What could cause an RPM jump, when letting off gas

Tiny
POS07IMPALA
  • MEMBER
  • 2007 CHEVROLET IMPALA
  • 155,000 MILES
I just want to know what could cause this. The rpms jump about 200-400 one time after letting off the gas and it only does it at cursing speeds above 30 or so, it happens almost 100% of the time at 45mph cruising.

And is 12.85v charging voltage at idle low for this car?
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Monday, March 12th, 2012 AT 2:41 PM

7 Replies

Tiny
CARADIODOC
  • EXPERT
Lockup torque converter is unlocking in preparation for coming to a stop. When the computer sees you press the gas again, it knows you changed your mind, and it locks back up for better fuel mileage. My '88 Grand Caravan does the same thing. You can prove this to yourself by cruising at a steady speed above the minimum lockup speed, (in my case, 43 mph), and tapping the brake pedal. You'll see the engine speed pop up, then go back down about two seconds later.
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Monday, March 12th, 2012 AT 2:47 PM
Tiny
POS07IMPALA
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Is there anything else that could cause that jump, and would a bad converter make the jump more noticable?
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Monday, March 12th, 2012 AT 2:50 PM
Tiny
CARADIODOC
  • EXPERT
Yes. 12.85 volts is too low. 13.75 to 14.75 volts is acceptable. If the voltage only drops at idle, which is when generators are less efficient, suspect a defective diode inside the generator. When one of the six is bad, you will lose exactly two thirds of the generator's rated capacity. A load test will prove that. You'll only be able to get near 40 amps from the common 120 amp generator at higher engine speeds. At lower speeds the generator won't be able to keep up with the demands of the electric fuel pump, ignition system, injectors, heater fan, and all of the unnecessary computers.

Word of warning. This is a very trouble-prone generator design used since the '87 models. It is real common to go through four to six of them in the life of the vehicle. To reduce the number of repeat failures, replace the perfectly good battery at the same time unless it's less than about two years old. These generators develop huge voltage spikes that can destroy the internal diodes and voltage regulator, and can interfere with computer sensor signals. As the battery ages, it loses its ability to dampen and absorb those spikes. Your old battery will work fine in an '86 or older car. It is likely what caused the diode to fail now.
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Monday, March 12th, 2012 AT 2:56 PM
Tiny
CARADIODOC
  • EXPERT
No. You have a problem when it doesn't unlock. GM had a huge problem with that in the '80s. The converter would stick and not unlock causing stalling at stop signs. The cheap fix was to unplug the connector for the lockup solenoid so it never locked and couldn't stick. That raised engine speed 200 rpm on the highway. Few people noticed the big drop in fuel mileage.

Yours is doing exactly what it is supposed to do. The only thing that can cause trouble is using the wrong transmission fluid. That can cause a harmless but irritating shudder for half a second each time it locks up. Again, the way to identify that is to cruise at a steady speed, then tap the brake pedal. Each time the converter locks up right after that, you'll feel the shudder if the wrong fluid is in there.

Another potential problem is caused by a dropout of the signal from the throttle position sensor. Usually that will set a fault code and turn on the Check Engine light, but if it occurs fast enough, it might not set a code. The clue is the converter will be unlocking intermittently when you're just moving the throttle a little bit. Normally you have to almost totally release the gas pedal before the computer knows you plan on coming to a stop. If you feel the unlocking with the cruise control set, the slight throttle adjustments it makes to hold the speed steady will cause the converter to unlock if there's a glitch in the throttle position sensor's signal. That's not real common. It seems to happen more on some diesel trucks.
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Monday, March 12th, 2012 AT 3:10 PM
Tiny
POS07IMPALA
  • MEMBER
Ok and would a alternator not generating enough power cause weird low power/hesitation?
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Monday, March 12th, 2012 AT 4:05 PM
Tiny
CARADIODOC
  • EXPERT
Your car can have well over 40 computers and they're all very susceptible to voltage fluctuations. When a diode is bad it would be unusual for there NOT to be a problem. The voltage spikes radiate into other nearby wires and "induce" voltage spikes into them. If those wires are for a sensor, the computer is going to get all kinds of weird signals.

To identify the generator as the cause of running problems, start with a fully-charged battery, then while the problem is occurring, unplug the small connector on the rear / side of the generator. That will turn it off. If the running problem clears up, the generator is the likely suspect.
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Monday, March 12th, 2012 AT 4:25 PM
Tiny
POS07IMPALA
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Thanks a lot for the info man!
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Monday, March 12th, 2012 AT 4:37 PM

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