Idle engine=dead engine on ground

[Arnie7781]

I taxied a 2007 C172SP out to the runway this morning. Temperature was about 40, airport elevation is 600ft. My student pulled the throttle back to idle at the hold short line and the engine sputtered and died. I started it back up, ran it up to 1200 RPM or so, then pulled the throttle back again. The engine quit again.

I tried restarting it twice (warm engine start then flooded start), but wasn't successful. Rather than burn out the starter, I decided to get towed back to the ramp. I read through the taxiing procedures in the POH and obviously it read, "lean for taxi." Is there anyway too rich of a mixture can lead to flooding and thus lead to the engine quitting? What are the odds that it's a larger issue with the fuel system?

 

[Roger, Roger]

Mechanics need to check the idle RPM and mixture adjustment. And yes, too rich of an idle mixture can cause the engine to quit when running at idle. However, a rich idle mixture usually shows up as fouled spark plugs (big RPM drop during mag check). The new injected 172s are especially susceptible. A low idle RPM is more likely to make the engine quit when you pull the throttle all the way back.

If your mechanic says the idle settings are good, next time you start, lean the mixture out until the RPM peaks (works best with the engine running about 1000 RPM). Also, when you're sitting still at the hold short line, run the engine up to about 1000 RPM. Makes the engine sound a lot happier when you're just sitting there.

 

[Arnie7781]

Mechanics need to check the idle RPM and mixture adjustment. And yes, too rich of an idle mixture can cause the engine to quit when running at idle. However, a rich idle mixture usually shows up as fouled spark plugs (big RPM drop during mag check). The new injected 172s are especially susceptible. A low idle RPM is more likely to make the engine quit when you pull the throttle all the way back.

If your mechanic says the idle settings are good, next time you start, lean the mixture out until the RPM peaks (works best with the engine running about 1000 RPM). Also, when you're sitting still at the hold short line, run the engine up to about 1000 RPM. Makes the engine sound a lot happier when you're just sitting there.

Yeah, I'll check with the mechanics tomorrow.

After instructing in Florida in July and August, I just didn't think it was possible to over enrich the mixture in February in Kentucky to the point of killing the engine. Worse case used to be get a little spark plug fouling, but burn it off in during the run up. Then again, I don't have much time in the newer 172s. I guess I'll be a bit more careful next time. Glad it happened when it did and not during a simulated engine failure.

 

[Blip16]

was the engine warm, in the green bar?

 

[Roger, Roger]

Sometimes its just the right combination of circumstances. I had the engine quit turning just after a student touched down on a night IFR x-country. Started up and ran just fine...

 

[Arnie7781]

was the engine warm, in the green bar?

Good question.

Honestly, we were pulling up to the hold short line when the student pulled the throttle idle. The engine cut off before the run up, which is when I really start paying attention to the temperature (right before the run up). So, I can't truthfully say I saw the oil temp. The whole scenario took .2 on the hobbs and the outside temp was 40, so I'd say it had time to warm up. 7 quarts in the pre-flight. Pressure was obviously fine.

Did I mention it was an intro flight?

 

[Arnie7781]

Sometimes its just the right combination of circumstances. I had the engine quit turning just after a student touched down on a night IFR x-country. Started up and ran just fine...

Agreed. Situations like these will keep you on your toes.

 

[Blip16]

Good question.

Honestly, we were pulling up to the hold short line when the student pulled the throttle idle. The engine cut off before the run up, which is when I really start paying attention to the temperature (right before the run up). So, I can't truthfully say I saw the oil temp. The whole scenario took .2 on the hobbs and the outside temp was 40, so I'd say it had time to warm up. 7 quarts in the pre-flight. Pressure was obviously fine.

Did I mention it was an intro flight?

might have be almost warm, or bottom of the arc. rapid throttle decrease and luke warm engine usually means death (for the engine). it does in the airplanes i fly and my bike (air cooled). this is the reason i will not take off w/o an engine in the green. .2 could be as low as 7 minutes total time, hardly enough for it to warm up yet.

could also be the idle setting too low, but probably a combo of both cold engine and idle setting.

 

[pilotsav27]

To the OP, where you located in KY?

 

[Douglas]

Good question.

Honestly, we were pulling up to the hold short line when the student pulled the throttle idle. The engine cut off before the run up, which is when I really start paying attention to the temperature (right before the run up). So, I can't truthfully say I saw the oil temp. The whole scenario took .2 on the hobbs and the outside temp was 40, so I'd say it had time to warm up. 7 quarts in the pre-flight. Pressure was obviously fine.

