Here’s everything you need to know about idle hours and how much is too many.

With the used car market in influx to say the least, a lot of used car shoppers, like myself, are looking for cars in unconventional places, and that means used car auctions open to the public.

A common and attractive choice for used car auction buyers are law enforcement cars or fleet vehicles used more in a patrol and work capacity more than actual driving.

I’m talking about police cars that do a lot of idling and city driving, fleet trucks on job sites that only drive a limited amount of mileage a day, or perhaps a car used for code enforcement where they do a lot of start and stop driving.

Take for example this 2013 Ford Taurus Police Interceptor I found browsing around for used cars. With the top bid under $5,000, under 100,000 miles on the odometer, and no major body damage, it’s an attractive option.

Under 100,000 miles sounds good, right? Not so!
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2013 Ford Taurus Police Interceptor. Cool!

That is, until I came across this odometer reading. Is 7,000+ hours of idle time too much? Too little? And, what does idle time affect?

Over 7,000 idle hours, is that bad?

Let’s dive into that.

How are idle hours defined?

Different manufacturers define idle hours differently.

For example, if you ask Ford Crown Victoria owners, they will tell you Fords record idle hours as time when your engine is running while you’re in Park or Neutral.

Ask Ford Super Duty owners and, on top of the shift lever being in Park or Neutral, some will theorize that idle hours also include speeds under 25 MPH and RPMs less than 1,500 rpm.

In general, idle hours count when your car is not moving with your engine running at low RPMs.

Engine Hours are the total amount of time the engine is on, driving in motion and idling.

That explains why a lot of police cars have relatively low mileage but thousands of hours of idle hours recorded.

What’s the idle hours to miles equivalent?

There is no hard and fast rule but, according to Government Fleet,

“According to research by Ford Motor Company, one hour of gasoline engine idle time equals 33 miles of driving. In other words, for vehicles with excessive engine idle time, such as a police or border-patrol unit, premature engine damage can occur without having driven many miles at all.”

This fact is corroborated by Ford Owner Manuals.

One idle hour for 33 miles of driving is an industry-wide accepted conversion, as General Motors apparently agrees with Ford’s research.

Returning to that 2013 Ford Taurus Interceptor, on top of the 93,197 driven miles, the idle hours adds an additional 236,907 additional miles.

In reality, this sub-100K mile Taurus is more of a 330,104 miles Taurus!

Makes that low winning bid more in perspective, right?

What do idle hours affect?

Since idle hours involve a car practically sitting still, idle hours largely affect anything engine and immediately-engine related, that means the engine, all accessory belts, sensors, and the exhaust system.

If you’re buying an actual pursuit rated used car, manufacturers have beefed up all the appropriate engine ancillaries, like equipping them heavy-duty alternators, heavy-duty starters, and larger batteries to handle the rigors of long idling, on top of hard driving.

Regardless, idling for a long period of time adds stress to the valvetrain, intake manifold, exhaust, injector tips and plugs. Since these cars are not driving hard for long periods of time, carbon can build up on those aforementioned engine components.

Idle hours also affects anything rubber under the hood including belts, hoses, and weather sealing. Whereas “cool” air enters and exits a moving car, anything rubber in an idling car soaks up all that engine heat.

On top of that, condensation builds up in the exhaust system and, if not enough moisture evaporates, exhaust rot eventually happens.

Buying a used car from a police fleet with a lot of idle hours is probably OK considering police cars go through recommended maintenance intervals per manufacturers recommendations.

Any other fleet vehicle from a non-government agency, I would be suspect of too many idle hours if the used car does not come with a complete service history.

Generally, if you can confirm good maintenance records, “as long as the engine is in good working order and proper fluids which are not broken down, high idle is no more damaging than high miles.

How many idle hours is OK on a used car?

There is no hard and fast number of how many idle hours is too much, and it’s all subjective, keeping the price in mind.

If you’re buying a used Crown Victoria, too many idle hours appears to be less of a factor of how long these cars last compared to other pursuit rated vehicles.

That being said, I’d start to take idle hours into consideration above 5,000 idle hours, generally considered on the higher side.

I’d really take that 5,000 idle hours into consideration if, for example, it’s a Ford Taurus with a high-performance 3.5L Ecoboost engine as, from what I’ve read, carbon buildup can cause premature turbo failure if carbon mitigation maintenance is not taken into consideration.

That 2013 Ford Taurus I found appears to be a naturally aspirated one so, if I was that winning bidder, I wouldn’t be worried about that turbo issue, at all.

As mentioned, you’ll have to do your research what’s considered too many idle hours for your particular year, make, and model you’re buying.


Idle hours aren’t equivalent, mile for mile, compared to actual odometer miles, but, they count for something.

The effects of idle hours, specifically too many idle hours, and all its harmful effects are mitigated by proper maintenance.

If you find a used car with a lot of idle hours, I’d take that into consideration but, would be slightly less concerned if I found out oil changes, coolant flushes, belts, and water pumps were all changed and addressed per the recommended maintenance schedule.

If there’s evidence of lack of maintenance, I’d bargain/bid accordingly to mitigate the higher likelihood of an engine failure.

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