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Unread 08-27-2010   #1 (permalink)
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Default 1991 Mustang 5.0-Oil Pressure goes up and down...

So i bought a 91 5.0 auto completly stock beautiful everything works..

so i was driving it and noticed the oil pressure was pretty low. right at the bottom on L

and it will randomly shoot up to the middle of the gauge like normal and then drop.

first i thought it was a bad oil pump but why would it randomly regulate..

and all the other gauges work just fine

the car has 108.000 miles runs great.. but the oil pressure concerns me. what can it be?
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Unread 08-27-2010   #2 (permalink)
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If it happens when you're cornering or heading up/down steep hills then it's just from the fluids natural movement and momentum. But if I had to guess, it's the lines, check for leaks, they can pop up randomly and seal again just as easy, happened in my old Jeep Wrangler all the time.
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Unread 08-27-2010   #3 (permalink)
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I had something similar happen to me on my 93 Fox. It was actually the oil pressure switch leaking out of the front. The plastic body of the switch failed and was leaking ever so slightly. Check that first. Its about a 15$ fix, but you will need a 23mm scocket as well. Do you see any fluids leaking around that area, or the oil pan?
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Unread 08-27-2010   #4 (permalink)
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I would install a mechanical oil pressure gauge just to eliminate any issues that might be in the electrical oil pressure sending unit. They are notoriously inaccurate in our cars.
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Unread 08-27-2010   #5 (permalink)
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A mechanical pressure gauge is a good idea as robs91fox stated. Although, from the past 4 Fox's I have owned, I could say that 99.9% of the time I had oil pressure issues was from a faulty pressure switch. The switch is unreliable, not the gauge.

Granted anything mechanical would be a direct reading and there would be no wires to lose resistance, which could result in erronous reading. But, the benefits of installing a mechanical pressure gauge vs. the cost is just not there. Ford designed it that way, and Boeing relys on electrical gauge indication 100% on all their jets (i know we are talking about cars, but same principles still apply).

It's a personal choice on whether to go mechanical vs. electrical. They both work as advertised.

Do you know if the engine was rebuilt recently? If so, it's common for builders to forget the galley plugs under the manifold. If the plugs are not installed, or installed improperly you can get a low pressure indication.
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Unread 08-27-2010   #6 (permalink)
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Id say its the switch too. On my Ranger, when its cool outside I get jumpy or just a no oil pressure on the guage, but do actually have it. Researching on Ranger forums, I found its a common problem with that particular sensor going bad.
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Unread 08-27-2010   #7 (permalink)
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When I bought my 87 GT convertible with 108K on it, the car was doing the very same thing. I thought it was my oil pump, but decided to change out the oil pressure switch/sensor next to the oil filter. 12 bucks from Autozone, and it fixed the problem. My gauge shows just over half when driving, and just drops slightly below half reading when idling and warm. No more up and down slow bouncing of the needle on the gauge. Try replacing the sender first.
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Unread 08-27-2010   #8 (permalink)
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Do the easy thing first, get a new sending unit and then make sure the rod it mounts to is clean and not clogged.
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Unread 08-27-2010   #9 (permalink)
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yeah there is no oil leaks what so ever.. and the engine was not rebuilt everything is still original.

so the sender is located by the oil filter? ill replace that first.
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Unread 08-27-2010   #10 (permalink)
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so i went out to my car and i unpluged the plug to the sending unit. and cleaned it and BAM.. all fixed.. hopefully. thanks guys!
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New Love 9/24/09 92 GT - Cherry, Torch Red - Presented in Edmundsinsideline.com 2011 5.0 Camparison! Check it out!
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