What is worse for the engine - idle vs. cold start

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Jul 17, 2003
New York
Let's say it's a day just running errands. Sunday morning - drive car for paper...shut off engine and pick up paper. Go grocery shopping on the *local jam packed* avenue. End up having to double park and move the car 2 - 3 times.

Is it better having 3-4 cold starts in a given hour VS. just letting the car idle while I do my errands.
Well, first of all those won't be "cold" starts. A cold start is when the engine is actually cold. It would probably be better just to leave it running...
I don't know about the laws in the various states, but I doubt leaving a car unattended with the engine running is legal. It's simply a liability.

I shut my engine off even if get in a traffic jam or if I have to wait at a really slow light. If I expect to sit still for more than 2 minutes, I shut the engine off. Haven'ever had a prematurely worn engine either...
My other half is usually do the shopping. I'm there usually playing escort & chauffeur

Originally posted by DZello:
I shut the engine.. Idling is wasting gasoline.

A starter costs less than the fuel you waste.

A starter cost about $100...or more than $300 if you can't replace it yourself. It cost about $1/hour in gas to idle...
I suppose in a warm climate - above freezing - you might save gasoline and this savings might even pay for the new starter. Perhaps not, have you priced a starter for a late-model lately?

As far as wear and tear, everytime you start the motor the mixture is momentarily rich until the O2 sensors can flip closed/open loop and lean it out. Though extended idle time tends to leave excess fuel in the crank too.

In consistently cold temps, say -30 C or colder, I have seen a few folks try to "save" gas by this constant routine of shutting off, starting up, shutting off, etc.

They don't make it through the first winter before they go back to the car, turn the key to start, and:

Unnn unnn unnn unn un un un un ... un ... un ... clickclickclick click

Another consideration is the crime rate. Say you leave it running, even if you have a spare key and lock the door - you walk out the store and the car is gone, with broken safety glass on the ground where they broke the window.

So in the end use common sense. I suppose it's a toss up between extended idle time and "saving" fuel.

Personally, I don't buy it that idling an engine is better for the engine than repeatedly starting it up. I can't recall wearing out a starter motor either. I'm also against wasting resources, especially unrenewable ones. And I really hate it if people keep their engines idling while gassing everyone behind them. Don't be an idler!

PS: chefwong, I like to carry my groceries myself.

A starter cost about $100...or more than $300 if you can't replace it yourself. It cost about $1/hour in gas to idle...

And after how many starts do you think your starter will fail ?

It can take a lot of time...A lot of years... If you add up all the gaz you waste, with the cash you save you can buy a new starter and lot more...
If its cold out, I'll let mine idle if I will be back in 15 minutes or so (such as just running into the deli). If its warm out, I always shut it off. It keeps the car warm on those ice cold winter days, and my GM 3.1L hates being cold (makes all kinds of terrible noises until it warms up).

I would never, ever, ever leave my key in the car however to let it continue to run. I installed a DesignTech AutoCommand 25522 when I got the car, which lets me both remote start it by locking the doors twice from the factory remote, and lets me let the car idle with the key out (a feature which I am in love with). You just hit a switch immediately before taking the key out, turn the ignition from "on" to "unlocked" and take the key out, and viola, the engine stays running. I have the optional time-out set to 15 minutes in case I'm longer than I expect.

Also, it's wired so that any activity other than a single door unlock with the key out will cut the motor. Pressing the brake or the gas, trying to move the shifter out of P--anything, will kill the motor immediately, and puts the system in security mode, so you can only restart the car with the key.
Ford starter motors are tested to 30K repeated starts. This equates to 10 yr, 150K miles of heavy duty use. I am sure other mfger's have same or tougher requirements.
I think if you leave it running and idle, you keep the oil temps up and burn off moisture. All the starts and shut-offs just cool the oil down so it never burns off moisture.
I agree with the comments that only the first start will bet the 'true cold start' as the engine will retain some heat for at least an hour....let alone an oil film thickness for several hours...

