"Surging" during normal acceleration, braking, and idling?

[CurseTheSky]

My dad recently gifted his 2003 Malibu 3.1L V6 with 140k miles to my girlfriend to replace her problematic '95 Eagle Vision. Even with 140k on the odometer, it still runs very smoothly and hasn't run into any major problems, just a few quirks like this one.

Today, her second day driving it, she said the car felt like it was "pulling forward" and "fighting her" when she was trying to brake. I drove the car afterwards, and indeed it does seem to be accelerating randomly. It only seems to happen after the car gets up to a certain speed and decelerates afterwards (50-65; we normally notice it after getting off the highway), and it's still intermittent after that. It might accelerate during one mile stretch and stop for the next three miles.

It's not hard acceleration, just enough to make it noticeable; during normal driving it's roughly the equivalent of pressing the gas an extra 1/5th of the way down, even during braking. I shifted it into neutral while it was in the middle of its phantom acceleration a few times, and it leveled out around 3-3.5k RPM. I also noticed that the cruise control mysteriously stopped working; I can hit the button to turn cruise "on," but setting the speed (which is what actually kicks cruise on) no longer works. The gas pedal doesn't stick or depress on it's own. Otherwise, the car accelerates normally, no noticeable power loss, odd shifts, etc.

The CEL isn't on, but the ABS light is. I believe the ABS light is just for a wheel bearing that's getting ready to go (the sensor is in the same area), since that's exactly the same thing that happened with my '03 Malibu (different car).

Any ideas? Computer? O2 sensor? Cruise malfunction? Sticking throttle cable / linkage? Poltergeist?!

Thanks.

 

[BlackTigers]

MAF? Maybe,,,?

Is the idle really jumpy? Or abnormally high or low at times?

 

[boomerang]

If it was mine I'd check the throttle body for carbon build up. It could be holding the throttle plate open. It's a very common problem for many modern cars.

 

[LTC8K6]

I'd check the IAC valve if it has one. This controls the idle speed.

 

[kalrith]

My wife had a Mazda MX-3 that would do this when the cruise control was activated. It would just randomly start accelerating until I tapped the brake to disengage the cruise. However, it never did this when we didn't have the cruise activated. If you're concerned that it's the cruise, then you could tape down the cruise-disengage button (have no clue what it's really called) underneath the brake pedal, and that should keep the cruise from working at all (assuming that the disengage button itself is working).

 

[LTC8K6]

If the cruise control uses a cable, you can disconnect it's throttle cable.

 

[exar333]

Did you check the floormat?

(/sarcasm)

+1 to try disabling cruise to see if this helps.

 

[CurseTheSky]

The throttle isn't jumpy / jerky, it's very steady. The best way to describe it is if you were driving with a small vehicle behind you, very lightly pushing on your bumper the whole time. You can still stop if needed, but it takes extra effort, and you end up braking constantly while just keeping up with the flow of traffic (coasting is impossible).

The cruise control does use a cable, so I'll look into disconnecting it. I'll also try pulling the cruise control fuse, which will hopefully disable the system. Otherwise I'll check MAF, IAC, and look into cleaning the throttle body. If that doesn't work, I'll get a priest and perform an exorcism.

Thanks guys.

 

[LTC8K6]

Could this be the ABS activating? When it quits, it could feel like a surge forward.

 

[CurseTheSky]

Originally posted by: LTC8K6
Could this be the ABS activating? When it quits, it could feel like a surge forward.

Nope, it actually revs to 3-3.5k RPM in neutral when it's exhibiting the problem.

 

[LTC8K6]

One other thing would be a bad Throttle Position Sensor, TPS.

 

[exdeath]

If the engine itself is actually surging or idling high, even in neutral, then it isn't a false sensor reading, there is actually extra air entering the engine allowing it to create power and speed up. False sensor outputs without an actual condition would cause stumbling and hesitation type problems, the engine wouldn't just accelerate smoothly because of a bad sensor without actually having air to speed up with.

Start with the throttle. If you can get it to misbehave while parked, reach in and force the throttle in the closed direction and see if moves any or dies down.

If you know how to use a scanner you can determine where the vacuum leak is based on the fuel trims, idle control position, throttle position, and MAF voltage. If the MAF voltage is increasing while the idle control counts are very low and fuel trims are normal, it indicates an "internal" leak at the throttle and air that is being metered (fuel adjusted by MAF), pointing to a throttle or IAC problem. If MAF voltage is low and fuel trim is high (fuel adjusted by O2), you have a vacuum leak introducing unmetered air behind the closed throttle/IAC.

 

[DilipVasan]

Sorry to post here again I'm facing the exact same issue on my Punto 1.3 this is a new car and the service people are clueless about this and there is no CEL . Can someone please provide me some pointers if you had been able to solve the problem as to what is causing this It would be really helpful as I don't want the service people to rip the car without any clue or overcharge me as I have no idea about the detailed mechanical stuff involved.

 

[DilipVasan]

Sorry to post here again I'm facing the exact same issue on my Punto 1.3 this is a new car and the service people are clueless about this and there is no CEL . Can someone please provide me some pointers if you had been able to solve the problem as to what is causing this It would be really helpful as I don't want the service people to rip the car without any clue or overcharge me as I have no idea about the detailed mechanical stuff involved.