a different kind of idling question

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Oct 6, 2005
i know most of the people here are against cold idling. but what about hot idling, like you get to your destination early and you just sit in the car with the radio on while the car is running to maintain heat or a/c. since the car is already warm it cant be as bad as cold idling, right? unburned fuel getting into the oil cant be as bad, right?
racer wrote: i forgot about the fuel economy aspect.

lets neglect fuel economy.

Oh, Oh. Another anti-SUV thread coming?

As far as the question...I always wondered about that. I ASSUMED that there was minimial battery charging and excessive engine bay heat build-up...but, as far as the engine parts themselves, I HEARD that the oil pressure is also barely adequate for some parts. I have no idea if this is true. It seems to my pea-sized brain that if the cooling system is in good working order, since the car is under very light workload, lubrication wouldn't be a problem. Engine bay heat and battery charging might be a problem, though.
why another anti-SUV thread, i have a neon. the exact opposite of an suv. fuel economy is 28 - 30 mpg on a bad day.
i understand idling = 0 mpg, but im not concerned with that. please neglect fuel economy in you responses.
Here's my rationalizing: I want to save the starter motor (and battery) over saving gas. (At ~0.2 gph idling, you're burning almost one cent per minute.) But I also want to avoid the cooling fan coming on, as this indicates too much heat building up at standstill. So it becomes a little balancing act of do I shut down and hot start a couple of minutes later? Or just let the cooling fan do its thing while straining the cooling system, but saving the starter? If it's hot and I need AC, do I look for parking in the shade instead? It seems each instance needs to be evaluated on a case by case basis.
Well, my 05 Corolla states not to idle for more than 20min as it could damage the catalyst in the catalytic converter. Wierd, I've never heard this before, but maybe for a lean burn engine design it could be an issue.
My van idled for 4 hours last summer while I was working setting up a mobile home, blowed a O ring on the water inlet and didnt know it with a/c on, temp guage was buried on Hot.
Still running fine although I know it took some life from the engine.
cant park in the shade, none available in my parking lot at school.

we would start my shadow every now and again it was sitting waiting for me to get my license. one time i forgot about it and the next day when i was feeding my dogs i realized that the car was still running. oops. no ill effects from it. just burned a lot of gas.

one time i forgot about it and the next day when i was feeding my dogs i realized that the car was still running. oops.


Anything else would have seized up. Should have done a UOA.
Good post.

I have an 05 F150 4x4 4.6 It runs about 10-13hrs a day. 4 hours of that is typical drive time, the rest is all idle time. Hot roadway, hot summer, a/c on max with me in the vehicle doing paperwork or the keys locked in it while its running and me out doing inspection on needed things.

My 03 f150 on my last job had typical run times of 14+ hours, sometimes it ran 24hrs straight at idle while I slept in the truck then back to work. 2.5 hours per day was interstate driving.

OCI for the 03 was every 5,000, on the 05 I'm doing every 3,000.

Never a problem with either one.

Once the summer hits, I'll rarely shut it off, maybe for 20 min while I grab lunch somewhere.

I don't care about gas mileage. I'm nto sweating my butt off while I'm doing paperwork. I have no office so the truck is my office.
fuel dilution would be a big problem even though the engine is already hot and more would get burned compared to when cold.
We ran the Class 8 diesels 24/7 in my last job, only shut down to fuel (about 20-minutes, about every other day).

At night, due to the heat, we ran the AC to keep the sleeper cool. Advice from our company shop was to run the High Idle feature to keep the oil pressure up, and the chance of wiping the bearings waaay down.

Due to health, I have to idle my DODGE Ram (V8-318) for long periods in the summer, when at a stop. I use the right foot to kick the rpms up to about 1,100 to keep oil pressure above 40 psi.

Daimler-Chrysler used to fit police-spec vehicles (from the 1970's forward) with High Idle controls for the same purpose: Keep alternator at cut-in point, keep oil pressure up, and keep engine cooling fan at higher speed to overcome the lack of airflow through compartment.

For a comparison, I normally average 22-24 mph for each tank of fuel (per elapsed time meter), and in summer this can drop to 11-12 mph. Fuel mileage drops from around 13+ to the low 11's.

This kind of hard-use causes me to select premium synthetic oil and synthetic media filter, plus the use of LC and FP (with ARX clean-ups per spec). Having until 2004 only owned non-computer-controlled cars, I can attest to fuel dilution in carb'd cars as being significant, and, though modern one's light-years better, still, I want to see little or no degradation over time and miles.
I remember reading somewhere that an engine at operating temp that is idling for long periods may not throw enough oil onto the cylinder walls because it is at such low rpm, causing cylinder wear. This may be related to fuel dilution also.
I worked and lived in Houston in the late 70's and warly 80's when the oil business was really going. Everyone got company cars and they were not treated very well. With temps and humidity over 80 many would leave the engines running in parking lots while eating or shopping. They didn't care one way or the other and besides, they just put everything on company credit cards. Who wants to come back to a hot car. When something went wrong with the car running in the parking lot, it really made a mess. I was the outcast because I never did that kind of stuff. I don't even use drive-thru's because I'd rather park my car (and turn off the engine). So if your car or truck is just a tool, who cars. If not, you might want to turn the engine off. If weather is a consideration, then you just have to make the car work for you.
Good post Larry.

Not a chance that I'd let my personal truck run all day long. I'd bring a generator and rig up a topper with a window unit.
I good test for this is that new Silverado with the built in 10k watt generator. Run that truck hard with & without generator use and see if it holds up.

It's hard to say, my TDI can idle all day long in the summer and use the fan rarely unless the A/C is on and even then it is much less then a gasser. So a big rig could tolerate it much more I imagine then a gas truck. I really want a high idle switch though for the winter. Even if only idling for 2 mins, it would make more heat at 1600-2000 then at 950.

My oil pressure is at spec though, 20 psi fully warm at 950 idle rpms. It should settle around 60 when cruising on the highway. I can't understand how some engines get away with anything less then 15psi, seems way to low.
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