What does it mean if a car's idle revs up while waiting at a red light?

Question

I'm thinking of buying a used car. When it is stopped at a red light, the idle revs up. If I shift into neutral, the idle stops revving. Does this mean there's a serious problem? I can't afford to put any real money into the car at this time. Thanks.

Answer 1

More than likely it is a vacuum leak. Just a simple hose replacement. The hardest part is finding it. You can start by pinching off vacuum lines and if the idle drops then the leak is probably in that line. Might also be the TPS (throttle position sensor) depending on the type of car should be under $40. If the check engine light is on then checking that out will help.

Answer 2

Mstigerlilies is obviously bs-ing you because a high reving engine will not cause the rad to blow lol!

Anyway, if the car raises the rev by a few notches (in other words, doesn't rev higher then 1000 rpm) then its normal if you have an automatci transmission because it revs up to keep the car from stalling. If the rpms are 'loping' (going up and down constantly) then you have a vacuum leak. Not a serious problem.

On the other hand, you should check your throttle position sensor and the oxygen sensor. These could be the problem. Again, not serious.

Answer 3

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RE:
What does it mean if a car's idle revs up while waiting at a red light?
I'm thinking of buying a used car. When it is stopped at a red light, the idle revs up. If I shift into neutral, the idle stops revving. Does this mean there's a serious problem? I can't afford to put any real money into the car at this time. Thanks.

Answer 4

I'm no mechanic, but I had a Honda Accord once that did this. I ignored the problem and the one day it revved up to to the red and the radiator exploded! This green goop blew up through the top of my hood and all over my windshield. The lady next to me told me that something was leaking from my car... thanks lady!
Anyway, the radiator ended up splitting at the seam and it was not repairable. We had to replace it.
See if you can take it in to a mechanic and have them do a diagnostic. These are usually very cheap ($20 or less) and they will tell you exactly what is wrong and how much it would cost to fix it (if you were to buy the car).
Even if it's rather expensive to get the problem fixed, it will probably not be as expensive as replacing the radiator and all of it's hoses.

Answer 5

it is probably your PCV valve which is very easy to change and also very cheap but, very inportant to the performance of your car. it is on the top of your valve cover and just pushes in by hand or pulled out to remove. It only costs about 6 dollars, when you buy it i am sure the parts man will show you were it goes and mybe even do this for you since it is so easy. the name is positive combustion ventelation valve, good luck

Answer 6

It means you have a vaccum leak. Check all of your vaccum lines. You will find air sucking somewhere. Sounds like a pretty large leak. It will be fairly obvious and you may even hear it whistle. Good luck.

Answer 7

I was gonna say It could be the oxigen sensor.. when the car gets hot it well stall out when you come to a stop. Id check that agian Just to be sure

Answer 8

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Oxygen Sensor is going bad it sounds like.

Answer 9

as long as it doesn't rev up too far it should be ok...as long as it's <1000rpm.

Answer 10

It may simply be that one of the vacuum lines is loose.