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Old 07-08-2019, 04:52 AM
Brynjminjones's Avatar
Brynjminjones Brynjminjones is offline
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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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  #2  
Old 07-08-2019, 07:00 AM
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SC/397 SC/397 is offline
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Like you noted - install a mechanical gauge on it and monitor it again.
A stuck Oil Pressure Relief Valve can cause a spike but I wouldn't think that it could cause fluctuations like that unless the spring is broke or something. I have never seen a broke one so that is unlikely. It does sound like a electrical issue in the sending unit or gauge to me.
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Old 07-08-2019, 01:57 PM
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brielly brielly is offline
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I doubt it anything serious. Either yoyur new sender is no good, bad guage or possibly a wiring issue. Your mechanic should be able to verify the correct pressure with a mechancal guage and narrow it doen from there.
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  #4  
Old 07-08-2019, 02:55 PM
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Brynjminjones Brynjminjones is offline
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Thanks both - I will make sure it gets verified with a proper gauge.

Here's a video I took immediately after starting it this morning. Ignore the pop noise as I hit the throttle - I think that must have been a coincidence as it didn't come from the engine.

You can however also hear a tick which has recently started and gets louder if I touch the throttle, although it sounds exhaust-y to me.

https://www.youtube.com/watch?v=C1c7...ature=youtu.be
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1997 Grand Cherokee (ZJ) 4.0
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Old 07-08-2019, 06:43 PM
johnsonic johnsonic is offline
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I think these senders are grounded via the threads - so if you used teflon tape it can break the ground intermittently. Happened to me...
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Old 07-08-2019, 09:30 PM
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ZackN920 ZackN920 is offline
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Looking at that video, i'd say the gauge or sender is flipping out. That is just way to sporadic and doesn't look like it would really match much with the engine speed. I wouldn't think that it would vary at all if it is just sitting there idling after a start.
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