h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Not wise unless you are there for a very long time - as most cars required much more fuel to start than to keep running...

This is an urban myth - it came up while the entry was in Peer Review. It doesn't take more fuel to start the car than to keep it running. It does take a lot out of the battery though - it can take around 20 minutes to replace the charge used by starting the car. Perhaps that's what you are thinking of?

Thanks for reading.

On many models of fuel injected cars, the injectors fire at full duty cycle for a number of seconds during startup. How much idling does it take to offset this? Not much, but the brief moment of rich running would be more of an emissions concern, i think. Furthermore, when the vehicle is started the PCM (Powertrain Control Module) moderates the airfuel mixture, erring on the rich side, to ensure easy starting and running until it can trust input from the Oxygen Sensor and fine tune the mixture for optimal performance and emmisions. This rich bias on startup justifies the first question on this string. How much idling would offset these concerns? I'm not sure. It would vary widely by model and year, and condition of vehicle. I think it would be very hard to justify any figure (ie. 30 seconds of idling) as a guideline that everyone should follow.

I would advise letting your engine idle for a short while after a cold start. Not too long, though. The piston rings have not formed a decent seal to the cylinder wall and an unacceptable amount of cumbustion gas gets driven into the crankcase when the engine is subjected to load. This shortens oil life and can pressurize the crankcase beyond what the PCV system can handle, creating oil leaks. Even more of a concern for engine longevity is that the valvetrain at the top of the engine doesn't see any oil for up to one and a half minutes on many vehicles. The figure tossed around the industry is that this cold startup starvation is responsible for 90 percent of engine wear.

In my Canadian climate I also see a lot of expensive repairs caused by winter "Start-n-go" drivers. Automatic transmission fluid, Power steering fluid and other oils are very thick at our low winter temperatures. Suddenly subjecting a power steering rack to 1600 psi (or an automatic transmission to 600 psi in reverse) before its fluid and thus seals and metal have had a chance to warm up beyond minus forty makes all sorts of neat and profitable things happen for your mechanic. I let my engine idle for a minute or two before driving, up to 5 minutes in the dead of winter. If I do have to take off earlier, I am carful to drive like old people **** for the first minute or so, and accept the fact that I am shaving tens of thousands of kilometers of my engine's optimal life.

Since I'm on a roll, Make sure you put your car through its paces every once in a while, too. I've had throttles stick wide open on me during test drives because they'd never been opened beyond 20 percent and the rest of the linkage was consequently rusted. I've performed hundresds of dollars of unnecessary repairs to vehicles whos braking systems had never been applied strong enough to get the rear brakes to move sufficiently to shake off the rust. I've tuned up countless engines that never got hot enough to burn the startup carbon off the spark plugs and catalytic convertor. I've even had to replace four wheel drive mechanisms that rusted solid because they were never used and oil and grease were never splashed around to lubricate the components. (Don't get me started on "Soccer Moms" who buy 60 thousand dollar one tonne four wheel drive gas pigs and never use it for anything more strenuous than taking groceries home from Safeway's... Wouldn't a CAA membership have been more cost effective?).

I don't want to be all negative about your artilce, though. I do agree with (and practice) almost all of it for 90 percent of my driving. And even if everyone followed it to the tee, we would be much better off for it as a planet. Plus, I would get to be a rich mechanic!

Ah well, it's written for people in the UK, where the temp doesn't generally get as low as Canada.

But thanks for the comments anyway! It's always useful to have more info. And thanks for reading.