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My car won't go past 55mph
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Troutman
New User
May 2, 2007, 12:00 PM
Post #1 of 5
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My car won't go past 55mph
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 Hello. I have a quick question: I have a 2002 Pontiac Grand Am with 103,000 miles. Lately, when I take the car up to highway speeds (over 55 mph ), the tachometer gets stuck and the car's engine will not stop revving. I can get the car to stop, I turn it off, and the tachometer works properly. There are no "Service Engine Soon" lights or any or clues as to what the problem might be. I've taken the car to several mechanics, and they all say that they can just start replacing parts and hopefully they will finally replace the correct part. Any ideas of which part makes sense to start replacing, before I pay for a new car? Already replaced the MAF sensor, that wasn't it.
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Tom Greenleaf
Ultimate Carjunky
/ Moderator
May 2, 2007, 5:54 PM
Post #2 of 5
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Re: My car won't go past 55mph
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I'm missing something here?? Do you mean the engine keeps reving to the actual speed but the tach doesn't go up with it. Does the speedometer say you are going faster or does this feel like it's slipping T
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Guest
Anonymous Poster
May 2, 2007, 7:37 PM
Post #3 of 5
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Re: My car won't go past 55mph
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The engine keeps revving up to 5000 rpm and goes down to 3000 rpm continually, but my foot is staying steady on the gas peddle. My speed is staying the same and the speedometer is working fine, it doesn't say I am going faster or fluctuates. Also every time I go up a hill I lose acceleration bad. I have to manually downshift to get the car to move.
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Tom Greenleaf
Ultimate Carjunky
/ Moderator
May 3, 2007, 4:57 AM
Post #4 of 5
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Re: My car won't go past 55mph
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OK-- This is an automatic - Right?
Either way just tossing parts is an expensive way to approach the problem. If mechanics are frustrated with nothing testing wrong in front of them they need to take this car for a ride with you and experience the problem which should at least narrow it down.
If you are willing to repair this yourself I would start with a vacuum test with a "T" at intake manifold vacuum source.
You can get a hand pump for making vacuum that has a gauge on top and you have a reading and a tool for checking other things and for future use.
Manifold vacuum should be around 18Hg at idle. It should drop off suddenly if you just rev up the engine quickly and return even with a momentary Hg of perhaps 22 and go back to the regular idle Hg.
Now -- hold engine RPM at 2000 and the reading should be the same as at idle. If lower it's a strong indication of an exhaust restriction. From there you would or a tech would then isolate where the restriction is. Then find out why because if a plugged converter is causing the restriction , for example, why is it plugged? It may fix the immediate problem but not the source of the problem.
Note: Hg = Mercury and it's a measure of vacuum called inches of Mercury or in. of Hg.
Again - just throwing parts at it is futile and with Murphy's luck it would be the last thing you try. Shops should be doing better than that,
T
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Guest
Anonymous Poster
May 4, 2007, 12:27 PM
Post #5 of 5
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Re: My car won't go past 55mph
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I did the process of elemination myself. I was just praying it was not the trans.
 My car is fixed.
My Cat. Converter was clogged bad. That's what is causing the chugging and why there was no codes. Could have save some more money if I would have known that first. LOL Oh well, at least now we're done chasing the problem.
I found out some more info. On the internet.
Look on the sticker on the driverside door it will tell you the date when the car was built. If it was built in 2001 up until i think the begining of 2002 you have at least a fighting argument, since they didn't change the "weak" parts until halfway through the production of the 2002 model year. Thus the 2002.5 grand am's. But some of the early model 02's had the same problems as the 01's. GM does warranty the Cat. converter up to 80,000 miles, but of course under Murphy's Law mine was at 103,000 miles. I hope this will someone else in the future.
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