Started to rev high at 6000 RPMs

Tiny
JEREMY74
  • MEMBER
  • 2005 CHEVROLET CAVALIER
  • 182,000 MILES
I have a 2005 Chevy cavalier. A few days ago it started to rev high at 6000 RPMs and then fell down to less than 1000 RPMs it started to lunge forward when in drive, and then stalled. Can you please help me?

I have checked the Battery and alternator they are both fine, and both new. My car a few days ago as soon as I started it it revs really high, then settles @ approx 1100-1400 RPMs. Then yesterday It started surging and in D tries to lunge forward and does the same in Reverse.

Then RPMs drop to 300-500 RPMs then pulsates for about a minute then stalls. Starts back up easily and repeats it's self. Could this be due to a clogged fuel filter?

Thanks Jeremy
Do you
have the same problem?
Yes
No
Sunday, October 30th, 2011 AT 4:06 PM

4 Replies

Tiny
CARADIODOC
  • EXPERT
The engine is getting too much fuel. A plugged fuel filter won't allow that to happen. Battery and generator have nothing to do with engine speed except that GM has a real miserable generator design that causes it to produce a lot of voltage spikes. Those spikes are responsible for repeat failure of the generator's internal diodes and voltage regulator, and they can interfere with various computer sensor signals. The battery dampens and absorbs those spikes but as they age, they lose their ability to do that. Replacing the perfectly good battery will prevent those problems. It will work fine in a 1986 or older model that has the better design generator. For intermittent running problems, if the generator is suspected as interfering with the computers, simply unplug the small connector on the side, then see if the running problems clear up.

A more common cause of high idle speed is a vacuum leak. Look for a leak in the tube between the mass air flow sensor and the throttle body. More likely, look for cracked or disconnected vacuum hoses on the intake manifold. You can use a spray bottle while the engine is still cold and running to spray water on the hoses and intake manifold gaskets. If the engine suddenly slows down or you see the water getting sucked in, that leak will allow air into the engine that the mass air flow sensor and Engine Computer don't know about. That will cause an increase in engine speed but not an increase in power.
Was this
answer
helpful?
Yes
No
Sunday, October 30th, 2011 AT 5:57 PM
Tiny
JEREMY74
  • MEMBER
Hi, thank you very much for your quick response I really appreciate it. I had tried the water, and as the car is running at idle the RPMs go from 1500-2000 up down up down. And when I put it in any gear it stalls. Do you have any other suggestions?
Thanks again.
Jeremy
Was this
answer
helpful?
Yes
No
Sunday, October 30th, 2011 AT 7:28 PM
Tiny
CARADIODOC
  • EXPERT
Try blocking the fresh air tube before the mass air flow sensor, if possible. If the engine continues to run, it has to be getting air from someplace else, and you have to figure out where.

If the engine stalls when you do that, the next step would be to connect a scanner that displays live sensor data while the engine is running. Of particular interest is the idle air control motor, (IAC). The Engine Computer sets that motor to various positions to open an air valve that lets air bypass the throttle blade. At the same time, it changes the amount of time it holds the injectors open during each pulse. That's how it controls idle speed. The IAC will be shown as a percentage or as the number of "steps" from 0 to 256. The higher the step or the higher the percentage, the higher the idle speed is being commanded to.

If you find the step real low, say below 25, the computer knows the idle speed is too fast and is trying unsuccessfully to reduce it. The IAC itself could be the cause. Electrically it is okay, otherwise the Engine Computer would detect the problem, set a fault code in memory, and turn on the Check Engine light, but carbon buildup could be preventing the motor / valve from responding properly.

If the step or percentage is real high, the computer is trying to raise idle speed in response to some other sensor reading. It usually takes an experienced mechanic to interpret those other sensor readings and to find the cause of the high idle speed. Ford had a lot of erratic idle speed problems caused by defective coolant temperature sensors. Idle speed is raised when the engine is cold, and the Engine Computer tried to respond to varying readings coming from that sensor.

With all of these things, it's still real hard to hit 6,000 rpm. Most engines will have severe "valve float" and misfires long before that speed is reached. Most manufacturers have gone to the insanely dangerous "throttle-by-wire" system that replaces the simple, reliable throttle cable with a computer, motor-driven throttle blade, and two position sensors. That is the system that put Toyota in the news last year, just like we predicted. If you have that system, leave the repairs all to the dealer. You don't want to become a party to any future lawsuits by tinkering with it yourself.
Was this
answer
helpful?
Yes
No
Sunday, October 30th, 2011 AT 9:42 PM
Tiny
JEREMY74
  • MEMBER
Thanks again so much.
Was this
answer
helpful?
Yes
No
Sunday, October 30th, 2011 AT 10:14 PM

Please login or register to post a reply.

Recommended Guides