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Cold Start

In the last couple of weeks I have had a few of my VW and Audi customers complain about “hard starts” from their vehicles in the morning. As the weather is finally starting to get colder, there are things that we can look for and check to try to prevent this from happening.

One of the the biggest problems we see is carbon building up on throttle plates. Gone are the days of a cable connected to a butterfly valve and manual fuel pump. Almost all late model fuel injected cars use a fly by wire throttle assembly. The throttle assembly is the heart of the intake system. Most people think that fuel is where the engine gets power but actually the amount of air that flows through the engine determines the amount of power produced. Fuel is just the catalyst and engines are just big air pumps. The more air we can force through the engine, the faster we can go. That is why we always hear about turbos and superchargers in advertising.

Getting back to the topic at hand, what exactly does the throttle assembly look like? Imagine that we take a normal soup can and cut off the top and bottom. We then drill a hole straight through the middle of the can and slide a shaft through the hole. Then we take the the lid and mount it to the shaft. If we were to rotate the shaft we could open and close the lid inside the can. If the lid is closed we are at idle and if the lid is all the way open we are at full throttle.

A fly by wire throttle body works the same way as any remote control toy (except it is not wireless). The gas pedal is like the joystick on a remote and the throttle motor is like the front wheels of remote control car. So, as you move the gas pedal the throttle plate will open and close.

One more important part of the throttle is the idle bypass valve or idle control motor. When the throttle plate is closed no air can go past the throttle. So the idle control motor will let air by pass the throttle plate when the car is starting or idling.

Over time carbon starts to build up inside of the throttle, throttle plate and idle valve. This is similar to plaque buildup in human arteries. Once too much carbon builds up, the throttle plate and idle motor can jam or get stuck in the wrong position. This will cause the computer to get confused during startup or during idle. These conditions will also be exponentially worse in cold conditions.

The good thing is that cleaning most throttle bodies usually is not that difficult. As for the VW and Audis we try to make it part of basic maintenance. Once the throttle body is cleaned the engine almost always starts easier and idles smoother.

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