Topic: Oil Pressure gauge reading incorrect

[kohr]

My oil pressure gauge today started reading wrong.  Any t/s tips would be appreciated.

Background:  I bought the car about 3 weeks ago and have started and run it 10+ times.  The oil pressure always read about 45-50 at idle and close to 70 when I got on the accelerator.

Today:  I started it to take it in for a muffler replace.  Gauges worked as normal.  I made one stop to get a quote from an auto shop.  As I left and started the car the oil pressure now at idle is about 20 and accelerated it maxes about 40.  What is odd also is the needle seams to be "jumpy" instead of a smooth movement.

The only other odd thing about today is it got down to the 20s but I ruled out weather because it worked when I 1st drove it.  The performance shop it is at now for muffler replace and dyno tune was going to see if it is something simple but I wanted to get some feedback as to what I can look for.  Thanks.

[Joker (§ir£Ğragon)]

How long were you letting it run the other times? The pressure will drop when it warms up.

[4speed76]

It could also be a faulty sending unit.

[ta78w72]

It's probably the sender, but you should test the gauge by grounding the sender wire and noting what the pressure reading is, then let the sender wire float and the reading should be opposite of the ground position.  If you get those readings, replace the sender.  If you don't, check the wiring and the gauge.

[kohr]

Thanks all.  I will try your suggestions once I get it back from the shop.

[Trevon]

Mine maxes out at 60 when its pretty cold.  Usually idles at about 20 when its warm and get to about 40 at rev's.  It moves around a lot when driving between 20 and 40; not "jumpy," just smooth rising and falling.  Doesn't seem to odd to me if thats the case with yours.

[kohr]

It is not a stock engine.  It is doing around 330 hp before I took it to get tuned up on the dino.  It does 45-50 @ idle when it is warmed up and around 70 when I am driving.  This is how I knew something looked wrong with the gauge because it worked fine after I warmed up the engine and drove it about 5 miles.  Once I restarted it after getting some quotes the gauge was not working correctly and nothing was wrong with the engine or how it was performing.

Something is wrong with it I just need to figure it out once I get it back from shop.

[4speed76]

I hope it's not a bearing failure, but a simple fix.

[kohr]

It's probably the sender, but you should test the gauge by grounding the sender wire and noting what the pressure reading is, then let the sender wire float and the reading should be opposite of the ground position.  If you get those readings, replace the sender.  If you don't, check the wiring and the gauge.

Thanks it tested fine using these tips.  Loose it went max (way past 80) and ground it went to zero.  I hope it is not going to be tough to get to since it is down above the oil filter about 6 inches and I have headers I have to work around.

At least it ran fine on the dyno and they got another 40 hp and torque out of it by rejetting carb and adjusting it for my altitude (it was running very rich before).

[ta78w72]

Try this:  Connect an ohm meter to the sender terminal (you need to also have the sender wire connected to the terminal too).  Start the engine and let it warm up.  Check the service manual for the PSI and associated ohms.  For a 78, you should measure 1 ohm at 0 PSI and 88 Ohms at 80 PSI.  I assume the ohms are consistent along the PSI range.  At various PSI readings note the ohms.  You can then take that data and figure out if your sender is registering correctly.

The needle being jumpy makes me suspicious of that sender.

The pressure fluctuating is pretty normal.  It should be high when you first start the engine.  Then it should drop as the engine warms up.  When you rev the engine the oil pressure should increase.

[kohr]

I might not have been clear.  I tested out the wire and the guage worked like you said so the sending unit is bad.

[ta78w72]

I agree, but the above procedure could double check that diagnosis.....but if it were my car, I'd just replace the sender like you'll probably do.  I guess I was trying to beat a dead horse a little too long.

[jjr]

 I recently had a gauge issue, and pulled out a Kent Moore universal
gauge tester that I found on ebay a coupla years ago.

 I was quite surprised to find that units that I though previously were
busted, in fact weren't.

 The function of resistance (ohms) is not very linear along the display.

 In other words, the manuals states a few samples of values, but you
cannot interpolate very much as it's not consistant.

 I had read once were these gauges were considered pretty sturdy,
over the years I didn't think so - but now I think I was wrong.



 Joe

[ta78w72]

I use a variable resister and an ohm meter.

I use to think these gauges were sturdy too.  I think they are, but after 30 years I think the failure rate is increasing.

[Amtrak]

when this happens I always install a mechanical gauge first to see what the actual oil pressure is and then change the sending unit next and see if it agrees with the mechanical guage reading