Possible Causes of Low Oil Pressure Reading:
* Low Oil Level: This is the most common and easiest to check. Check your dipstick immediately. If the oil level is low, add the correct type and amount of oil specified in your owner's manual. Then, check the pressure again *after* running the engine briefly (but don't let it run if the pressure remains low).
* Failing Oil Pressure Sensor: The sensor itself might be faulty, giving an inaccurate reading. This is a possibility given your description of the pressure fluctuating.
* Worn Oil Pump: The oil pump might be failing to effectively circulate oil. This is a more serious mechanical issue.
* Clogged Oil Pick-up Tube: The tube that draws oil from the oil pan might be clogged with sludge or debris, preventing the pump from getting enough oil.
* Blocked Oil Filter: A clogged oil filter restricts oil flow. Replace it immediately, as this is a relatively inexpensive check.
* Worn Engine Bearings: Worn bearings in the engine allow oil to leak out of the critical areas of lubrication, leading to low pressure. This is a major repair.
* Leaky Oil Cooler: If your engine has an external oil cooler, a leak in it will reduce oil pressure.
* Thick Oil: Using oil that's too thick for the temperature can also result in low oil pressure.
Locating the Oil Pressure Sensor:
The oil pressure sensor on a 1991 Ford Aerostar 4.0L V6 is typically located on the engine block near the oil filter. It's usually a small, threaded sensor with one or two wires connected to it.
To pinpoint its location:
1. Consult your owner's manual: It might have a diagram showing the location of the sensor.
2. Use a repair manual: A Haynes or Chilton repair manual for your specific year and model will have detailed diagrams and instructions. These are available online and at auto parts stores.
3. Online forums: Search online forums dedicated to Ford Aerostars. Someone might have posted pictures or descriptions of the sensor's location.
What to do NOW:
1. Check your oil level IMMEDIATELY.
2. Do NOT drive the vehicle until you've determined the cause of the low oil pressure. Continuing to drive could cause catastrophic engine damage.
3. If the oil level is low, add the correct oil. But again, *do not drive it until you have investigated the cause of the low pressure*.
4. Replace the oil filter. It's inexpensive and could solve the problem.
5. Consider having a mechanic diagnose the problem if you are not comfortable with mechanical work. Low oil pressure is serious.
Ignoring low oil pressure can lead to a costly engine rebuild or replacement. Address this issue promptly.
