Car manuals say: "Don't let your engine run idle for long"

[mb]

How cold is "super cold"? Should you let your car warm up for a few minutes when it's below 15? Below zero? If you live in California you may never have to let your car warm up but if you're in Minnesota or Michigan there are a few months of very cold weather every year.

Basically I mean any temperature when you reach the point where you would idle the car just for the sake of warming the interior anyway.

But above that, there is no point. For example if it's 40F outside, start the car and go. There's no need to sit there and wait for the engine to warm up. I'm not suggesting you should drive it hard right away, but there's absolutely no need to wait 5 minutes.

If it's freezing outside and your windows have inches of snow/ice on it then you'll probably be warming it up first anyway.

 

[LTC8K6]

How long I warm up my car.

Above freezing - just long enough to see that the gauges look correct and no warning lights are on.

Just below freezing - about 30 seconds.

Well below freezing - 1-2 minutes.

Friggin cold - long enough to have some heat.

 

[AstroGuardian]

This happens for a couple of reasons that I am aware of:

1) Piston speed is low, so there is a longer time for air/fuel mixture to slip past the piston ring.

2) Timing is retarded, so again there is a longer amount of time that there is unburnt fuel in the cylinder, and therefore more time for it to slip past.

3) Idle is generally at a very low MAP (manifold absolute pressure) and your PCV (positive crankcase vent) brings the crank case pressure down very low. This creates a larger pressure difference across the piston rings, which results in more air/fuel being pushed across the piston rings.

Does that make sense?

The stress that piston rings see (I believe) are not large enough to create significantly more leakage.

Yes you are right. It makes whole lot of sense. I haven't think about that. Which means that piston rings are more efficient during fast strokes but bigger pressure than idling when the stroke is slower but also pressure is lower... right?

 

[AstroGuardian]

Incorrect.

Maybe a full minute or two on those super cold mornings (or however long it takes for the interior to warm up anyway), but for the other 360 days of the year, it is not needed. By the time you buckle your seat belt and release your parking brake (if applied), you're good to go.

Next you'll be telling us we need to inflate our tires to sidewall.

I don't understand your point. I also say that you should wait for like 20-30 seconds when the temp outside is like -20 or -10 (which happens for AT LEAST three months during winter in middle Europe)

For every other warm day you should wait like 10 seconds for the oil to start circulating and lube everything up before you put some stress on the parts... right?

 

[JCH13]

Yes you are right. It makes whole lot of sense. I haven't think about that. Which means that piston rings are more efficient during fast strokes but bigger pressure than idling when the stroke is slower but also pressure is lower... right?

Yes, I believe it is related to "choked flow" around the piston rings, not to get too technical or anything.

 

[skyking]

Sorry i forgot to mention that it is a manual transmission vehicle which means that when in neutral there is zero stress to the transmission...

Why do you say that the powertrain is not designed to idle? I mean a machine is machine, it should work better with less stress than with more stress...

What does exhaust system temperature have to do with anything?

Zero stress on the transmission? that would be parked and shut off. Idling it is at the very least spinning the input shaft.
It was designed to drive down the road. If it were designed to idle it would be sitting on a pallet on a dock somewhere, producing some fixed amount of power all the time.
Cool exhaust = condensation = corrosion.
Idling is at the very edge of the engine's operating range. The fuel/air ratio is not as good as it is under a load. This causes fouling of plugs, O2 sensors, etc.

 

[mb]

I don't understand your point. I also say that you should wait for like 20-30 seconds when the temp outside is like -20 or -10 (which happens for AT LEAST three months during winter in middle Europe)

For every other warm day you should wait like 10 seconds for the oil to start circulating and lube everything up before you put some stress on the parts... right?

ugh.. I guess I didn't word it clearly, my bad.

What I mean is this: when it is really cold, you are likely to idle the car to warm the interior. Your engine will also warm up, likely before the interior does. Whenever you feel the interior is comfortable enough to drive, you can drive. You don't need to wait X more minutes.

On any other day where you don't feel the need to idle your car to warm up the interior (meaning it's not friggin cold outside), you can just get in and go without waiting to warm the engine. You can probably wait 10 seconds if you really feel like it, but there is no need to wait even 1 minute.

 

[AstroGuardian]

ugh.. I guess I didn't word it clearly, my bad.

What I mean is this: when it is really cold, you are likely to idle the car to warm the interior. Your engine will also warm up, likely before the interior does. Whenever you feel the interior is comfortable enough to drive, you can drive. You don't need to wait X more minutes.

On any other day where you don't feel the need to idle your car to warm up the interior (meaning it's not friggin cold outside), you can just get in and go without waiting to warm the engine. You can probably wait 10 seconds if you really feel like it, but there is no need to wait even 1 minute.

