Prodigious

A few weeks ago, my oil pressure started to get a little low at idle when the oil was hot. By a little low I mean the top of the red or even a little lower than that sometimes. Engine temp is fine, and it didn't do this until a few weeks ago, and oil pressure climbs with the rpm like normal, and it has plenty of pressure under throttle even when the oil is hot. Just idling for some reason it goes low. The idle seems to be completely normal though, you wouldn't even think there was a problem until you look at the gauge.

The other issue I sometimes have is that when I have my transmission in drive, the one with overdrive disabled only though, after I let off the throttle after heavy acceleration it almost always backfires once or twice. It does the same thing sometimes when I get back on the throttle. I don't know if I should post a new thread for this problem or not, so I'll just leave it here for now.

Any advice or opinions on either problem would be much appreciated, thanks guys.

L-Train8

I'm seeing the same thing, oil pressure-wise. When the engine gets to operating temperature, the oil pressure drops at idle. Not quite to in-the-red on the gauge, but much lower than it has normally been at for the last 3 years.

The last oil change, I switched to synthetic oil, but other than that, I haven't changed anything.

Could there be some kind of blockage in the oiling system? Could the oil pump be damaged?

Prodigious

Mine is even stranger than that, it only happens when the motor has been running for a while at operating temperature. This just started out of nowhere, and it's only at idle. The gauge will climb to perfectly healthy numbers when driving, and I mean perfectly healthy, even if it's been running at operating temperature for a long period of time. But when I stop at a light, it will be fine at around 15 psi for the first minute or two, but after that it will slowly dip lower until it's about at the top of the red. It won't go any lower than that though, but it just confuses me why it started doing that all of a sudden. I would assume that it's the sending unit to the gauge, but it climbs the way it's supposed to under acceleration, so I'm guessing it's not that. The only other thing that I can think of that would cause low oil pressure readings is worn bearings, but I don't think that it would be this sudden. It never went that low before, and my motor only has about 110,000 on the rebuild, so I don't think they would wear that much by now anyways. However it hasn't done it in about a week now, so maybe it was just a fluke. I'll continue observing the gauge under all driving conditions and see if it happens again.

sofakingdom

iTrader: (1)

No "blockage" in the oil system, with the possible exception of the filter.

First thing to try is, change the oil pressure sending unit.

3 things are about to happen: someone will come on here and tell you to go buy and hook up a mechanical gauge; and someone will tell you about "10 psi per 1000 RPM"; and someone will tell you to pour thicker oil in your engine.

Ignore them. Not because they're "wrong" or any such; just, none of them fits the situation at hand.

The situation being, you have a LOW GAUGE READING, not necessarily low oil pressure. Quite possibly 2 COMPLETELY different things.

The reason you should ignore about "mech gauge" is that all it will tell you is whether your factory gauge is reading right. No problem there, as far as it goes. Problem is, the underlying facts: the main one being, the gauge VIRTUALLY NEVER fails that way. It stops working, but doesn't suddenly (or randomly or intermittently or gradually or any other way) become inaccurate. Therefore you can assume that if the gauge works, it itself is good. So after you buy, install, and get readings from the mech gauge, you either (a) have a gauge that's reading wrong, or (b) an oil pressure problem. If the gauge is reading wrong, the thing that makes it read wrong, is the oil pressure sending unit. If you go through all that expense and work (damn I hate that word...) and arrive at conclusion (a), you now know that you need to change the oil pressure sending unit. If on the other hand you arrive at conclusion (b), you need an engine, because about the only way the oil pressure in this type of motor goes away, is from excessive bearing clearance, usually caused by wear. There are other causes such as the regulator getting stuck or the filter clogged or the pickup tube fallen off, but they produce entirely different symptoms and are readily identifiable.

Bypass all the "mech gauge" crap and just replace the OPSU. There's probably better than a 50% chance that's what's wrong in the first place, anyway. If that clears up your gauge reading, you just performed both the troubleshooting and the repair in one eeeeeeezy step, and are now completely through. If the problem persists, ... motor time.

The "10 psi per 1000 RPM" thing refers to a minimum recommended pressure for survival under racing conditions. IN NO WAY does it apply to a street-driven engine. It's best to consider that the oil pressure should be "normal", whatever that happens to be, for the engine type in question. For the V8s in these cars, something around 25 - 30 at idle and 50ish at cruise, fully hot with THE CORRECT oil in it (10W-30), is "normal".

CORRECT oil refers to the viscosity that produces the desired lubrication (film strength) in the bearings, and the desired FLOW through the bearings for cooling. Excessively thick oil which will make the gauge read higher, DOES NOT necessarily either lubricate or cool better. Stick with THE CORRECT viscosity to optimize oil system performance, NOT put something wrong in it just to fake the gauge into reading higher.

Prodigious

Thank you for your help, I will start with replacing the sending unit and see if there is any change. As far as the gauge goes, what psi exactly is the first yellow line on the gauge? The one right after the red? I always assumed that it was 15 psi, but I would like to make sure I'm not mistaken. That is usually right where my car is at on the gauge when it is idling hot, so if it is 15 psi, then I hope my sending unit is bad... lol. Because I'm not sure 10 psi under the normal is very good at all. But at the same time, 10 psi less oil pressure does sound fairly normal, as far as my knowledge goes at least, for a motor with over 100K on it. But after this long and this many miles, maybe the sending unit has been bad ever since I got the car 6 months ago.

The oil in the car is 10W-30, I only put in the weight that the motor is supposed to use.

L-Train8

Thanks, Sofa. That was excellent advice. I replaced the sending unit and the gauge is displaying properly again.

This thread: helped me figure out how to do it, since neither my Chilton's nor my Pointiac shop manual told me where the damn thing was. It was a little hard to get to, since it's above the lip of the top of the oil filter. After 5 minutes of dicking around, I was able to get the old one out with some slip-joint pliers. A bit trickier than it looked at first, but still a simple job that didn't take much time at all. I spent more time getting the car on jack stands than I did replacing the sending unit.