360 Strange oil pressure behaviour

[Brynjminjones]

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?

[SC/397]

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.

[brielly]

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.

[Brynjminjones]

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

[johnsonic]

I think these senders are grounded via the threads - so if you used teflon tape it can break the ground intermittently. Happened to me...

[ZackN920]

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.

[Brynjminjones]

Thanks, I'm waiting to get the Wagoneer back to investigate further. After dropping it off at the mechanic, we realised that the threads on the oil pan drain plug are stripped and it started spewing oil everywhere!

The strange thing with this bouncing gauge is that it still changes with oil temperature. Once it's warmed up, the range it bounces over moved much lower at idle.
The moment I sped up to highway speed it went back up to 70-80 kind of range.

[Brynjminjones]

One small update - the mechanic it's with has had the relief spring out, and said it was completely free, so it looks like that shouldn't be a problem.
If that's free, is there any other way the pressure could go this high?

It's looking more and more like the sender.

[babywag]

bad wiring/connection in circuit.
on 86-up if disconnect sender wire OP pegs on gauge.

[Brynjminjones]

Thank you - I knew that it pegged if disconnected, but the fact that this isn't simply pegging is what threw me!

Anyway, today it has started jumping around even without the engine running, so I know for sure it's the sender.
I drove it about 15 miles and at one point the gauge even started working correctly, although not for long.

I will be putting a new sender in ASAP.

I've also confirmed that my engine tick is definitely the exhaust manifold, so it's all-round a pretty good Waggy day!

[babywag]

unlikely it is the sender more likely it is a wiring issue with those symptoms.

[Brynjminjones]

Originally Posted by babywag

unlikely it is the sender more likely it is a wiring issue with those symptoms.

I hadn't actually considered that yet! I will try grounding out the wire to see what it does.
If it goes to 0 and stays there, I assume the wiring is good?

[Brynjminjones]

Right, I've verified that the wiring is good and installed a new sender. This new (different) sender reads lower than the previous one did though!

Anyway, I finally got my mechanical gauge hooked up (temporarily) and here's what it's doing:

Cold start: 57psi
Normal driving once warm: 25-50psi
Hot idle (in Park): 20psi
Hot idle (in Drive): 15psi

All of these figures are very stable with no fluctuation. Hot readings were after about 30 minutes of driving.
Using 10w-40 oil.

Is that about what I should be seeing with a new (1k miles) rebuild?

[SC/397]

Which weight oil are you using?

[Brynjminjones]

I'm using 10w-40 oil.

I've just been for a longer drive - about 40 minutes at varied speeds and 20 minutes at 75mph.

At 75mph, oil pressure stayed at 45psi. After the high speed stretch, idle oil pressure was about 12-13 before coming back up to 15 once the oil cooled down a touch again.

This seems a touch low for a new rebuild to me - should I be concerned?

[SC/397]

Agreed, It is a little lower that I would like it for a new engine. Not a fix but, I would run 20w50 in the summer months and see if it stays a little higher. If it starts going lower then you might have to open it up and check clearances and detail the oil pump system. Rang-a-stang had a similar problem. He starts talking about it on page 12.
http://www.ifsja.org/forums/vb/showt...185193&page=12

[Brynjminjones]

Thanks, I'll have a read through that thread in the morning.

I was previously running 20w-50, but it really seemed like it took a long time to circulate on startup. It quiets down much quicker with 10w-40.

I should mention that the timing cover was replaced during the rebuild and we put new Melling oil pump gears in. The oil filter adapter is original and wasn't touched.
I'm not sure if any of the oil pump clearances were checked either.
I'm thinking about ordering a mid-plate - would that be a good move?


Also, my engine builder is replacing the oil pan for me in a couple of weeks due to a stripped drain plug. Is there anything worthwhile to check whilst the pan is off?

[wiley-moeracing]

Those pressures are not bad, I would run it. It could be a little higher on a blueprinted motor but just an average rebuild, not bad.

[babywag]

Originally Posted by Brynjminjones

I should mention that the timing cover was replaced during the rebuild and we put new Melling oil pump gears in. The oil filter adapter is original and wasn't touched.
I'm not sure if any of the oil pump clearances were checked either.
I'm thinking about ordering a mid-plate - would that be a good move?

The end clearance on gears if out of spec will lower pressure some.
Simply installing a mid plate will not change end clearance and probably do nothing for PSI.

I'd just run it and keep an eye on it.

[SC/397]

I am not a fan of the mid plate. Just have the oil pump cover lapped to get a nice finish.

No matter who's name is on the box, there is only one oil pump kit for the AMC V8 which is the K85 Melling. No need to change that.

If he has the oil pan off make sure that he has something to replace the button on the bottom of the pick-up screen. See page 16:
http://rebsamcandjeep.proboards.com/...builds?page=16
Never use a Fram Filter. I like the B9 or B39 Baldwin or the ACDelco PF24.