Totally agreed. Warming up case closed

 

[AstroGuardian]

Zero stress on the transmission? that would be parked and shut off. Idling it is at the very least spinning the input shaft.
It was designed to drive down the road. If it were designed to idle it would be sitting on a pallet on a dock somewhere, producing some fixed amount of power all the time.
Cool exhaust = condensation = corrosion.
Idling is at the very edge of the engine's operating range. The fuel/air ratio is not as good as it is under a load. This causes fouling of plugs, O2 sensors, etc.

That too makes sense. But why it's not explained in the manuals? And it says about bad things to the engine not the exhaust system.
But other things that i heard here are probably correct.

Feel much .... umm.. less confused now than i was previously

Thanks guys

 

[DivideBYZero]

Zero stress on the transmission? that would be parked and shut off. Idling it is at the very least spinning the input shaft.
It was designed to drive down the road. If it were designed to idle it would be sitting on a pallet on a dock somewhere, producing some fixed amount of power all the time.
Cool exhaust = condensation = corrosion.
Idling is at the very edge of the engine's operating range. The fuel/air ratio is not as good as it is under a load. This causes fouling of plugs, O2 sensors, etc.

Bollocks. MAF sensors, O2 sensors and fuel computers are there to ensure optimum air/fuel ratios at all operating speeds and loads.

 

[skyking]

Bollocks. MAF sensors, O2 sensors and fuel computers are there to ensure optimum air/fuel ratios at all operating speeds and loads.

Bollocks back at you!
The engine is indeed at the very edge of operating: there is a single sensor on most cars, but the intake and exhaust system are not designed nor are they tested for the best balance at such a low power setting. This means that the balance between individual cylinders won't be as good as it is at the normal operating range.
Think about any car you have left idle for a long while. It will have a slight stutter or 'clearing of its throat' when you throttle up or go. I can't think of any car I have driven that does not load up just a little bit when left idling too long.

 

[LTC8K6]

Bollocks. MAF sensors, O2 sensors and fuel computers are there to ensure optimum air/fuel ratios at all operating speeds and loads.

Open loop...

Closed loop...

:biggrin:

 

[Howard]

That too makes sense. But why it's not explained in the manuals?

Why does anything ever have to be explained?

 

[exdeath]

Bollocks back at you!
The engine is indeed at the very edge of operating: there is a single sensor on most cars, but the intake and exhaust system are not designed nor are they tested for the best balance at such a low power setting. This means that the balance between individual cylinders won't be as good as it is at the normal operating range.
Think about any car you have left idle for a long while. It will have a slight stutter or 'clearing of its throat' when you throttle up or go. I can't think of any car I have driven that does not load up just a little bit when left idling too long.

This is wrong.

Not sure what you are talking about by "edge of operating" and "balance between individual cylinders"... but:

The AFR on a standard fuel injected engine (no direct injection required) can reach 16:1 and higher and 40+ deg spark advance on an idling engine with no load, making it much more efficient than it would be under load or cruise in the 9:1 to 12:1 range, regardless of any other tuning parameters. Engines are extremely efficient at idle.

The stumble when you make a sudden throttle transition is called a transient and it is accommodated for via the throttle position sensor and an accelerator shot. Additionally there is another factor called wall film compensation for fuel that accumulates in the ports which suddenly breaks free off the port walls when the airflow is suddenly increased. No properly tuned and maintained EFI engine should stumble at all when throttling up from idle regardless how long it sits idling.

When an engine reaches operating temperature, it stays there. You have a thermostat and automatic fans that will maintain an absolute max coolant temperature of around 200 deg. It will maintain this indefinitely regardless if the car is moving or not. You will not exceed this temperature idling. Nor will you fall far below this temperature driving, as the fans will stay off and the thermostat will close in an attempt to KEEP this temperature.

Mechanical wear and tear is negligible idling compared to WOT and starting. And other theories such as oil/fuel contamination are also bogus. Cylinder pressure is lowest when idling with no load resulting in minimal blow by, and, as described above, the mixture at idle is so lean that there is minimal fuel in the combustion chamber TO blow by or cause bore wash.

There is no logical reason against idling a car other than wasting gas and creating unnecessary emissions.
.

 

[dust]

Lol, you guys are worried about idling for as long as required even during warm up, and here in the Middle East there are cars left idling FULL DAY, just because it's way too hot to stay without ac and switching off for as little as five minutes would turn your car into an oven.. Hell there are trucks with chillers being left running all night in ques at customs just because they have load that requires cooling.

I kinda believe these issues have been long dealt with.

What are you guys doing in traffic? Switch off every five minutes or so?

 

[Thommo]

Letting engines idle for excessive time can glaze the bores and compression is lost. Don't hear of it in modern cars but I was told of a case where a Cat Diesel on a drilling rig had a faulty battery so the drillers didn't switch it off o/night. In the morning blowby pressure in the sump had blown oil everywhere.

Fixed by blowing some "Bon Ami" (an abrasive kitchen cleaner) down the intake.

 

[WhoBeDaPlaya]

There is really zero need to do that. Unless it is brutally cold outside you can safely drive off within 10-15 seconds after starting the engine. Just don't romp on it until it is up to normal operating temperature.

