2002 Mercury Mountaineer Low Oil Pressure

  • V8
  • AWD
  • 120,000 MILES
1 month ago the low oil pressure light and gauge kept going on in my truck, I brought it in to mechanic and he changed the low oil pressure sensor switch, I would like to note that this truck idled and drove like a new truck, you couldn't even hear it run. A few weeks after that the truck needed a new tie rod and links, we brought it in and they fixed that problem, in the paperwork a mechanic noted that the low oil pressure sensor and gauge were reacting. My husband then told them that they had just fixed this problem, which they didn't realize until they checked their computer. The mechanic then told me not to worry about that when they get the new part in they would call us and change it, that sometime the sensor switch gets clogged with debris. He told me to keep driving it, a few days later my trucks engine seized, now they want to put in a new engine. What would have been the proper diagnosis and procedure that could have avoided all this money and time that is now being spent on this, and do you believe that they are at fault?
Do you
have the same problem?
Sunday, November 7th, 2010 AT 11:32 AM

1 Reply

Hi john casale. Welcome to the forum. The first problem is assuming someone else did something wrong. We always look for someone else to blame and that is the mechanic who touched the vehicle weeks or months earlier. They are used to hearing the "ever since" syndrome.

The second problem is things always get mixed up between the mechanic who understands cars but has little communication / customer skills, the service adviser who has great customer and communication skills but knows relatively little about cars, and the customer who knows very little about how insanely complex cars are today and doesn't want to hear bad news.

First of all, two different sensors told you the oil pressure was low. That is proof the engine was already coming apart inside. The damage was already occurring but no one knew it yet. There are two ways to approach a low oil pressure indication. The most reliable way is to remove the pressure sending unit and temporarily replace it with a very accurate mechanical gauge to see what the actual pressure is. That takes time which means money. Mechanics understand that customers don't like to spend money so they look for a less expensive route. That would be simply replacing the sending unit to see if that corrects the problem. It apparently did on your truck so there would be no reason to look further.

Oil pressure sending units have been the major cause of gauges reading low and lights flickering on for decades. About 95 percent of the time just the sending unit is defective, and it is not unheard of for new ones to act up too.

I suspect the comment about debris in the new part came from the service adviser, not the mechanic. There had better not be debris floating around in the oil system; that's what oil filters are for. If there is, it came from engine parts that are disintegrating, namely engine bearings. Worn bearings allow pressurized oil to run past them easier than normal. That results in low oil pressure. The pressure will usually read fine when the engine is still cold and the oil is thick and doesn't flow so freely. As it warms up and thins out, it runs out of the worn bearings easier so normal pressure can not be maintained. That also means less or no oil is being sprayed onto the cylinder walls. That's what causes pistons to overheat and seize.

Had the mechanic performed the mechanical oil pressure test at your first visit, the news would not have been any better. He would have identified the low pressure unless he only tested it while the engine was cold. On some engines that are still running properly, it is possible to just replace the crankshaft and connecting rod bearings without removing the engine from the truck. That would get the oil pressure back up to normal but it would not address the wear that has already taken place on the pistons and cylinder walls. There will also still be metal particles circulating in the oil passages. Those particles can lodge in the soft metal of the new bearings and start chewing them up again. The repair might buy you as little as a few days of extra engine life, but more likely will get you a few years.

An engine that is running unusually smoothly and quietly is a sign of low compression caused by worn piston rings. Those worn rings cause less than normal friction so the engine doesn't have to work so hard to maintain idle speed. They also allow a lot of "blowby" where exhaust gases sneak past them into the oil. That forms sludge in the oil more rapidly than normal and reduces the oil's lubricating properties. That accelerates the wear on engine bearings.

My guess is the service adviser told you there might be debris in the new sending unit to justify replacing it at no charge without admitting they sold you (what he thought was) a defective part. It's one of those little white lies that keeps customers happy and is otherwise harmless. The problem is the fib sounds plausible because it really is something that can happen but only on very rare occasions. When it does, as appears to be the case here, the disintegration of parts is already in progress. Most people would not believe a mechanic who said, "your engine is running very smoothly and quietly, there is debris in the oil, your engine is in the process of self-destructing, and you need a new one". We would assume he was trying to rip us off.

What led up to the engine seizing? Did it make any unusual noises? Any smoke from the tail pipe? Did it gradually lose power? Did it overheat? If it suddenly stopped running without making any noise, similar to just turning off the ignition switch, you may have a stalling problem. That is very common on any brand of vehicle and is way different than a seized engine.

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Sunday, November 7th, 2010 AT 3:20 PM

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