2014 Nissan Altima S 2.5 Stalls At Idle

One of my good customers called with a description of stalling after a start at idle, battery light and a lot of corrosion at the battery terminals. I thought the stalling problem might be too low an idle rpm and suggested the one foot for braking and one foot for keeping it just off idle to raise the rpms. That worked well enough to get the car to the shop. When I drove the car inside there was no stalling or low rpm. In fact it ran very well but there was a check engine light. A light means a code. A code might be a clue to what had been going on.

A P0101 MAF sensor code was stored and actually is a pretty common fault code on these cars. So common in fact there is a technical service bulletin that suggests an updated PCV valve and a possible ecm flash update, depending on ecm part number.  NHTSA ID: 10119177
TSB ID: NTB16-030a 

That being said, nothing about the known P0101 problem would explain the stalling complaint. Also, the manner of driving with one foot on brake and the other on the accelerator could have set the code. In the few minutes I was with key on checking for codes the car battery voltage had lowered enough that the car powered down to save battery voltage. This brought me to what Danny (customer) had said about the battery condition. I decided to let the P0101 aside for now. I had another idea about the original complaint and its cause. I should explain that a car has a learned memory or an updated one that stores slight adjustments over time to compensate for wear or tear that has taken place since it has been getting older. If you reset the memory it goes back to before the learned behaviors and then has to relearn the adapts. One very common thing to happen over time is a buildup of carbon at the air intake throttle body and throttle plate. The buildup was slow and the idle strategy adapted to it to keep the car idling at a proper rpm. Clearing the adapts on a car that has a dirty throttle (carbon) will cause the car to stall at idle until the relearn adapts OR until you clean the carbon. If you clean the throttle the idle will not have to be relearned but will already match the reset strategy. I suggest cleaning the throttle after a reset. 

The battery here had not been disconnected and the battery had not failed to crank the car. You might wonder then why am I thinking of a problem with throttle carbon buildup and memory resets. A weak battery can drop voltage low enough during a crank to reset the learned adapts. Seriously. I've seen it more on a Dodge than other models so they may be more sensitive to it. It isn't just reset either. Some codes can set on a start that were a result of battery voltage dropping too much during an engine crank. The rule of thumb here, for me anyway, is that if the voltage drops lower than 10 volts during cranking its a problem. 

The plan was now to see if this battery was dropping too low on a crank and if the throttle plate was carbon dirty. Those two things and I'd have evidence this was the problem. 

The battery voltage starts at the left, the drop is down to 7.84 during the crank and then once the engine starts the voltage is output of the alternator. So yes the voltage is going low enough to reset the adapts and I do need a battery. But I need to see carbon buildup on the throttle to explain the stalling. 

Especially sensitive to any carbon is the edge of that throttle plate. Cleaning the throttle and replacing the battery should be the fix. At least it can't hurt. 

Cranking voltage with a new battery.

The P0101. I cleared the code and after 3 trips it had not reappeared even as a pending code. I decided not to pursue further. 

Thanks for reading!

Kenny@ggauto.repair