Help! Oil Pressure guage tacking out!!

[cheftl13]

hey guys I have an issue that needs to be addressed asap. I had an oil pump installed last year, the pump is reading 40psi at the motor. The guage starts at 40psi, but once the car is on the road, it jummps to 80psi and stays tacked out. Once in awhile it jumps back down to 40psi, and then back up. Any thoughts?

 

[bill81vette]

have you checked to see if your sending unit is working properly???
that's kind of hi for a small block Chevy....
unless it was a high pressure pump...
what else was done to the engine??? rebuilt?new bearings/heads????

 

[Crimson Thunder]

Be careful, running that high of oil pressure can split a cheap oil filter.

 

[JohnZ]

hey guys I have an issue that needs to be addressed asap. I had an oil pump installed last year, the pump is reading 40psi at the motor. The guage starts at 40psi, but once the car is on the road, it jummps to 80psi and stays tacked out. Once in awhile it jumps back down to 40psi, and then back up. Any thoughts?

Sounds like the oil pressure sending unit is going south (yours is electric, not a direct connection like earlier cars). Real oil pressure doesn't jump back and forth between 40 and 80 psi.

 

[cheftl13]

Sounds like the oil pressure sending unit is going south (yours is electric, not a direct connection like earlier cars). Real oil pressure doesn't jump back and forth between 40 and 80 psi.

Thanks for the info John! Is it possible that it could be the tach itself? Just trying to narrow down my options before changing out a bunch of parts.

 

[cheftl13]

have you checked to see if your sending unit is working properly???
that's kind of hi for a small block Chevy....
unless it was a high pressure pump...
what else was done to the engine??? rebuilt?new bearings/heads????

thanks bill, I will check out the sending unit, the engine is orginal with 58000 original parts. Just replaced the oil pump last year. The tach was pushing to 80 when the pump blew, but never went back to normal after replacing it. When putting a gauge at the motor, pressure is right at 40psi, so sounds like it may be the unit.

 

[Bob Chadwick]

I had the same problem with my 77. Replaced the sending unit and the connector and had the oil gauge rebuilt. Never was able to track the problem down. I suspected an intermittent ground issue in the circuit, but like I said, I was never able to track it down.

 

[JohnZ]

Thanks for the info John! Is it possible that it could be the tach itself? Just trying to narrow down my options before changing out a bunch of parts.

Are you talking about the tachometer or the oil pressure gauge? Sending units are cheap, and take five minutes to change.

 

[Crimson Thunder]

What kind of oil pump did you use when you replaced the pump?

 

[cheftl13]

Gentlemen,....we have a winner!!!! Oil pressure sending unit, it was! Pressure is back to 40psi. Thank you to all who responded and saved me a ton of money and time. $20 part, and 15 minutes install time=peace of mind!

 

[Juskie77]

Gentlemen,....we have a winner!!!! Oil pressure sending unit, it was! Pressure is back to 40psi. Thank you to all who responded and saved me a ton of money and time. $20 part, and 15 minutes install time=peace of mind!

I'd like to jump into this discussion with my 77. Been in a 3 year restoration and been troubleshooting some electrical problems. on the oil pressure gauge, with a new oil pump (engine rebuilt), and a new sending unit, the gauge doesn't read zero when the car is off, and when it is running, the pressure sits at almost 80, like 70 or so. I unplugged the sending unit and the gauge pegs, way past 80, all the way to the right at 3 o'clock. A new oil pump was installed. It is a Melling PEM Replacement Oil Pump. Is it possible the new sending unit is bad or is it the gauge?

 

[TimAT]

I'd like to jump into this discussion with my 77. Been in a 3 year restoration and been troubleshooting some electrical problems. on the oil pressure gauge, with a new oil pump (engine rebuilt), and a new sending unit, the gauge doesn't read zero when the car is off, and when it is running, the pressure sits at almost 80, like 70 or so. i'm gonna try unplugging the sending unit and see what happens. Is it possible a new sending unit is bad or is it the gauge?

Quick and dirty test- Unplug the sender and turn the key on. Gauge should zing right past full to the stop. Then connect a 12v testlight between the sender wire and a good ground. That should make the gauge drop to about 1/2 scale. I used that test for years when I was playing like a dealer mechanic. Never has failed. If the gauge works like described, it's the sender. If not, it's the gauge or wire to the sender- (usually the gauge).

 

[Juskie77]

Quick and dirty test- Unplug the sender and turn the key on. Gauge should zing right past full to the stop. Then connect a 12v testlight between the sender wire and a good ground. That should make the gauge drop to about 1/2 scale. I used that test for years when I was playing like a dealer mechanic. Never has failed. If the gauge works like described, it's the sender. If not, it's the gauge or wire to the sender- (usually the gauge).

Ok, I unplugged the sender and turned the key on and the gauge needle went all the way right past the 80. When I connected the 12v testlight, it did not go to half scale, only rose up from the all the way to the right position to the 80 mark on the gauge....so is the problem the gauge or the sender?

 

[TimAT]

I'd say the sender. The gauge responds to the resistance in the circuit provided by the testlight. You could use a meter and check the resistance your testlight has. The GM gauges work from 0-90 ohms. Unhooked from the sender, the gauge sees an open circuit, so it goes full scale. The testlight is a cheap quick way to add some resistance into the circuit. An 1893 bulb has about 45 ohms resistance- half of the 90 ohms that would make the gauge register "0".

 

[Juskie77]

I'd say the sender. The gauge responds to the resistance in the circuit provided by the testlight. You could use a meter and check the resistance your testlight has. The GM gauges work from 0-90 ohms. Unhooked from the sender, the gauge sees an open circuit, so it goes full scale. The testlight is a cheap quick way to add some resistance into the circuit. An 1893 bulb has about 45 ohms resistance- half of the 90 ohms that would make the gauge register "0".

New sender installed and the gauge goes to zero when its off and runs about 60 psi when running. I think that is normal right?

Thanks for the tips!

 

[TimAT]

I call 60 PSI good. One of those places that too much is way better than not enough.
I run a minimum of 60, but Ol' Red has a ZL-1 oil pump and straight 30 weight oil too. And all the bearing clearances are right in the middle. I''ve seen it up around 85 cold.