Did I mention it was an intro flight?

You could have been in the green arc, but in my experience with that airplane in those temps, .2 taxiing doesn't get the needle off the peg.

I would put my money (very little money) on it idled to low and died because of it, not due to temp.

 

[BrewMaster]

Almost all my single-engine experience has been in a 172N/P/R/RG, and I have found that anything below 1000rpm on most models will idle fairly rough or 'not happy.' What I mean is I don't think temp or maybe even mixture were the factor, allthough leaning right on start is not a bad idea. I would say that idling too low was the issue. We actually have 1000rpm on our checklists for idle to keep that from happening.

Lean it a little bit on start up, not necessarily to peak, but a few twists, and keep it at or around 1000rpm. That is what we do and have never had an issue.

 

[deadstick]

Didn't Cessna/Lyc put out something a couple of years ago about seasonal mixture/throttle adjustments on the 172R/S to prevent this?

 

[Arnie7781]

Sorry if I haven't been clear, but the engine quit as soon as the trottle was brought back to idle, almost as though the mixture had been pulled. It was not a situation where we were holding short of the runway for an extended period of time at low idle. My student pullled the throttle back, applied brakes and the engine quit. It was no different than the time it takes to perform an idle check during the run up.

I've been doing idle checks on 172SP models (mostly '01, '03) for a while now, and I understand that the engine doesn't really like that, BUT until yesterday I've never had one quit on me.

 

[braunpilot]

To answer deadstick, yes, there have been I believe SB's out talking about seasonal changes. I fly 210's & Barons around they cessna does talk about doing 4 seasonal changes to the FF. With that being said though, it could also be just a low idle RPM thing. If the idle is set to low it will die no matter what the mixture setting is. I have had a couple of 172's and 210's quit while doing the runnup. Just means that I tell the mechanics. I even had one where I would actually lean the mixture on departure because otherwise I wasn't getting full power. It took another year before mx got that issue sorted out. Not their fault. Lyc had issues with the IO-550 STC for some 210's.

 

[Chief Captain]

When I used to fly the 172R and SP, I'd usually go darn near .3 before it was warm enough to do the runup. Where I fly, the temps rarely fall below 70.

 

[nosehair]

I've been doing idle checks on 172SP models (mostly '01, '03) for a while now, and I understand that the engine doesn't really like that, BUT until yesterday I've never had one quit on me.

...so now, you'll really do idle checks, won't you?

I didn't used to do them 'til I had one quit on me in the air when I pulled it back to idle.

Now, the idle check is right up there with the mag check.

 

[trafficinsight]

Lyc had issues with the IO-550 STC for some 210's.

I can see why, since it's a Continental engine.

But seriously.

This is one of those things that you should tell the mechanic, but I wouldn't lose sleep over. Idle should be in the ballpark of 650 to 700, WARM.... like real warm... like after the flight warm. until the temps are up there's no guarantee it'll idle at all.

If there really is something wrong with the idle circuit, the engine wont run at all below about 1300rpm.

Remember that all the idle speed setting is is a stop on the throttle travel... so if it dies when you pull it allllllllll the way back... don't pull it back that far

 

[Roger, Roger]

If there really is something wrong with the idle circuit, the engine wont run at all below about 1300rpm.

Remember, this is an injected engine soo...no separate idle circuit. Just an adjustment of the throttle fuel valve linkage length to set idle fuel flow

 

[trafficinsight]

Remember, this is an injected engine soo...no separate idle circuit. Just an adjustment of the throttle fuel valve linkage length to set idle fuel flow

That's idle mixture, the idle speed is still just a stop on the throttle.

at any rate, you're right, it's not a separate idle circuit, but it's an idle circuit just the same.

What I probably should have said is, if you are pulling it back and it's dropping below that 650-700 range, or whatever the idle speed for the airplane is, and then dying, just don't pull it that far back. If it'll run at 650-700 then it's idling fine. Check it again after the flight and if it's still too low tell someone about it. Alternately, I suppose, you could run it until the temps are in the normal operating range and then check it.

 

[mhcasey]

This happened to be every once in a while in recips when it was cold. My guess is the fuel is just not vaporizing readily at that temp, so when you pull the throttle back all the way there's nothing to burn. I would definitely talk it over with the mechanics, but this could just be one of those "know your airplane" deals. I used to fly a 182 that always had low temp and high pressure on the ground, but after a few minutes of flying it was fine.

Can any of our friends from up north chime in?