Piston Slap: Putting the Brakes on Auto Start/Stop?

Hagerty Community member HelenC writes:

Sajeev, my question is how do you disable the stop/start feature on a Jeep Grand Cherokee? It would be a tremendous service to your readers if you explained how to do it on not only Jeep products but GM and Ford.

I know the car companies have not come up with such an iterative feature since they put those seat belt starter interlocks on cars back in 1974. However it was easy to disable by just cutting all the wires under the seat. This aggravation may be more complicated to find which wires to cut so all your help would be appreciated.

Sajeev answers:

Turns out I got this one wrong, as there are modules that defeat this system. But remember these can void your vehicle’s warranty, so let the buyer beware.

Before I reply, please permit me to weasel out of a proper answer. Because getting a representative vehicle from all manufacturers, performing the task (either manually or electronically) to verify for myself, and then subsequently getting banned from all media fleets would kinda tank (what’s left of) my career as an automotive journalist. I’m only kinda joking, as doing this on not-media fleet vehicles would also be too cost prohibitive. And with that out of the way…

Let me instead point you in the right direction to disable auto start/stop in your vehicle. And this “pointing” is more valuable over time, as vehicle systems change over time, but the people who work on them generally do not.

• Call or email ECU tuners (Ford, GM, Chrysler) to see if they can remove auto start/stop from their software. This is often possible, but only they will know as they have access to the software and the ability to do what you want.
• Ask a dealership’s service department if start/stop can be removed either with their software, or unplugging/bypassing a wiring harness. (I assume the answer will be no, but it never hurts to ask.)
• Find out who makes the start/stop starter unit in your application. (Denso is a well known brand in this space.) Once you know the brand you can see what inputs it takes from the rest of the car to operate. Maybe you can disable start/stop without destroying the rest of the starter.

My advice tries to cover all bases, but honestly, you only need the first option. An ECU tuner will know more than anyone else, as modern cars use multiplex wiring that ensures any physical modification will take out more things than just the start/stop feature.

I suspect a tuner can make the system default to “off” so you can hit the button and turn it on if you tire of getting 0 MPG in traffic. Problem is, the tuner’s work isn’t free. It will cost hundreds of dollars, but it’s worth it if you also load a tune that allows for more power (premium fuel sometimes required), eliminate the throttle/torque management software, and firm up the transmission shifts.

Almost all of my EFI-powered vehicles have been tuned by a professional in the aftermarket, and I recommend it for more reasons than just disabling a start/stop feature. But that should get you exactly what you need, no matter what make or model we are discussing.

(For the record, I have no problem with this feature in late model vehicles. Times have changed, its operation is relatively seamless, and eliminating periods of 0 MPG when stuck in suburban sprawl has financial and ecological merit.)

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