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2008 Mercury Grand Marquis - Unexpected acceleration

unexpected car acceleration when slowing down

What was your mechanic’s diagnosis?

were you wearing boots at the time and accidentally hit the accelerator instead of the brake?

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Dirty throttle body or faulty IAC can do this.

Whenever someone claims that an unintended acceleration occurred, people always trot out this explanation, and the automakers themselves attempt to blame floor mats or careless drivers for the resulting accidents. However, there is a major hole in these excuses: these unintended acceleration events almost always occur on vehicles equipped with a “drive by wire” throttle body.

If floor mats or clumsy-footed drivers were to blame, then logically the problem would be just as prevalent on older cars with a physical connection from the accelerator pedal to the throttle body. The fact that it’s almost never a problem on the older design tells me that what most likely occurs–despite the automakers’ excuses and speculation–is that a worn spot in the accelerator pedal position sensor can result in “potentiometer noise” which tells the PCM that much more acceleration is desired than what the driver intends.

If this scenario occurs, there is no inherent safety like there would be with a traditional cabled throttle body. If the throttle position sensor on a vehicle with a cabled throttle body fails in such a way as to specify rapid acceleration, while the driver is barely pressing (or not pressing) the accelerator, then the air/fuel mixture will become too rich, and the engine will stall. It won’t “run away” because the mostly closed throttle plate will prevent that. With drive-by-wire, the computer controls the amount of fuel and air provided to the engine, so this won’t happen.

Even a brake override feature in the PCM is a poor substitute for the inherent safety provided by the older design, but obviously much better than nothing. For this reason, I would only buy an older vehicle with a physical connection between the accelerator pedal and the throttle body. Newer is not always better, and sometimes it’s a lot worse.

"bcohen2010 Newer is not always better, and sometimes it’s a lot worse.

Agree 100% new is not always better sometimes it is worse.

Almost all cases are traced to driver error or jammed floor mats, sometimes stuck throttle bodies that needed cleaning. Eliminate those 3 possibilities first.


I asked a question…didn’t say it explained the issue. When you have 13.5 feet like I do, this is a possibility to look into.

I think they did find that in some cases, the floor mats did get bunched up cause a problem

I think you’re worrying more about an extremely rare occurrence because of your fear of modern technology. I’ve never had this happen to me (despite my big feet :slight_smile: ) despite driving cars from the 80s-today. (also have never had my power windows fail, or my automatic locks go bad. The cylinder in my ignition in my Focus went bad, so my father-in-law and I jury rigged a push button start system instead! And that was still going no problems up until it met it’s demise.

Looks like that Ford lawsuit was dismissed:

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I agree, but there’s one other very important thing to add.

I tell anyone that brings this issue up that you can always push the shift lever into N. There’s nothing to stop you, just whack it into N. No buttons, no levers, nothing. It will slide there easily. If you are worried about the engine “blowing up”, first of all are you more worried about the engine than you are about dying? Second, it won’t “blow up” because all the tech stuff that makes this runaway possible will keep the engine from over revving. So just put it in N, let it keep running so you have power steering and brakes, and coast to safety. When you are stopped completely, turn off the engine with the key.

We make ZERO attempts to actually train people in how to handle emergencies in cars. It’s beyond stupid.


We make ZERO attempts to actually train people in how to handle emergencies in cars. It’s beyond stupid.

Agree I have also seen preventable accident’s by people forgettig to use the E brake and what the E stand’s for they seem to think it is a parking brake only.

We could go on and on about this. Instead people are carefully taught how to parallel park. How about how to survive a blown out tire. How to regain control of a car if you nod off and wake up on the grassy center divide. What to do if another vehicle hits your and you are still moving. How to survive aquaplaning. Lots of topics, and lots of people have been injured or died because no one ever took the time to help.

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I agree with you 100% IMHO people are wanting the car to do it all for them I also thig that every time I see people making stupid mistakes that there should be a required driving course after they get two ticket’s. I blame cell phones for much of the stupidity I see on the road.

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I don’t know how you feel about it but I think it is worse in town than it is on the open road.