Oil pressure Sending unit questions

AHSOM50

New Member
Jan 19, 2006
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Mill Creek, WA
Hi- This is my first post here!
:SNSign:

I just picked up an 89 5.0 GT ragtop with 82k miles on it.

After the car is warm, the Oil Pressure is running low- down around the Second mark when cruising, and the first mark when idling (bottom white mark, not the bottom red mark)

I did a search and read through a lot of posts here and it sounds like the sending units going bad are a pretty common thing... and that sounds good because an engine with 82k miles should NOT have low O/P....

My question is- in some of the posts, I read that the sending unit is basically just an On/Off switch, and even though there is a guage- it does not acutally fluctuate with RPM...

Mine DOES fluctuate with RPM right now- it acts just like all the O/P Guages I have ever seen- reads higher when cold- and goes up and down with RPM, and reads a little lower when idling warm.

Do the replacement senders NOT fluctuate?
I dont' just want to stick in an "On/Off" sender that will reads good right up until it drops below 5lbs (like an idiot light does)

anyone have any details on this?

BTW: Just changed the oil and still ahve the same results.
its running fresh 10w30 & A Napa Gold filter

Thanks-
Ben
 
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The stock gauge is very hysteretic. If you dont want to install an aftermarket gauge, a new sender (for 10-15 bucks) is worth doing.

The needle on the stock can move around, but the calibration is not really there.
Good luck.
 
Well, you can't really make a good call about your oil pressure without a real gauge (Autometer, etc.) The gauge isn't an on/off switch like the later Mustangs are. Those just sit in the middle as long as there's over 6psi hitting the sending unit.

The foxbody's have "functional" ones that actually move, but like Hissin said, they aren't exactly the most accurate (especially since there aren't numbers to go by, just a H and L.)

Depending on the condition of the motor, I would try perhaps running a thicker oil, like an Xw40. This should help your low pressure at hot idle if indeed it is low.
 
So... I guess i fixed it-
After reading a bunch of posts about it- i went out- pulled the wire off the sending unit- tapped it a coupld times with a wrench- put the wire back on...

No more low oil readings... now it reads good- Just dips a hair below the mid mark now when idling at temp.

So I guess the sending units really are pretty flaky.
Oh well- I think that low O/P reading was the reason I got a good deal on the car- so I can't complain too much! :D

Thanks!
Ben
 
I had a similar issue with my '90. On a cold start, the oil pressure would be halfway b/t the white line above the mid line. After about 20 misn of driving the oil pressure would drop to midline. At a stop it would drop to the line under the midline. I changed the oil pressure sending unit, and oil pump, and the car still has a flux in oil pressure. Not as much now, normally less than a line difference as compared to the line and a half difference earlier, but it still fluxuates...

Moral of the story is get an aftermarket guage :nice:

To answer your question, at least in my experience, changing the oil pressure sending unit will still let the guage fluxuate based on oil temp.