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Oil Pressure Gauge Question

  #1  
Old 03-04-2014
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Location: San Diego
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Oil Pressure Gauge Question

Hello all,
So I have appx 1,500 miles on the 2002 Ford Ranger I recently purchased. I just replaced the oil and filter. I went with Full Synthetic (Valvoline... they didn't have Mobile 1 in 5W20 in stock.... i'll switch to that first chance I get).
Today, after driving a couple hours, i was at idle and and noticed the oil pressure gauge "twitching."
I did a little research online. First, I have learned that the gauge on these is basically a glorified dummy light (only registered pressure and goes to half point).
Second, I have found countless forum discussions of this indicator twitch issue, suggesting it's pretty common.
My concerns:

1) Having just changed to Synthetic, I worry that the two prior owners (it now has 81,000 miles) never used Synthetic... that now having it could dislodge debree... and this debree could clog the filter and impede oil flow.

2) That I could have overfilled the engine with oil. Most of what I have read suggests that these 3.0L V6's use apps 4.5 quarts of oil. I used the entirety of the 5.0 Quart jug I purchased. However, I carefully checked the dip stick while filling. And today it still hits right at the max line. So not sure why it used all 5 quarts. And not sure if I had in fact overfilled if this could cause the gauge to twitch.

Any advice appreciated!
 
  #2  
Old 03-05-2014
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Location: Vancouver, BC
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Moving oil pressure gauge needle would be an electrical "twitch".

The oil pressure switch closes at 6psi(putting gauge needle at around 1/2) and opens below 6psi(needle drops to low)

The switch is a ground, which is why it just has 1 wire, when switch closes the engine block ground is passed to the gauge.
The gauge also has a ground, but with two resistors inside, so with key on engine off the oil pressure needle will come up to low.
When engine is started and oil pressure is 6psi, the switch closes and this new ground is connected between the two resistors so needle only has 1/2 the resistance and needle moves up on the gauge.

So a couple of things can cause a 'twichy' needle, the switch wire/connector might be loose or dirty.
People often add sealing tape on the threads of 1 wire senders, bad idea, the threads, at least at the bottom, need to be clean and bare to get a good ground.
Either the engine ground and instrument panel ground might be loose.

The alternator might be fluctuating voltage, you can test this with a voltmeter at the battery with engine running
 
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