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My girlfriend has a 1997 Pontiac Bonneville SSE (3.8L GM 3800-II, non-supercharged). It ran low on coolant a few weeks ago, and started to overheat. I refilled with coolant, but the next time I was in the car, I noticed that the oil pressure gauge (which, according to the shop manual, should read around 60 PSI) has been hanging around 80 PSI, often increasing to 100 or higher (gauge goes to 120 PSI) and sometimes dipping down to 40 or so.

The gauge movement seems somewhat random to me. It will stay around 80 for a while, then fairly quickly climb up to 100+, sometimes hang there for a while, sometimes just drop back down to 80. It also seems to bob up and down quite a bit.

I know I've heard that the oil pressure senders in these cars are known to go bad, but I've got less than 500 miles on this oil, and would rather not have to change again so soon, especially since I run Mobil1 fully synthetic in it.

Can anyone think of a mechanical cause (or anything other than a bad oil pressure sender) that would cause these erratic oil pressure changes?

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The oil pressure should change quite dramatically depending on the load on the engine. This is normal if you are driving under varying loads. –  jzd Jul 14 '11 at 11:07
    
I'm aware of that. All of this movement happens on the highway, on a straightaway, while at the same speed and RPM. But this JUST STARTED HAPPENING. The needle never went over 60 PSI or so before, and now it's pegged at times. –  Jason Antman Jul 16 '11 at 14:18

1 Answer 1

As @jzd commented, oil pressure should change with engine load.

At idle, your oil pressure is at its lowest, when accelerating it will be at its highest and cruising it should steady out.

Also ambient temperatures effect oil pressure depending on the weight of oil you are using.

Here is an explanation I gave a club member on our forum of a similar question
http://zcluboftexas.org/zcotforum/index.php?topic=176.msg862#msg862

When I first bought my Z31 on the drive home I had a "freak out" moment when I got off of the freeway and was sitting at a red light, looked down and saw my oil pressure gauge sitting at 1 notch from the bottom.

I was used to all my previous cars' gauges staying at the same levels once warmed up, not fluctuating like that.

I quickly pulled over, cooled it down, checked the oil, sat there thinking "crap what do I do" heh.

I started paging through the manual and found this:

"The pressure is normal if the gauge remains within the range of from 2nd to 12th lights corresponding to the engine rpm" (it says lights because I have a digital dash, on an analog it would be between the low and high range marks).

So based on my experience with my Z31 and what Ed said, lower pressure on the gauge is fine at idle (just not too low, the needle shouldn't be sitting all the way at the bottom probably).

Depending on ambient and engine temperatures and what weight oil your using, your pressure will be slightly different on throttle, but should be somewhere in the middle I would suspect.

For example, my normal pressure moved up 1-2 notches when I moved from 20w40 to 20w50 last week.

Z31s are the only sports car I've ever owned, but my assumption is this is one of the differences between owning a "sporty car" and a "sports car", your readouts are more informative because you are expected to be pushing the car in more extreme situations.

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