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Injection Engine Start-up science (or metaphysics !)

Hey there.

Flying on two injected engine equipped aircraft (DR500 with IO360 and TB20 with IO540), I have no problems with starting a cold engine, but I have a lot of troubles starting a hot engine.

I searched a lot of information in the POH of these aircraft and on the web, but I am far from being a mechanic amateur, so I will ask some (probably candid) questions.

First of all : when do you consider your engine hot ?
I saw and heard anything and everything :
- engine hot if turned off for less than 20 minutes (cold after 20 minutes)
- cold if oil temp is below yellow mark (hot above yellow mark)
- cold if you can keep your hand put on the engine (sic).

If I start a cold engine (like really cold, first start of the day), and taxi to the fuel station (so 3’ between start and shutdown).
Do you consider it cold or hot ?

A few weeks back, I tried to start the DR500 using the “hot engine” procedure. (engine shutdown for less than 10 minutes, still in the green area, so hotter than hot).
First try : engine coughed, mixture on, but no catch, and starter turned in the wind.
Second try : hot procedure again. Nothing.
Third try : flooded procedure. Nothing.

At this point, my chief pilot came to see me. He manually turned the propeller a few times, to “clear it”.
He applied hot engine start procedure, and the engine started in a 1/4 of second.

So I am trying to understand what I am doing wrong here.
I am following the procedure “at the letter”.

I naturally tend to think that if the engine does not start, it means it does not have enough fuel.
I think there can be my mistake, and on an hot injection engine, less fuel injection is better than too much.

What’s the purpose of hand turning the propeller ? clearing all the excessive fuel in the cylinders ?

If you have any tips or advices, you are welcome.


My policy is this: if in doubt, presume hot.
It’s better to fail to start with a hot start procedure that to fail to start with a cold start procedure.
Because you are trying to prevent “flooding” the engine with too much fuel so risking fouling the plugs or worse still, an engine fire.
If it doesn’t start using hot start procedure then you can usually be fairly confident that it will then start with the cold start procedure but you cannot be so sure the other way around.

Forever learning

Sorry to sound unhelpful, but all these “hot starts threads” go pretty much the same way. Everybody posts “his” own procedure which “always works” (which he uses in his very own aircraft but thinks it has general validity) and in the end, nothing has been learned.

The reality is that every aircraft is different. Now I don’t mean aircraft types, and not even specific variants. Every single aircraft is different. Reasons are the condition and setup of the ignition system and fuel injection system / carburettor. Hence, it can even change after the same aircraft has had maintenance on these components.
So, the only way is to get intimate with one’s specific aircraft and understand how it really ticks. You can almost always forget what is written in the POH.

Regarding flooding: some aircraft are very prone to flooding, others are impossible to flood. GENERALLY, the IO-360 (which can indeed be a bitch to start when hot) is one that is rather easy to flood in my experience with Arrows and Mooneys.
In any case, turning a prop “a few times” by hand will likely do nothing to bring a truely flooded engine back to a state where it can start. It takes much more than a few turns.

Last Edited by boscomantico at 02 Jan 16:41
Mainz (EDFZ), Germany

In general terms, people squirt too much fuel into a hot engine. There’s also a tendency to thrust the red forward at the first cough – be gentle.

To me, if the oil temp gauge is not on the bottom stop then consider the engine hot.

Fly safely
Various UK. Operate throughout Europe and Middle East, United Kingdom

I consider an injected engine hot anytime it has been running in the last 1-2 hours, even if it was just to taxi to the pump. That is mainly because it is much easier to add fuel later than to get it out, so I’d rather err on the side of too little fuel.