True. I just hit the remote start right as I'm gathering my stuff for work.
Coming back from work, I do it to help cool the car down.
Probably exaggerated the idle time - more like a minute or two.

In between, I just usually start moving after my navi loads and I hit the legal nagscreen, buckle up, etc. If it's cold, no punching it of course.

 

[AstroGuardian]

Why does anything ever have to be explained?

Because of techy guys like me

 

[AstroGuardian]

This is wrong.

Not sure what you are talking about by "edge of operating" and "balance between individual cylinders"... but:

The AFR on a standard fuel injected engine (no direct injection required) can reach 16:1 and higher and 40+ deg spark advance on an idling engine with no load, making it much more efficient than it would be under load or cruise in the 9:1 to 12:1 range, regardless of any other tuning parameters. Engines are extremely efficient at idle.

The stumble when you make a sudden throttle transition is called a transient and it is accommodated for via the throttle position sensor and an accelerator shot. Additionally there is another factor called wall film compensation for fuel that accumulates in the ports which suddenly breaks free off the port walls when the airflow is suddenly increased. No properly tuned and maintained EFI engine should stumble at all when throttling up from idle regardless how long it sits idling.

When an engine reaches operating temperature, it stays there. You have a thermostat and automatic fans that will maintain an absolute max coolant temperature of around 200 deg. It will maintain this indefinitely regardless if the car is moving or not. You will not exceed this temperature idling. Nor will you fall far below this temperature driving, as the fans will stay off and the thermostat will close in an attempt to KEEP this temperature.

Mechanical wear and tear is negligible idling compared to WOT and starting. And other theories such as oil/fuel contamination are also bogus. Cylinder pressure is lowest when idling with no load resulting in minimal blow by, and, as described above, the mixture at idle is so lean that there is minimal fuel in the combustion chamber TO blow by or cause bore wash.

There is no logical reason against idling a car other than wasting gas and creating unnecessary emissions.
.

Hmmm... looks like there are some factors in effect but their influence is minimal enough not to consider them. That's a conclusion i would draw from this discussion.

 

[Zenmervolt]

Zero stress on the transmission? that would be parked and shut off. Idling it is at the very least spinning the input shaft.

And unless something is wrong with the design, this is still the lowest amount of stress possible short of the car being shut off. A manual transmission in neutral or even an automatic in neutral, sees very little loading; so little, in fact, that unless something is malfunctioning, it's never going to be a worry no matter how long the car idles.

It was designed to drive down the road. If it were designed to idle it would be sitting on a pallet on a dock somewhere, producing some fixed amount of power all the time.

It was designed to withstand typical operation as a car. This includes extended periods of stop-and-go driving and owners idling the car to warm up the interior in cold climates.

Cool exhaust = condensation = corrosion.

The engine and exhaust still reach operating temperature when idling. The idea that the exhaust somehow stays cold is ridiculous. It will take longer to warm up, but it's not like it's going to stay cold forever.

Idling is at the very edge of the engine's operating range. The fuel/air ratio is not as good as it is under a load. This causes fouling of plugs, O2 sensors, etc.

Unless you had a complete and utter idiot programming the ECU, any EFI system will maintain a near-perfect air/fuel ratio even at idle. The idea that idle mixture is substantially imperfect is a relic from the days of carburetors; a modern EFI system can idle indefinitely without fouling anything.

The biggest issues from idling with modern cars are, firstly that it's just not efficient or necessary so you end up wasting a lot of fuel (around 0.5 gallons/hour with a warm engine) and secondly that the extra stress on the cooling system (specifically the fans) may cause issues. The electric fans in modern cars are not designed to run continuously and extended idling could cause them to fail, which would then lead to overheating.

ZV

 

[LTC8K6]

Isn't the car in open loop mode when it's warming up? Thus making the a/f mixture a fixed setting off of a map? No feedback in use.

 

[JCH13]

Many are, yes. I've got a handy little Dashhawk that shows me AFR information and it's a *little* rich, around 13.8 or so, but the car will switch to closed-loop after it warms up and increase the AFR to around 15.0 after a few minutes.

 

[JeffreyLebowski]

I usually let my car idle for 3-5 mins in the mornings and after work, then drive it easy for the first couple of miles.

Why?

In fuel injected engines there is no reason to let it idle for more than 15-20 seconds to get the oil flowing.

 

[JeffreyLebowski]

On the other hand, you shouldn't stop and start your car too much either. I made that mistake while waiting in line for a car wash. Rather than idling for ~3 minutes while each car ahead of me got washed, I just shut the engine off. Third or fourth time, the car wouldn't start. Luckily it did eventually start after it sat around for a few minutes. Scared me though.

That is more likely heat soak with your starter.

 

[Zenmervolt]

Isn't the car in open loop mode when it's warming up? Thus making the a/f mixture a fixed setting off of a map? No feedback in use.

Yes, but it'll be in open loop during warm up whether it's idling or not and it still would need an ECU programmed by an idiot to have really wonky a/f ratios even in open loop.

ZV