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Discussion Starter · #1 · (Edited)
We're about to head out of town tomorrow for a spring break trip. I changed the oil yesterday and today we started getting low oil pressure indicator when sitting at a stop light in drive. I think it is an electrical problem but wanted to run it by you guys before heading out.

2012 F-150, 5.0 L V8
150,000 miles
It's got the dummy gauge that pretends to say what the pressure is between L to H

- Intermittently (maybe half the time) the oil pressure drops to L and the car dings when sitting at a stop light in park with Idle at 500 RPM. Only does it when warm.

- I swapped the oil pressure sensor/switch for a new one. Problem persisted.

- I let the car sit for 1 hour and checked the dip stick 3 times. Oil level is right at the top of the hashmarks so it's got the right amount of oil.

- I bought a new mechanical oil pressure gauge and connected it up to the hole that the sensor was in. It read 28 lbs at 500 RPM idle with the truck in drive. Went up to 80 lbs when my wife revved it in park.

- I connected the new sensor to an air pressure regulator on shop air. I slowly increased the pressure from zero and measured the resistance/continuity through the sensor. It is a one wire sensor that connects to ground when there is enough pressure. This one connected to ground at 5 PSI. So it would seem this sensor for the dummy gauge will say "good oil pressure" with anything above 5 PSI.

- So I've got a sensor that will show "good oil pressure" with as little as 5 PSI. And my mechanical gauge shows 28 PSI at 500 RPM.

- There is an oil leak somewhere above that has the whole wire and connector drenched in oil. But I checked the resistance between the connector and about 3 inches up the wire from it (stabbed probe into wire) and got 0.3 ohms so the connector resistance seems fine.

- I was NOT able to read the oil pressure with my mechanical gauge and see the dummy light show "no oil pressure" at the same time. I didn't have the fittings I'd need to hook that up. So I can't 100% positively say that I've observed good oil pressure at the same time the dummy light was going off. The car did sit for a good 30 minutes before testing with the mechanical gauge. And before I had to drive it for maybe 5 minutes and then come to a stop before getting the dummy light to chime. But I can't see how warming it up a bit more and driving it around for 5 minutes could cause that big of a drop in oil pressure.


Not sure if it is something electrical or what. Before changing the oil, the old oil (with 10,000 miles on it) produced a bit higher oil pressure at idle. The wiring was bad but the pressure was just enough at idle to keep the computer thinking everything is OK. Then I changed the oil and the pressure is now a bit lower at idle. And it is just on the threshold and throwing the low pressure indicator now This seems like a long shot to me because the resistance of the sensor went down to 1 ohm when it triggered at 5 PSI and the resistance stayed at 1 ohm even as I went up to 40 PSI of air pressure in the test. And the 28 PSI I'm seeing at idle is way above the trigger threshold of the sensor.

So I'm kinda stumped. I think the truck can make the trip. It seems doubtful that the state of the motor would change much in one day after changing the oil.

All else I could think of was a bad oil filter maybe? Using Motorcraft FL 500S like always.


Thanks!!
 

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Yes this is a common problem.

Bad sensor (seems to happen during colder temps) or air around the sensor(bad threads/leaking sensor) can give bad readings.

Wipe everything down with a rag, then mix up a 50/50 simple green/water mix and wipe the stuff down using a disposable tooth brush in those tight areas, then wet everything down with a rag/water, before wiping everything dry. I like to hit things with silver paint(if not the entire block) to see leaks easy. But if it is a pain to get to then the above is good enough.

Start the truck and just let the thing idle for 5 minutes, then slowly increase rpm to 2500 over 4 seconds and hold it for a second.
Remove your foot from the accelerator FAST letting the thing throttle body slap closed(expect your low oil light to come on here).
Then hit the gas hard to get up to 4500 before letting off FAST again letting the throttle body slap closed again. Then let the engine idle for a minute or two before shutting off the truck and letting it cool. Doing this with the older oil will let you see it easier. Once figured out a change is recommended.

This causes pressure spikes and if you have a oil leak this will expose them on the first time most of the time.

10,000 miles is a bit much for me. I change my oil and filter(AC or WIX) on all my rides at 3,000 with full synthetic(high mileage) Walmart oil($15/5quarts it is one of the few things I buy at Walmart). My engines are clean when tore apart(usually a head gasket/transmission) and most of them make it to 250 to 350k without any oil issues with credit going to more frequent oil changes.
While it may seem a waste of money to do this at 3,000 instead of 5 or 7 or 10. It also gets me under the thing to check other things. Catching a burnt wire shroud (which this could be) or a leak before it becomes a bigger issue is worth the additional expense.
 

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Discussion Starter · #7 ·
Changed the filter this morning. So far so good. One thing I hadn't mentioned was before I tested the resistance of the wire/connector, I sprayed it out real good with brake clean. It was super oily in there. Maybe that fixed it? I have made a few trips this morning and haven't had the low pressure indicator yet.

I packed tools and stuff to let me mount the oil pressure gauge outside the windshield on the cowl. So if it gives me a lot of alarms on the way down there I may take an afternoon and hook up the gauge. Then at least I can watch the pressure while driving. Its camping so there isn't much else to do :)
 

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10K is a bit much... unless it's full synthetic. That will technically last that long, especially in a n EFI engine in reasonably good condition. But.... the filter usually won't. Any trash in the oil gets caught in the filter, and the filter can clog or become a restriction and open the bypass valve, then the oil is no longer filtered. So if you're going to run that many miles at least change the filter at 5K and top off. Won't hurt to change the filter and top off a 3K or so (two filter/tops between changes).

AMSOIL still sells a dual filter system (or use a single bypass filter and the original filter in combo). They used to advertise "never change your oil!" when using they bypass or dual system. BUT... they recommend changing the filters and topping off at the factory recommended change interval. The 2 micron bypass/second filter is a big 1 quart filter. If you use their two filter system the standard filter is a 1 quart filter also. So you're "changing" just about half to 1/3 of the oil anyway with the two filter system (assuming a typical 5 quart V-8... newer V-8s typically use 6 quarts), 1.5 -2 if you use the single bypass with the stock filter (depending on capacity of stock filter, usually 1/2 quart, sometimes more). It's still a good system and works well -- without the typical oil changes -- ever.
 

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Also remember that with the Ford CAN bus wiring there is no direct connection between the sensor and the gauge. The sensor provides input to the computer, and the computer tells the gauges what to display.

In both GM and Ford vehicles there have been issues with “lazy” stepper motors in the gauges. Computer tries to step the gauge needle up to the right reading but the stepper motor is slow and never gets the gauges up to the correct reading. Happens more often in cold weather.

You can use something like a Scangauge 2 to get the readings directly from the computer, and use that to verify the dash gauges.

Bruce
 
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