Ignition will happen anytime the fuel/air mixture in the cylinders is in the combustible range and you add a spark – too little fuel and too much air won’t work, but too much fuel with too little air won’t work either. When your chief pilot turned through the prop (with the mixture in idle cutoff?), he added air and sucked out whatever air/fuel mixture was in the cylinders, in order to remove excess fuel. The key to a successful start is knowing how much air and fuel you have put into the engine, and if that probably put you on the lean side (add more fuel with the primer if the engine doesn’t start) or the rich side (suck out fuel with the starter or by turning the prop with the mixture in idle cutoff and throttle wide open).

In the SR22 and also on other Continental engined planes, I would say I have developed a relatively good feel for how much I want to prime in what conditions, i.e. how much fuel I want to add according to temperature and density altitude. (Other planes like the Seneca even have a diagram with the recommended priming durations in the AFM/POH.) There is a hot start procedure for the IO550N that works well for me, which consists in opening the throttle 1 cm, advancing the mixture to full rich, then with one hand on the starter switching on the boost pump to “low boost” and listening for the pitch of the fuel pump. As soon as it changes in tone (after about 1-2 seconds) as it starts delivering fuel, I turn the starter, and usually the engine will start right away. I don’t know if this would work in the DR500, but it is probably started with the boost pump off, so this wouldn’t work?

Lycomings can be rather hard or impossible to hot start due to vapor lock in the fuel lines. Sometimes the only way is to wait for it to cool down, or to flood it with the boost pump to clear the vapor and hope that the flooded start procedure will work.

One thing that has baffled me in French aeroclubs (or is it just mine) is their insistence on priming carburetted engines with the throttle lever instead of the primer. That is not according to the POH, it is a fire risk, it is less effective and harder on the engine, so I don’t understand why anyone would insist on doing it that way?

Last Edited by Rwy20 at 02 Jan 16:50

Some engines seem to work using the method of recirculating the fuel as a coolant (recirculating it in ICO) But not all of them,. EG this principle worked well in a Seneca V, but not in a SR22.

Your mileage may vary…

Fuel system purge:
-Mixture cut-off.
-Props forward.
-Full throttle.
-Run the fuel pump for 30 to 60 seconds (40 is usually fine) with the mixture cut-off, then off.

Then use cold start procedure

Last Edited by Pilot-H at 02 Jan 17:01

After listening to and reading about the magic tricks of the hot start, I went back to the POH.
My not-cold starts (that ‘s when I apply them – everything when the enginge has not cooled down to ambient temperature) have sinde worked out well.
That’s with a IO540.

If I have taken (“hot”) a bit too far down the temperature scale, then bringing mix forward during the second starter run has always done the trick.
ahem almost always, I forgot once to set the fuel selector…

(The previous Conti IO360 in a tight cowling did have vapor lock issues and would require extended pump action with open throttle and closed mixture.)

What I do hear and see a lot, is folks not even trying to follow the POH, as if it were useless.
Cannot confimr that sentiment——-yet ;-)

EDM_, Germany

Thanks all for your feedbacks.

ch.ess wrote:

If I have taken (“hot”) a bit too far down the temperature scale, then bringing mix forward during the second starter run has always done the trick.
ahem almost always, I forgot once to set the fuel selector…

I am not sure to understand. Can you re-explain ?


Basically – everything not a cold start, I treat as “hot” from the procedure.
That means mixture cut-out and throttle about 1/4. When engine comes alive, mixture rich (and reduced throttle).

If I come back after say 4 hours (as has happened) and still do a “hot start”, and the enginge has not started after 1 or 2 prop turns, then I enrich the mix before the engine is alive. That has done the trick.

If you are not quick enough to move the mix forward, it feels better to stop the starter and start again after 15 secs.

(Amended: As Peter mentions later, the stronger/faster Skytec starter has further improved the starting performance)

Last Edited by ch.ess at 02 Jan 18:19
EDM_, Germany

Interestingly, for the C210 I fly mostly at the moment, the cold start procedure works perfectly well even when the engine is most definitely hot. Just two days ago dropped a friend off after a one hour flight and only shut down to let him disembark. Engine – a Conti IO 520-L – immediately came back to life using the standard procedure.

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