Warm idle question

[Swift101]

Hey Everyone,

We all know idling to warm up a cold engine isn't the best idea due to fuel dilution and other issues. But what about idling after driving for an hour? Would I be wrong to assume it's less harmful since everything is up to operating temperature?

-Thanks

 

[4wheeldog]

Just wastes gas. I do it only when the outside temp is really uncomfortable. Not particularly a problem, as long as it does not overheat.

 

[Cressida]

I believe that is a good assumption.

Using my Scanguage 2, I recently noted that my 1.8L Toyota engine idled (warmed up / normal operating temp) at 700 rpm and consistently consumes 0.17 gph of fuel.

Fwiw, I don't understand how there can be much fuel savings with the hybrid engines that turn off the engine at stop signs since you can see that the amount of fuel being used for one minute of idle time is almost nothing.

 

[HerrStig]

Happens in the frozen north when stuck in traffic jams.

 

[rslifkin]

Idling once the engine is warm is still not ideal due to lower combustion temps (can foul spark plugs or cause other issues in some motors). But it doesn't cause much outright wear, unlike idling on a cold engine.

Ideally, if it needs to be kept idling for whatever reason, the idle speed should be raised a few hundred rpm over the normal idle (keeps combustion temps and oil pressure up a bit better and provides better alternator output).

This reminds me, I should hook up the scanner setup and watch my injector pulse width to see how much more fuel the Jeep burns at high idle (1000 rpm) compared to normal idle (680 rpm) when warmed up. Probably not a big difference, as it's in open loop at ~13.5:1 AFR at normal idle and closed loop and ~15.5:1 AFR at high idle and it's also out of the idle timing maps at high idle (and running 11* more timing than curb idle, IIRC).

 

[SwampSurvivor]

And some engines won't lube the "top end" as well idling. I don't like shutting my Jeep off when on a trail, but I'll do so if I am going to be sitting for a while as the Jeep 4.0s won't lubricate their cam bearings and lifters. Once I install the high idle switch, won't be a problem.

 

[zach1900]

Idling is hard on valve springs. No inertia.

 

[rslifkin]

Originally Posted By: Miller88
And some engines won't lube the "top end" as well idling. I don't like shutting my Jeep off when on a trail, but I'll do so if I am going to be sitting for a while as the Jeep 4.0s won't lubricate their cam bearings and lifters. Once I install the high idle switch, won't be a problem.


I was very happy when I found the option in my tuning software to enable this feature in the ZJ (and I then wired up the switch). Had to disable a few PCM integrity checks to avoid a CEL, but it works just like an XJ (the PCM was sitting there thinking "this feature is enabled, but I'm not a 4.0 XJ, so it shouldn't be. I must be broken!")

I'll admit I may have over-used it a little at a rallycross event last winter when it was about 0* F outside. Kept the ZJ running the entire night, just parked it and threw it to high idle between runs to keep the engine and cabin warm. Only times I shut it down were before the first set of runs (during registration and driver meeting) and during the dinner break (it ran continuously for ~2.5 hours before dinner and at least 4 hours after, including the hour drive home).

 

[bigt61]

Originally Posted By: zach1900
Idling is hard on valve springs. No inertia.

What?

 

[meborder]

Originally Posted By: zach1900
Idling is hard on valve springs. No inertia.

That's probably the dumbest thing I've ever heard.

 

[Johnny2Bad]

Lengthy Engine Idle is severe service in a gas motor, and I can say from experience where extreme cold means you do run the motor for the full work shift, it significantly reduces overall engine life. I'm talking blown engines @ 80,000 Mi.

 

[CT8]

Originally Posted By: zach1900
Idling is hard on valve springs. No inertia.
What inertia are you talking about?

 

[mctmatt]

I swear that I read you need to let your car Idle for a few minutes after start up before you put it into gear instead of just starting the car and driving off.

 

[LotI]

Originally Posted By: mctmatt
I swear that I read you need to let your car Idle for a few minutes after start up before you put it into gear instead of just starting the car and driving off.

In your owners manual? You don't need to recall, just go read it.

 

[Garak]

My G37's manual calls for a 30 second warmup.

 

[rslifkin]

Some newer cars idle very, very high right at startup when cold. On those, I'd generally let it idle for 30 seconds or so to let the revs come down before driving it.

In the Jeep, unless it's particularly cold out, it gets a couple seconds once it hits a stable idle and then off we go. If it's below 20* or so, I'll give it an extra 30 seconds. It only gets longer than that if I need heat for the defrosters before it's safe to drive.

 

[SwampSurvivor]

When it's cold out I usually try to let the Focus drop below 2500RPM. Trying to drive in traffic with an engine that absolutely REFUSES to run at below 2500 rpm (especially with a manual transmission) can be interesting.

Also makes it hard to shift when the transmission lube is like molasses and the engine races up to 2500, 3000 when the clutch is pressed.

 

[BRZED]

Originally Posted By: meborder
Originally Posted By: zach1900
Idling is hard on valve springs. No inertia.

That's probably the dumbest thing I've ever heard.


What Zach means is that inertia of the valve uncreases with RPM. At idle, the valve pring pushes with its full force against the cam lobe, causing increased wear. The higher the RPM, the higher the intertia of the valve, thus less pressure and decreased wear on the cam lobe. When the spring can no longer overcome the inertia of the valve, valve float occurs, which deteriorates performance.

 

[zach1900]

Originally Posted By: meborder
Originally Posted By: zach1900
Idling is hard on valve springs. No inertia.

That's probably the dumbest thing I've ever heard.
Why thank you.

 

[zach1900]

Nothing heats an engine up like driving it gently, If it's below 30F i actually have to drive the Camry to get the needle to move up otherwise i don't think it ever does, or at least it won't in under 30 minutes.