Two For One

The 2011 Impala came in to the shop for a rough idle and P0300 misfire code. The shop determined that the cause of the concern was throttle body coking. After cleaning the throttle body the engine had a high idle, around 1700 RPM, and wouldn’t drop, even after a battery disconnect.

Coking comes from certain emission control components that capture crankcase oil vapors and small amounts of exhaust fumes and re-directs them back thru the intake system of the internal combustion engine. Too much time in stop and go traffic or idling isn’t the best for an internal combustion engine. This type of driving/operation can significantly carbon/coke up their engine very quickly. Another bad habit is hard acceleration. This over fuels the engine and leads to more problems including more carbon/coking build-up.

“So, how do we bring the idle speed back down?” Raul asked.

“First, realize that then engine computer learns the airflow through the throttle body to calculate the correct idle speed and stores the calculations in its memory.”

“Yeah, but can’t it compensate for coking?”

“It does,” I explained, “But up to a point. Then when you cleaned the coking, you changed the throttle body airflow rate.”

“Right—but doesn’t the engine computer learn the new values on its own?”

“It may take several drive cycles to learn out the coking, Raul. But you can also use a scan tool to reset all the learned values back to zero.”

“My scan tool doesn’t have anything like that,” he said.

That’s why I brought mine to his shop. I consulted the service information because the relearn procedures can be different depending on the vehicle.

“To reset the learned values to zero, we turn the ignition ON, engine OFF,” I said, finding the Idle Learn Reset procedure in the Module Setup menu on the scan tool.

“Next, we start the engine and monitor the throttle idle airflow compensation data PID.” The engine was already idling normally now and the data PID was at zero, which was where it belonged. I checked for trouble codes; there were none, and Raul took the vehicle out on a short road test. He was all smiles when he returned.

“The Impala runs great now.”

That one was easy. But Raul wasn’t going to let me off that easily.

“Hey, as long as you’re here, Bob, I have another one for you.”

Here it comes. “What is it?”

“I’ve been fighting with a 2009 C4500 eating up heater cores all the time,” he said. “We did a coolant flush first and that didn’t fix it. I switched coolants and put a ground on the heater core but that didn’t help.”

I was writing this all down in my notepad, of course. The memory isn’t as good as it used to be. “Get rid of the heater core ground,” I said. Back when electrolysis first cropped up as a cooling system problem, some technicians tried to re-route stray voltages to battery ground. But it merely accelerated the problem. I asked what else had been done to the truck.

“We installed a water shutoff valve to shut off flow to the heater core when not in use, but we still have issues with the truck. How about if we install a copper and brass heater core?”

“That would be treating the symptom but not the disease,” I said.

“We checked for bulletins and other documents but found nothing.”

I wrote that down. “Did you check for voltage in the coolant?”

Raul nodded. “A tenth of a volt, maybe less,” he said. We touched the negative voltmeter probe to battery ground and pot the positive lead into the coolant. We didn’t touch any metal, either.”

“Did you check for both DC and AC voltages?” I asked.

Jim, one of the technicians, asked me why they had to check for AC voltage.

“Keep in mind that you could have a bad engine block heater or faulty alternator diode,” I explained. “Did you check all the grounds?”

“They’re all good,” Jim said.

“Let’s check them statically,” I said. “Turn off all accessories. Turn ignition on, but do not start the engine.” I had them check with the ground probe of the meter to battery ground, engine ground and vehicle ground, sequentially. They found one ground connection that needed cleaning. We continued our testing.

“Check the accessories without using the on/off switch on the vehicle instrument panel, use a jumper wire to ground,” I instructed. Next, we turned on all the accessories. We tested again with the ground probe of the meter to battery ground, engine ground and vehicle ground. We were coming up with about two-hundred milli-volts.

“The spec calls for .4 volts or less,” I said.

“Can static buildup cause problems?” Raul asked.

“Rubber-mounted driveline components, a squirrel cage spinning in a plastic HVAC case when the blower motor isn’t properly grounded, and even tires.”

“Tires?” Jim asked.

“Tires?” Raul asked.

“A while back, a certain tire company got complaints from drivers who kept getting zapped whenever they got out of their vehicles. The tires were producing a static buildup when the vehicle was driven under certain conditions.”

“Maybe that’s the problem here,” Raul offered.

“What’s this truck used for, Raul?” I asked.

“Deliveries—it’s a delivery truck.”

“Lots of stop-and-start driving?”

“Right,” he said.

“Did you check for coolant voltage while cranking the engine?”

Bingo.

In cases of electrolysis, a defective or missing ground on an electrical component causes the electricity to seek the path of least resistance whenever the component is energized. Sometimes the path of least resistance is a radiator or maybe a heater hose, or the radiator or heater core. As the current draw of the poorly grounded accessory increases, so does the destructiveness of electrolysis. A poorly grounded engine and starter motor can flow enough current through the cooling system to blast apart a heater or radiator in a matter of weeks or even days, depending on how often the vehicle is driven with stops and starts. 

It was a long weekend. Closing up shop.