I'm not sure just when Ford went from a "real" oil pressure gauge to a idiot
light with an analog readout, but it probably was late 80s for the truck lines.
If there's a button type sender at the oil pressure output port, it's a fake
gauge that closes a switch around 7 psi and then reopens it at say, 5 psi. Put a
mechanical gauge on the oil pressure port and see if the readings are consistent
with the factory "gauge". If so, you have problems, ranging from a dirty sump
(oil pickup screen gets clogged) to worn main bearings or crankshaft. If not,
it's the gauge itself, the sender, or the sender wiring.
No, they had idiot lights back in 1949 or so; I'm talking about the fake gauge
that's driven by an on-off switch like the idiot light, but appears to be an
actual needle-type gauge. That piece of trickery didn't happen until the late
1980s on Ford trucks, and Ford-made cars with apparent gauges in the early
Well I have an '82 pickup or bronco, or an 89 pickup, take your pick, with a
factory idiot GAUGE, not the light! If you need a picture the gauge and
it's sending unit, I'll gladly post if on a binary Usenet group for you. I
don't have my 71 truck anymore, The oil pressure gauge in that one may have
been after market.
Check the sender on the '89. It's most likely the button-type, and the "gauge"
in the cab is a fake. If it is a button (snap-switch) sender, look at one of the
Ford truck websites for a how-to conversion to a real gauge, or if it doesn't
bother you, do nothing.
With the audible noise, likely the gauge is NOT lying to you. As
another poster told you, probably sludge is being sucked up to the oil pump
pickup screen & causing engine to starve. Shutting off engine allows sludge
to 'fall away' from the screen & let pressure go back up when you next start
it--then the cycle repeats.
Some will say to have pan removed, remove & clean/replace screen/oil
pump/oil pan. Much quicker, cheaper, and quite effective procedure would be
to drain oil on Friday evening. Replace plug. Pour in 4-5-6 qts.--equal
amount to # of qts of oil it takes-- of carb cleaner (the kind that is used
to soak entire carbs in). On Monday evening, drain oil pan COMPLETELY.
This brings out now-diluted sludge along with the carb cleaner which has now
washed the pan & oil-pump screen and to some degree, the pickup tube.
Replace plug & refill with oil and change oil filter too--just to say
you did. Drive truck for few minutes until it reaches full operating
temperature, bring it back and change oil & filter again IMMEDIATELY WHILE
Problem solved! It works, I know, 'cause I've done it several times!
Donate $ saved to your local church/charity. s
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