Hey folks, as mentioned elsewhere I've recently bought an '84 Camaro Z28. After driving it for a few days, there are some issues and generally strange things going on. One of these is the oil pressure; it seems completely random and only vaguely based on RPM and engine temp. The car needs a new thermostat but will sit at about 160-170F while driving (the cooling fan doesn't turn on by itself). The oil pressure at idle while at this temperature varies from 20-40psi on the gauge. Is this normal? Is the gauge just completely wrong? I haven't noticed any unusual engine noise, power is great, but the wildly varying oil pressure has me worried.

 

Try a new sending unit.

 

The only way to know the exact oil pressure is to install a mechanical gauge on the engine. Chances are you have a failing sending unit. The gauge itself doesn't fail very often. When it does fail, it will just quit working.

 

It's accuracy is relative, my isn't "correct", but it shows me a health relationship of what the oil pressure in the engine is doing and that's really all the factory gauge is designed to tell you. If you want it to be "accurate" it needs to be calibrated with known parameters.

Like these other two said though, you should troubleshoot what you have. Could be one, could be all three, it's impossible for us to know without being there. That being said, it is probably the sender as that is the most common point of failure. But getting a mechanical gauge on the engine to make sure that isn't the problem, since it is the most expensive part to fix.

 

If the oil sending unit begins to fail, it usually fails in an open circuit which causes the oil pressure gauge to peg itself.

So if its jumping around and not tied to Engine RPM, then its likely you have some kind of short in the wiring, or the sending unit is failing.

 

The early sending units (especially) are extra craptastic. I wouldn't bet on the accuracy of the gauge, but funny activity is often the big silver bell sender.

 

99.9999% of the time, the problem with the stock gauge system is the sender. (assuming of course that the wire is good... I'm going to go out on a limb and assume that you're smart enough that you checked that before posting on the Internet about it)

The other .0001% of the time it's also the sender. I don't believe I've ever seen the gauge itself "go bad" in any car of any age, model, or condition.

Putting a mechanical gauge on it is a great idea, until you do the math. So I'll do it for you:

1. Buy mech gauge.
2. Figure out where on your motor to hook it up to.
3. Remove plug, fitting, etc. from said place. Note that quite often, the only convenient place will be, to remove the stock sending unit.
4. Run line into pass compartment.
5. Mount gauge.
6. Discover that, in fact, your stock gauge system is bad.
7. Read the above post and realize that it's almost certainly because the sender has failed.
8. Disconnect and remove all of the mech gauge parts.
9. Buy sending unit.
10. Install new sending unit.

OK now, pop quiz:

What good did steps 1 - 8 do you, given that you already know what's wrong with it? You can skip directly to step 9 and bypass all the rest. One of 2 things will happen: either it will straighten up and fly right, in which case, you've fixed the problem with a minimum of expense and effort; or, it will continue to act exactly the same, in which case there's something wrong with the oil pressure and therefore the motor's internals.

To answer your first question, NO the factory gauge is not particularly "accurate". Like, NO, the local agency that certifies things like gasoline pumps and grocery store meat scales, is NOT going to allow you to use it for trade purposes. Definitely not in that league of things. It's not really a whole lot more than a fancy pretty eye-candy idiot light. (trim package) However, you don't really need "accuracy", anyway; does it really make any difference whether your OP is 49 psi, or 52 psi, while you're driving to work in the morning? Really? REALLY? Now, the difference between 49 psi and 12 psi is a WHOLE OTHER MATTER; and the factory gauge is good enough for that. Likewise, if the thing has been reading 38 psi during your commute for the last 80,000 miles that you've owned the car, but this morning is started jumping to 60 intermittently while driving down the same stretch of road and then fell to 0 and stayed there while driving home after work but the engine seems just fine - no knocks, ticking, funny smells, or any other signs of distress - then you can be fairly sure there's nothing wrong with the OP itself, but instead, the gauge has flaked. See the odds of the cause of this being the sending unit. Or OTOH you get out there and start test-driving it enthusiastically after some tuning, and suddenly the OP goes from 45 psi to 18 psi cruising and below 5 psi at idle and there starts being a funny tapping sound coming from somewhere down in the deep belly of the motor, then the gauge has done its job. It told you something broke. All it's really intended to do, is to tell you if something has dramatically changed. And in fact that's all that about 95% of car owners need; is to be warned when there's a problem. The absolute accuracy of the gauge, the "number" it shows you, is largely irrelevant.

Now, that said, if you're RACING the car, having an absolutely accurate and dependable OP gauge is totally necessary. But that doesn't sound like what's going on here.

Try a new sending unit. FIRST.

 

Well, not really. My oil pressure gauge shows me the pressure while I'm idling, warming up the engine or just testing stuff. Once on the track, I never see my gauge. Gauge is mechanical with a steel braid line inside the car. Rule book states that oil lines inside the car can't be made of plastic. Most tracks even during street legal nights will fail a car if it has a plastic line inside the car. Upgrade to copper or steel braid if you want a mechanical gauge inside the car.

When on the track, I rely on a large warning light to tell me if pressure is too low and even then, it's still good for a while. I use an 15 psi sending unit so the warning light comes on when pressure drops below 15 PSI. Engine also uses an accumulator so if pressure drops from a lack of oil in the pan, the accumulator dumps up to 3 quarts more into the system until the pan volume increases and the pump fills the accumulator again or at least gives me some time to shut the engine off.

The gauge is nice to see an exact number but while racing, I use a data logger to let me know where the pressure is during a run. I don't have time to look at a gauge.

Same goes for my tach. It's a playback tach so I can play back a run when I'm back in the pits but during a run, I don't see the tach during a run. I see a shift light to tell me when to shift. Including the tach, I have 5 gauges in the car and I never look at any of them while racing. A quick glance after a burnout then get ready to race.

 

Speaking of tachometers - mine died the day I bought the car. I have a special kind of luck with cars. Thanks for the help, guys. I assume by "check the wire" you mean the sending unit wire - it's fine. There is, however, evidence of water getting into the back of the dashboard. The VIN plate is very corroded and has moisture on the viewing glass. Yes, I've already ordered a seal kit for everything but the hatch - I knew what I was getting into when I bought the car.