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Old 07-08-2019, 04:52 AM
Brynjminjones's Avatar
Brynjminjones Brynjminjones is offline
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Join Date: Jun 11, 2017
Location: Derbyshire, England
Posts: 188
360 Strange oil pressure behaviour

It feels like I'm asking constant oil pressure questions on here, but I've got another one.

The oil pressure in my 1991 Grand Wagoneer with a newly rebuilt 360 started acting strangely yesterday.
Engine has about 1000 miles on it since rebuild and has a new oil pressure sender.

We did a ~200 mile trip in it yesterday and the pressure was great the whole way, except it was consistently about 5psi higher than usual, hitting about 60psi driving at 70mph. Before now it would sit at about 50-55psi.
I didn't think much of this.

Once we came off the highway I noticed that the pressure started to bounce all over the place, but smoothed out some if I sped up again.


Got home and did an oil and filter change. Started it up and the pressure was normal until the new oil warmed up, when it started bouncing again.
Took it for a drive, oil pressure bouncing everywhere. I floored it to see what happened and the pressure went back to normal.
It then acted completely normally for the rest of the drive.

This morning I started it up and the pressure went all the way to 80, but was making huge jumps from 40-80. As the oil warmed up the average reading slowly dropped and became more steady, but it would still go up to 70+ at highway speeds, which it has never done before.
At idle it would bounce around 30 but was very unsteady.


I'd love to think that the new, previously working sender has randomly quit, but the higher than normal pressure yesterday morning, plus the fact that the pressure went stable again for a while after driving it hard yesterday makes me think that this might be a bigger issue.

It's with a mechanic now (already booked in for other reasons), so I've asked him to verify the pressure with a mechanical gauge.

Assuming the gauge isn't at fault (I wouldn't be that lucky), where should I start?
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1991 Grand Wagoneer - Hunter Green

1998 Cherokee (XJ) 4.0
1997 Grand Cherokee (ZJ) 4.0
1974 Ford F100 390
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