No oil pressure when cold

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Dec 14, 2003 | 11:58 AM

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No oil pressure when cold

2001 F150XLT SC Auto 120000 miles
Yesterday was a very cold morning (-5c) and when I started up I noticed the oil guage would not come up. After 10 seconds I shut down and restarted and it came up immediatley. This morning it was 20 degrees warmer and it stayed down after 3 restarts. I left it idle because engine was not making any unusual noise, opened the hood, it stayed quiet, no valve lifter noise, and after a minute or so looked at the guage again and it was OK, in it's usual spot over halfway up. Could this be a sender problem? It only does it on a cold start. I'm using Motorcraft 5w20 semi-synth oil. With 120000 miles on the truck do you think I should try switching to 10w30 for winter use? The truck uses no oil, the coolant level has always stayed the same, it still runs like brand new. TIA for your help.

Dec 14, 2003 | 06:29 PM

Bob Ayers

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First, welcome to FTE!!!!

You didn't mention checking it, so I'm going to ask the most obvious question, does it have oil in it?? If it does, then I would
suspect the oil pressure sending unit (switch). I don't know if you
are aware, or not, but your truck has an "idiot light" type of oil
pressure gauge. The oil pressure sending unit is a normally open
switch, that closes with about 7PSI of pressure, completing the
ground path circuit of the gauge, causing it to register about 1/2 scale. It DOES NOT show fluctuations on oil pressure.

Dec 15, 2003 | 08:29 PM

travlnman

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From: North Carolina

I agree, also I would not go with a heavier oil it will only make your problem worse in those kinds of temps. Stick with the 5w30, if you think it's too cold for that, Mobil makes a 0w30. I bet if you asked some friends and co-workers, their all using 5w/or10w. Also if it has been unusually wet and cold, I would suspect that you have a bad connection at the sending unit causing a temp loss of signal. If there's no oil flowing at that temp she's a gonna rattle... a lot.

Dec 17, 2003 | 11:32 PM

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Quote:

Originally posted by Bob Ayers

The oil pressure sending unit is a normally open
switch, that closes with about 7PSI of pressure, completing the
ground path circuit of the gauge, causing it to register about 1/2 scale. It DOES NOT show fluctuations on oil pressure.

I wasn't aware of that, I know they used to use a variable resistor/diaphram sending unit with the electric guages. What's the primary use for the guage, then, peace of mind? They may as well paint a line on the guage. I guess it's just as well though, I had to replace sending units on my 89 F150 302 every two years, a friend rebuilt his lower end first and then found out it was only his sender.

Dec 18, 2003 | 05:19 AM

Bob Ayers

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Supposedly, there is a 20 ohm resistor in series with the gauge that sets the current when the oil pressure switch closes. This
current causes the gauge to read half-scale. I have a 1999 Ranger 3.0L FFV, and I could not find the resistor, so all I did to
see fluctuations in oil pressure, was to change out the sending
unit "switch", with a "REAL" analog type of sending unit. If you
have the 20 ohm resistor, you need to short it out (wire across it) when you convert to the analog type of sending unit....

Dec 18, 2003 | 06:19 AM

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Does the idiot light have it's own switch?

Dec 18, 2003 | 06:34 AM

Bob Ayers

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From: Durham, NC

You should have a "check gauge" light, which uses a circuit that has input from all your gauges. Changing from a switch to an
analog type of oil pressure sending unit will not effect the "check
gauge" light.