For years, the oil pressure gauge in my 68 J-code XR7 always seemed to start at about 50 psi cold and then drop to halfway between the 25 and 50 markings when warm. As I was driving, I could watch the needle move back toward the 50 mark as RPM increased. But for the last month or so, I’ve noticed that the needle now starts out at the halfway point when cold, then drops to 25 psi when warm. Also, now the needle barely moves in response to engine RPM. I know these gauges are notoriously inaccurate and that I need to install a trusted mechanical gauge to see what’s really happening, but is this something I should be worried about at all? The motor runs fine, temp stays cool and I can’t hear any knocking, clicking, rattling, etc. On my most recent long distance trips, she burned only about a 1/2 quart of oil over the course of about 2000 miles. The oil looks clean and does not smell of gasoline. The only thing I did differently with the last oil change (about a month ago) was to add some of that zinc additive stuff.
First some basic questions.
What oil (brand, weight etc) is in the engine and is it the same exactly as you were running before the oil change?
Did you change the filter and did you use a different filter? What filter are you using (brand, number etc) and if different what was the previous one?
Exactly what was the additive you put in (brand, number etc)?
My 68 XR7 had a bad oil pressure gauge for 18 years! I replaced the pressure switch on the engine many years ago to see if that would solve the problem…to no avail. Checked the pressure with a mechanical gauge and the pressure was fine. My guage read 10 psi when cold and zero when warn. I just recently changed out the motor and got a used replacement oil pressure gauge from WCCC and when I fired up the motor the pressure was good (over 40 psi) until I ran the motor for about 10 hours. Then the gauge showed zero again. What I foubd was that a piece of teflon tape got into the hole on the switch to tell the gauge how to move the needle through the resistance of the switch. Just a thought…don’t rule out the basics!! Including the wire connections on the switch and the gauge.
This is frustrating! I tried to take the sending unit off so I could hook up a mechanical gauge and managed to round off the hex part. No problem, I thought, I’ll just unscrew the whole extension/sending unit together. Wrong–because of the angle of the sending unit, I could only turn it a quarter turn then it hits the filter. I guess I’ll just wait until it’s time for an oil change and have at it when the filter is not in the way.
For what it’s worth, the last oil change went from 10w-30 to 10w-40. I think the zinc additive was STP brand but I’m not 100% certain. I always use motorcraft filters.
Last night I noticed light shining in the glove box and found that one of the oil gauge lights had fallen out of the housing. I know that these gauges operate by heat and so I wonder if the extra cooling from the open hole could cause a lower reading?
The lights in the guages are just lights to see the guages in low light situations. They have nothing to do with the operation of the guage. The guage reading increases as the electrical resistance of the sending unit decreases from the oil pushing against the sending unit internals.
I had a 68 XR-7 that the oil gauge never went above 25 and hung around 0 when idle and hot. A 67 std. that had a flickering oil light at idle. These cars were work/winter cars so I wasn’t to concerned. I was curious enough to drop the oil pans on both and found the oil pickup screens were mostly plugged with goo. Probably poor oil change schedule over the years. Cleaning the screen on the 68 brought the gauge up to between 25/50 and never went to 0 again. The 67 had over 100k miles so I replaced the oil pump while I was in there. The light never flickered again.
Sending unit was the culprit. All is well again with my oil pressures. 