Car Won’t Idle After Dead Battery (Simple Free Fix)


There’s never a good time to have a flat battery, and just when you get that sorted you find the stupid engine won’t idle! I feel your pain, this is exactly what happened to me.

So why won’t the car idle after a dead battery? When the car battery goes dead, power to the on-board computer is interrupted. This loss of voltage causes the ECM to lose track of various sensors and actuators.

In this post, I’ll outline what sensors and actuators cause the most problems and how you can fix them. I’ll also show you how you can avoid this happening again.

Why Won’t The Car Idle Smoothly?

ECU

You already know that disconnecting the battery has caused the onboard computer to lose track of various sensors and actuators.

But what does that actually mean? Well, once upon a time the accelerator pedal of your car was connected to a carburetor by a cable, pressing the pedal pulled on the cable that pulled on the carburetor lever and you moved on down the road.

Throttle pedal

Well them days are long gone, everything (or almost) is run electronically. That means instead of a big old cable attached to your throttle pedal, the throttle pedal itself is a sensor that is simply telling the onboard computer how far you’ve pressed it. The system is known as a fly-by-wire.

The ECU, in turn, makes a few calculations and sends a message to the throttle body. The throttle body is an electronic stepper motor that opens the throttle flap very precisely, it’s sort of a modern-day carburetor if you like.

Throttle body
Throttle Body

The accelerator pedal and throttle body are in constant communication with the ECU and the ECU knows what exactly each is doing and what position they are in. The ECU needs this constant feed of information so it can respond quickly to drivers commands.

In addition, the clever ECU adapts the position of the throttle body over time to allow for wear and tear on the component and also for the natural build-up of carbon on the throttle plate.

The issue arises, as you know when power is lost and the ECU forgets the adapted throttle body position. So it simply tries to reset the throttle body to new factory settings, and that’s what’s causing the erratic idling.

How To Fix Erratic Idling

Scan tool

Most cars will relearn the throttle body position themselves, it may take a week of driving and of course won’t be pleasant to drive, especially in traffic.

The relearn procedure is usually pretty simple but it’s advisable to google your exact model as relearn procedures often have exact steps that are timed. You won’t do any damage by not getting it right, it just may not work.

Relearn procedure

The following is a GM relearn procedure but it’s also a fairly standard throttle relearn procedure.

  1. Idle the engine until reaches operating temperature
  2. Turn engine off for at least one minute
  3. Idle the engine again for at least five minutes
  4. Drive the vehicle at a range of speeds using various acceleration/deceleration cycles (on and off throttle)

It is possible for fault codes and a check engine light to come on, if so just clear them and repeat the drive cycle again.

What Sensors & Actuators Commonly Need Calibration?

car heater controls

Here’s a list of common actuators and sensors that require calibration after a battery has been disconnected.

  • Electric windows
  • Hvac
  • Electric steering
  • Accelerator pedal
  • Throttle Body
  • Radio presets
  • Time clock
  • Electric seat memory

Most of these sensors and actuators should relearn themselves given enough drive cycles. But here’s a quick rundown on how you can calibrate some of them right now.

Electric windows

Simply let each window down in turn and hold the button down for at least four seconds. Now send each window up in turn and again hold the button for at least four seconds. That’s it!

Hvac

Move through each of the vent positions and hold them there for five seconds.

Electric steering

With the engine running lock the steering from one side to the other and hold for five seconds, now move back to the center position. That’s it!

Accelerator pedal

With the ignition turned on, press the accelerator pedal to 1/4, then 1/2 way and finally flat to the floor. Now turn off the ignition for at least one minute.

Radio & Time

Are usually pretty simple, your drivers manual will tell you how.

How To Prevent This Happening Again

Batteries aren’t made to last anymore, a car battery will now only function for about three to four years. I recommend changing your battery after the third year.

Having a good battery is always important, especially in modern cars, where computers manage everything from the engine to the heater.

A modern car can have easy 30 plus control modules and they all need to communicate with each other. They use high and low voltages which are transferred into code, the code is understood by all the linked control units.

Mess with the voltages and the message gets lost in translation. The symptoms vary from model to model but you’ll find some features like windows, transmission, and throttle acting a little weird.

Anyway, you already know how to handle them, but to prevent this from happening in the future you’ll need to keep the computers powered, even though you may need to replace the battery.

KAM

A useful piece of kit called a KAM (Keep Alive Memory) tool or battery maintainer is used to keep the computer alive. It’s a power pack that plugs into the cigarette lighter/ 12v power outlet. An adapted data link connector can also be used to supply power to the onboard computers.

That’s it, couldn’t be easier, now you’re safe to remove the battery and all computer settings and calibrations are safe.

This supply is only used to power the battery, not start the car or power windows or lights, etc.

Related Questions

What increases the value of a classic car? Original details like matching engine and chassis numbers will increase the value of a vehicle, but overall condition and scarcity will have the greatest effect on value.

John Cunningham

John Cunningham is an Automotive Technician and writer on Rustyautos.com. I've been a mechanic for over twenty-five years, and I've worked for GM, Volvo, Volkswagen, Landrover, and Jaguar dealerships. My passion is cars. I use my knowledge and experience to write articles that help fellow gear-heads with all aspects of car ownership, including buying advice, maintenance, and troubleshooting.

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