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rob's garage


I recently had the oil and filter changed on my 1999 4.0 litre Ford Ranger. I splurged and had all the fluids replaced.

After I left the shop I noticed a change in the oil pressure - my oil was reading too high. It usually reads in the middle of the gauge, somewhere between the "R" and "M" in the word "NORMAL". The reason I noticed a change, was because the needle was pointing between the "A" and the "L".

I turned around and took it back to the shop immediately. They thought there was too much oil in the engine, so they drained a little out then said I was good to go.

Back on the road I had the same reading so I took my truck back again. A different tech checked it over and said the oil pressure sensor was faulty. They replaced it but no change, still reading A and L.

With the hot weather we're having, would this affect the oil pressure? My shop and I are stumped - what's up with my Ranger?


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Two things Roger:

Firstly, I can honestly say that I've never heard oil pressure readings referred to as "R" and "M" as a normal gauge reading and second, here we go again - the "ol shotgun approach" to a repair. I've said it before but I'll reiterate here - If you replace enough stuff, you should be able to fix a problem. Not unlike jacking up the radiator cap and driving a new car under it.

Secondly, it surprises me that in this day and age with access to training, not to mention the Internet, that technicians still can't take a systematic approach to diagnosis. Case in point: Here I am performing via e-mail, what should take place in a shop.

First, we have to understand what creates oil pressure in the first place. Roger, this may be simplistic, but you may have to illustrate this to your mechanic. In this instance, pressure is the result of resistance to the flow of oil. Actually, pressure is the result of resistance to pretty much anything - here's where your demonstration comes in.

Consider a child's wagon. Push against it. As long as it isn't up against anything solid, it will move with little effort (or very little pressure) because there isn't much resistance. Now push against your house. It shouldn't move much - or at all. It will take a lot of effort (pressure) to make it move because there is tonnes of resistance.

Alright, back to your problem. Oil pressure in an engine reacts in the same manner. Blaise Pascal, a 17th century French scientist, created Pascal's Law of hydraulics. He calculated that there was relationship between the force applied to a liquid that was up against a resistance. I have over-simplified this, but you get the idea.

So Roger, to have your oil pressure increase, a few things had to have happened:

  1. The resistance to the flow had to increase. Most of the time this is caused by reduced bearing clearances. This is really bad because it means that some or all of the bearings inside your engine are packing their bags and heading south. Next stop: your bank, to take out lots of money to for an engine re-build.
  2. The oil got very cold. We can rule this out because as you stated, it's hot out there. When it gets hot, oil gets thin and when it's cold out, oil thickens. Thin oil flows easier than cold oil resulting in lower oil pressure.
  3. Someone changed the viscosity of your oil. Although you haven't said what grade of oil was put in your engine, this is the most likely cause of your heart-to-heart with your mechanic.
  4. An oil filter with a malfunctioning by-pass valve may have been installed.

You see, oil grades have quite an effect on oil pressure. Check that little dust-covered book in your glove box; your truck should be using 5W-30 motor oil. Unfortunately, it is common for many oil change stores to buy 10W-30 or 20W-50 in bulk. It is possible that the really thick stuff - the 20W-50 - was poured into your engine. Just like cold oil, thicker grades of oil are harder to pump, therefore, higher oil pressure. This is the easy one Roger. Take this info back to your shop and ask the question. Find out what oil was installed. Failing that, have them replace the oil filter.

It has to be something simple if your oil pressure was R-M when you drove in, but A-L when you drove out.

By the way Roger, the R-M reading should be 40-60 pounds per square inch (Pascal's Law again) at 2,000 rpm.

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