So I'm with the idling wastes fuel crowd...and is unneccessary just like starting the car (eg. remote starter) and idling for 1/2 hr. to warm up...just plain silly.
Letting an engine idle for extended periods (more than 5-10 minutes) is the worst thing you can do for an engine. The oiling to the pistons/cylinders in minimized, combustion is at its least efficient, and "cylinder wash" (raw fuel and unspent components of gasoline washing down the cyl walls and diluting the lubrication) is at it greatest. Unless you have a real good reason to leave it running,I'd shut it off. To say that you are trying to extend the life of you starter/battery isnt a good reson. The few starts you save over the life of the starter is of no consequesnce especially when there is no way to quantify your results. There is no way you can be certian you are gaining any battery or starter life but you can be assured you are increasing wear on your engne.
Again you have to use common sense.

If your ambient temps are -30 C or colder, I will guarantee that start/off/start/off/start/off in city driving will NOT make it through the first winter before something goes haywire.

The best thing is to run the motor at 1,200-1,500 RPM. A dead idle allows condensation. For carbs, it was easy to rig up a hand throttle. Kussmaul makes high-idle kits for popular EFI motors.

That is an awesome tip of the black box that allows idling but kills the motor if the shifter is moved out of Park. Thanks!

Ford starters are "design tested?" HA! My 1984 Ford F-150 went through starters every 3 years due to a poorly designed armature arm that BROKE, so the starter would spin but the pinion would not engage the flywheel. Another "better idea."

For HD diesel motors, it's a general rule that usually below around -20 C you do NOT shut them off unless you can plug them in. They have a high-idle function too, even the newer electronic HD injection.

How come Police cars, taxis, Delivery trucks, Ambulances ect last so long if they are idling most of the time? It must not be too bad for them.

How come Police cars, taxis, Delivery trucks, Ambulances ect last so long if they are idling most of the time? It must not be too bad for them.

How long do those last?

One thing is for sure: their engines get rarely or never really cold, and that's a big plus. However, it does not mean the same advantage will apply to your daily driver.

Originally posted by JonS:
How come Police cars, taxis, Delivery trucks, Ambulances ect last so long if they are idling most of the time? It must not be too bad for them.

I'm not sure about taxi fleets, but Emergency fleets are usually equipped with Fast Idle boxes from Kussmaul or others.

Not too often you see an ambulance or fire truck switched off at the scene. Sometimes they run for 1-6 hours.

To shut if off with the strobes flashing would leave the battery dead in about 5 minutes. For rescue equipment, the PTO must power the hydraulics, air system, and pumps.

Also don't forget for most emergency and rescue equipment, you can expect the crew to race from scene to scene, so a motor wouldn't last long if shut off, cooled off, then restarted and immediately at WOT.

Or would you want your EMT driver to "gently" drive to your massive coronary to help prolong the life of the ambulance motor?

So with most emergency equipment, the driver pokes the "FAST IDLE" button at the scene and the motor immediately jumps to 1,300-1,500 RPM and stays there.

The newer equipment is fully automatic, no button to push. It monitors the batteries and load demands, once the voltage drops the fast idle kicks in.

On either Kussmaul system there is a safety override so when you step on the brake pedal, it drops back to low idle so you don't shift your expensive Allison automatic transmission into Drive at 1,500 RPM. That could be spectacular.

Most new electronic fuel injection HD diesel motors are factory equipped to provide fast idle through the SAE J1939 data interface bus. The Cummins equipment I operate is so equipped, just have to plug in the appropriate black box.

This is perhaps why a few folks I know got used surplus Chevy Caprice '94-'96 police cars at auction. They were cheap, easy to rebuild, and they added the Kussmaul box. So in temps colder than -30 C, they hit the button and the LT1 motor runs at 1,500 RPM.

i usually leave it running while i run to the store and it takes only upto 5-10min. I didn't know idling was that bad on your engine!! I'll have to buy one of those automatic idlers.
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