Last month a customer brought in her 2011 Volkswagen Jetta SE with a 2.5-liter, five-cylinder engine. The customer said her check-engine light was on and she felt a rough idle on cold starts.
We connected the car to the Volkswagen factory tool (ODIS) and pulled two codes: P3078 (throttle control air flow at idle too low) and P0106 (manifold/barometric pressure sensor implausible signal). The codes led us to test the engine’s air intake system.
We first conducted a smoke test on the intake system. We do this by using a tool that dispenses pressurized smoke into the intake. If there is a vacuum leak, we see the smoke escaping through that leak. The vacuum test did not reveal any leaks.
Feeling the pressure
We then checked the barometric pressure sensor. Before I describe the test, let me explain barometric pressure. Barometric, or atmospheric, pressure is the pressure within the Earth’s atmosphere. Standard atmospheric pressure is 29.9212 inches of mercury (Hg) or 14.696 pounds per square inch (psi). Strong high pressure from a hurricane could measure 30.70 Hg, for example, and low pressure from a hurricane could be measured as 27.30 Hg.
Either way, we always convert our measurements back to psi. We disconnected the barometric pressure sensor and connected leads to the barometric pressure sensor. The barometric sensor read 14.682 psi. This measurement told us that at that moment the sensor was OK. Yet as we know, sensors in cars can work intermittently.
Another issue that causes problems with Volkswagens’ cold idle is a dirty throttle body.
In past articles, I have described how the positive crankcase ventilation, or breather system, works. It pulls unwanted oil and fuel vapors from the crankcase and returns them to the intake system to be reused. This in turn leaves an oil residue behind on the throttle body, intake manifold and intake hoses.
We knew the next step was to remove the throttle body to check for carbon buildup.
If there is a lot of carbon buildup, the throttle plate will not close all the way. If that happens, the throttle position sensor gets confused and does not correct the air fuel ratio and idle speed.
Once we removed the throttle body, we found a large amount of carbon and oil. We then realized that we also had to remove the barometric pressure sensor because it sits under the throttle on the intake. When we got the barometric pressure sensor off, oil poured out of the passage. The barometric pressure sensor was completely covered in oil.
What we found in the end was that the oil vapors from the engine’s breather system were collecting in the intake. When the customer would go around corners, the oil was sloshing around – collecting in the throttle and in the barometric pressure sensor.
After further checks, we found a bulletin from Volkswagen (No. 11-14-23) pertaining to these faults. It states that the barometric pressure sensor had to be replaced, the throttle body cleaned and the car’s software updated.
We performed all three procedures – plus one more. We also cleaned the inside of the intake the best we could.
We were unclear what the software update was for. However, we think if the computer sees those two same codes again, it will take a different form of action or code.
Unfortunately, we have found that Volkswagen engines typically burn and use a larger amount of oil compared to other engines we’ve worked on. Because the breather system on this car checked out OK, we hope the customer will not have to deal with this problem again. Either way, the car ran great and started smoothly on cold mornings.