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Old 05-09-2011, 05:08 PM
Coondawg Coondawg is offline
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oil pressure gauge fluctuation

Hey y'all.

Today, my oil pressure gauge started swinging. At idle and off-idle it will bounce on the L and climb to the middle of the range at highway speed but slowly drops back to L and bounces. The oil level is good and it's not smoking.

I'm thinking it may be a faulty oil pressure sending unit but how likely is it that the gauge on the dash is bad? Possibly a bad oil pump? Any other trouble I need to look at?

It's a 1995 F-150 with 302 & AT.

Also, I don't think it's related but never know, I had it in the shop for a rear fuel pump swap and water pump swap. I know they broke the bottom two bolts on the front cover and had to remove the front cover the get them out.

The oil pressure issue happened a couple hours after I left the shop.
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Old 05-09-2011, 06:02 PM
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check the wiring going to the sender that would be the first step. next would be to replace the sender unit.

if you are really worried put a mechanical gauge on there. the stock gauges arent really a true gauge.
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Old 05-09-2011, 06:35 PM
Coondawg Coondawg is offline
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That is what I'm thinking. I had an 80 F-100 that most of the instruments didn't work on so put mechanicals in the cab.

Anyway, since this truck is computer controlled, will putting in an aftermarket gauge cause any trouble for the computer?
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Old 05-09-2011, 07:31 PM
Coondawg Coondawg is offline
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Update;

I just played with it some more. It seems as though there was a lot of grease inside the connector at the wire. So I cleaned that up and the gauge doesn't go to L now except on initial start up then runs back mid-range rather quickly. The gauge doesn't move with an increase in engine rpm like it did before either. Now it just stays at one point even with the engine off.

So, I guess maybe they bumped it when the where removing the front cover.

Anyway, I'm not going to drive it until I get a mechanical gauge to test it with.
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Old 05-13-2011, 01:17 AM
Dannym Dannym is offline
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I had the same problem- thought I'd fixed it by cleaning/tightening the gauge head, but really it's a waste of time.

Don't worry too much. IF you run out of oil pressure, your lifters will start clicking. You'll hear it.

I replaced it with a true gauge. I fear ripping the hose- which actually carries oil to the gauge- quite a bit, because it could leave you unable to drive, it'd spray out oil unless you pinch it with vise grips or whatever.

The gauge is nice, but it varies so much it IS difficult to conclude anything from it. It goes pretty low when warm and at a stop, and much higher on the road, and changes with RPM. Basically, if you don't hear the lifters getting noisy, it's enough, and I'm not sure what good this gauge does if I pay it no attention and would only rely on engine sound.
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Old 05-13-2011, 09:58 AM
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my 88 had problems just the same, the connector was getting loose and eventually fell off (which freaked me out)

is it stuck on L or somewhere else?

like kemical said check wiring, then the sender, and i agree about a mechanical. and sorry dan, but i wouldn't want to listen to lifters as a sign of low pressure, stuff wears out at an alarming rate w/o enough oil.

the senders weren't that great in these trucks, ford had a technical bulletin (although your 95 might be past the dates, but i know my 88 was included in it) on them that involved replacing the sending unit and installing an inline resistor with them.
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Old 05-13-2011, 01:46 PM
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The senders in some models was merely a switch, if it is above the bare minimum, regardless of rpm, it would go to "middle", otherwise it goes low- at which point you'll probably hear the lifters anyways.

I heard this was because Ford had people coming in asking for warranty work about perceived problems with oil pressure going up and down. In short, it scared people.

It would fail to catch the notable indication of a problem where the oil pressure fails to rise at higher RPM, but stays above the idle pressure it deems "good". This seems rare though- AFAIK, there's only TWO primary problem cases here.

One, the engine bearings have failed and are horribly leaking so much much oil from the high pressure side of the oil system to the low side that it can't deliver minimum oil pressure at idle when warm, but it typically builds pressure with any application of gas.

Two, the oil pump shaft breaks and you get no oil pressure at all, zip, zilch.

There's a low-pressure condition where the oil system gets clogged. It's plausible IF the stock nylon-coated timing sprocket's still there, because that thing can shed plastic and clog the oil pickup screen. I suppose an oil filter could get clogged, but I'm guessing you're not the type to see how many years you can go without changing the filter.

There are some high-oil-pressure conditions, but AFAIK that should only be a funky problem from an ill-conceived modification or extreme repair goof.

It's just hard to get much from the gauge, because you look at it and say "is that low?" and without taking into account engine temp and rpm there's no way to tell. But for the most part, the primary problem, bearing failure, doesn't even show low oil pressure at driving rpm, only idle. The bearing failure is not an immediate "if you don't catch it within 2 minutes your engine will die for lack of lube". People hear the lifters at idle but pressure pops right back up to "good" with rpm and continue to drive it for some time. You can rev it up in neutral at a stoplight and presto, oil pressure goes back to normal!

You might assume that you'll have more warning of bearing failure with a gauge, but I gotta note, with 30W oil in a warm engine and a clean, low idle, it's WAY at the bottom of the gauge as "normal". At a glance, it would appear as "zero", but in fact, it could be like 5-10psi. My gauge doesn't have a number printed until 20psi, and only one "tick" mark below that. So you're gonna have to look really carefully, consider engine temp, how low the idle actually is, look straight-on at the gauge (looking sideways, hard to say exactly what the needle is above), and guesstimate the number to say "hey, this is low." In short, "normal" for warm idle is so low it's nearly unreadable anyways, even with a mechanical gauge. But it still won't matter much because unless the lifters get noisy, it' still ok.

There's not much you can do anyways because engine bearings are basically a teardown/rebuild thing. You're gonna say "ok, but I NEED to know if it's developing a problem before I drive to Florida, needing a rebuild in a strange town is a disaster!" From what I can tell, even if it develops a problem in the first hour of a road trip, drops to zero at idle and lifters chatter, you can STILL drive it a good long time before it actually fries the engine. And you can dump some 50W oil in there to thicken it and make it harder to leak through the bearings at idle.
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Old 05-13-2011, 02:24 PM
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I fully agree with Dan, except for that last statement (just because im stubborn and i would kill it and call for somebody to come get me, and I would be crying to hard to drive anyways...) the one exception would be if it was an emergency of course.

but Dan is completely right about the factory gauge. its a pos and more of a switch than a guage.

there are aftermarket electric oil pressure gauges though, accurate and you don't have to run the oil line (my highboys gauge likes to leak right onto the clutch)
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Old 05-14-2011, 09:26 PM
Coondawg Coondawg is offline
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Well, time for another update.

I installed an Auto Meter mechanical gauge yesterday morning. After getting everything back together I fired up the engine and it had 48psi at cold start. It dropped to 42psi for a few minutes will idling in the driveway. When I pulled out into the street it was on 40psi and stayed there for a couple miles. When I stopped at the first red light it dropped to 30psi, stayed for a few minutes then dropped to 22psi before I accelerated again. Upon acceleration, it went back up to 35psi but then quickly dropped again until it reached 15psi, which it would hold as long as the engine was at highway speed. Upon stopping again, it dropped to 2psi and never went up again.

So, I said enough is enough (the odometer is over 316,000 mi at this point), and took it into a local Ford dealer who gave me $950 for it as a trade in on a 2005 F-150 STX 4X4. I gave $750 for the truck 6 months ago so I feel I was very blessed.
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Old 05-14-2011, 09:26 PM
 
 
 
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