have found over the years. Ha!

7T1vette

Black wires are usually electrical 'ground' connections. Can't really see that wire, but it could be the one for the wiper motor which connects at one of the starter motor bolts.

cu79vette

Thanks. The wire was cut to about an inch. I believe I see the wire you mentioned but will confirm.

One ground wire that is cut and I haven't for the end is the ground wire for the underhood light. I assume that is not the same wire.

gozo

Sorry for dropping into this discussion. I have similar problem, And just maybe Willcox will respond. 1977 corvette, new oil sending unit and new reproduction oil pressure gauge. (Electric-not oil) Gage is all the way to the right and stays there, if I ground the dr. Blue wire at the sending turn key on the gauge goes to “0”
I have checked the dr. Blue wire on ohms and checks good and not grounded. The printed circuit looks good no fried tracers. Any help would be greatly appreciated,,, is it possible that I have two bad sending units?. Can the sending unit be checked on the bench?