While driving home this morning, I noticed my car’s engine was sort-of ‘revving’ at stopsigns, causing my car to shudder and my overly paranoid mind to start worrying. I checked all the handy guages at the next stop sign, and the only thing that I noticed that -might- be odd was this: The oil pressure drops to about half its normal level when I’m at a stop. I have no idea if this would cause such a problem, or even if it -is- an anomoly or such. Anyone have any ideas if 1) this pressure change is bad, or 2) what else could be causing the sudden ‘revving’?
Hopefully Rick or GaryT will chime in, but I’ll give it a shot anyway.
First of all, abnormal oil pressure should have nothing to do with the engine surging. Oil pressure is simply a result of your engine turning, and I am not aware of any feedback device that controls RPM based on oil pressure.
Secondly, oil pressure does tend to vary somewhat with RPM. This will depend on the type of engine, the type of oil pump, and the wear inside the engine. (An engine will wear over time, resulting in less pressure drop in the bearings.)
Having said that, we have a couple GM vehicles, and all of them exhibit the characteristic you’re describing – at low RPM, the oil pressure is fairly low, and at high RPM it is quite a bit higher. I’ve always figured it was normal for each engine to do this.
You may or may not have a problem with your car, but it’s completely normal for your oil pressure to follow your engines RPMs.
At a certain RPM it’ll stop going up because there’s a little valve that releases to keep it from going any higher.
1.) it is completely normal for oil pressure to change with RPM and temperature. Oil pressure will usually be lower when the engine is idling and warm. As long as it is in the normal range, it is probably not a concern.
2.) For your second query it would be most helpful to have more details, such as the year, make and model, engine, etc.
When you describe the engine “revving”, does it actually increase in RPM at the stop or is the engine idling higher than normal all the time once you get underway? Is your check engine light on? Does it exhibit the same behavior in neutral? It might be something as simple as a sticky throttle cable/linkage, but more info is needed to try and start diagnosing the problem.
The oil pump is sized so as to provide sufficient oil pressure at all rpm. Low rpm doesn’t require as much oil as does high.
Many engines have an oil pressure regulator. When the engine rpm rises, over pressure is avoided by having a spring loaded ball valve at the output of the pump. This valve opens at the desired regulated pressure and returns some of the pump output to the sump. The pump output is thus regulated at some preset pressure.
Sorry I’m late. This work stuff really cuts into my dope time. I got to do something about that.
Anyway as has been pointed out, oil pressure does vary with both temp and engine RPM. It will also vary with level, and how old the oil is. Older oil, in my personal experience on cars that had oil pressure gauges, runs a lower average pressure than fresh oil.
With all that said it is very unlikely that the low oil pressure is the cause of any surging, but more likely just the side effect of running the engine at idle.
If you want to go further we need make model year, engine size and as good a description of the problem as you can give.
