IO-360 ‘warm engine’ start on M20J

[Tiny18Driver]

Hi all-

Does anyone have thoughts on starting a sort-of warm engine that’s most likely in vapor lock? 

Hot start— check, no issues. Standard cold start— check no issues. Flooded start— check, no issues. An engine that has sat for about 2 hours— kicking my butt.

Previous chats and the internets suggest going with a hot engine start (throttle - slightly in, mixture - cutoff, no boost). That’s not really working. My Plan B has been to boost it to a flooded state (checklist style with throttle cracked, and mixture rich) then doing the AFM Flooded Engine Start. Also not really working. 

Now I’ve seen a suggestion to boost it to a flooded state by: Mixture - Rich, Throttle - FULL OPEN, Boost pump - ON 3-5 seconds, then crank with Throttle - ~1000RPM, Mixture - Idle/Cutoff.

I guess it sort of makes sense to boost with throttle fuel open to purge fuel lines of vapor, but that’s a new one to me… so I now open the floor for suggestions.

Cheers!

[Mooney in Oz]

5 minutes ago, Tiny18Driver said:

I guess it sort of makes sense to boost with throttle fuel open to purge fuel lines of vapor, but that’s a new one to me… so I now open the floor for suggestions.

If mine has been shutdown for longer than about 20-30 minutes, I fully open the throttle, give it a quick half a second boost pump squirt of fuel, close the throttle then do a normal start.

This method generally works for me provided I don’t over do it with the boost pump.

[Ragsf15e]

I generally did them in this order when warmish…

1. Hot start

2. Normal prime, normal start

3. Flood it, flooded start.

 Don’t remember getting to #3 more than once or twice in 10 years.  
Fyi, I always prime with throttle full open, mix full rich as that’s the most flow.

[Joe Linnebur]

Like Mooney in Oz said above, in those ‘intermediate’ situations I tend to go rich mixture, throttle full, then a few second burst of the fuel pump. Then I do a hot start technique (throttle open a little more than normal, mixture lean). Seems to light off within 10 blades. A little throttle “in and out action” sometimes is needed to get the air/fuel ratio right, but that might be our engine.

[MikeOH]

For anything but a cold stat my technique is to leave the throttle where it was at shutdown (1100 rpm), NO boost pump, mixture ICO, start cranking and WAIT 2 to 3 seconds to see if it lights off...if not, then I SLOWLY start advancing mixture until it starts.  This works for me for anything from a few minutes to a couple of hours after shutdown.

[0TreeLemur]

21 minutes ago, MikeOH said:

For anything but a cold stat my technique is to leave the throttle where it was at shutdown (1100 rpm), NO boost pump, mixture ICO, start cranking and WAIT 2 to 3 seconds to see if it lights off...if not, then I SLOWLY start advancing mixture until it starts.  This works for me for anything from a few minutes to a couple of hours after shutdown.

This is what I do too.   And it allmost always works.   There is a video by Don Maxwell about hot-starting a Mooney that I really enjoyed and thought was educational.  It's linked here on this site multiple places I'm sure.  In that video he said "Lycoming engines always start flooded."   I took that to heart and it relieved my stress about starting a hot or warm engine.

[Z W]

Here's how Continental says to do it, from their "Tips on Engine Care" publication. Works every time. Full documentation attached and worth the read. Lycoming people may have a different experience.

Hot Start Procedures - Continental.pdf

[Shadrach]

5 hours ago, Z W said:

Here's how Continental says to do it, from their "Tips on Engine Care" publication. Works every time. Full documentation attached and worth the read. Lycoming people may have a different experience.

Hot Start Procedures - Continental.pdf 994.51 kB · 0 downloads

You would expect people to have different experiences, given the differences between the Bendix RSA style fuel system and the Continental fuel system. Lycoming’s have no return line from the servo as there is with the Continental. This means that you cannot circulate fuel through the servo. It’s a one-way street from the tank to the injectors. This does not mean that it is difficult to hot start an injected Lycoming engine, it just means that one must use procedures oriented towards the Bedix RSA style fuel system.  Challenging hot starts are often related to ignition issues. These engines really aren’t that hard to start when hot. I’m proof that it doesn’t take a genius.

[GeeBee]

1 hour ago, Shadrach said:

You would expect people to have different experiences, given the differences between the Bendix RSA style fuel system, and the Continental fuel system. Lycoming have no return line from the servo as there is with the Continental. This means that you cannot circulate fuel through the servo. It’s a one-way street from the tank to the injectors. This does not mean that it is difficult to hot start an injected Lycoming engine, it just means that one must use procedures oriented towards the Bedix RSA style fuel system.  Challenging hot starts are often related to ignition issues. These engines really aren’t that hard to start when hot. I’m proof that it doesn’t take a genius.

That is correct. Lycoming and Continental require different approaches.

[McMooney]

i do exactly as you've been told,

 

hot start, if that doesn't work, flood it.

[Ragsf15e]

13 minutes ago, McMooney said:

i do exactly as you've been told,

 

hot start, if that doesn't work, flood it.

Just be careful.  If you have to flood it often because it won’t start hot/warm, you’re doing a lot more starts than needed with excessive fuel in the engine.  It’s in the exhaust, maybe dripping down the intakes, etc.  there’s maybe a slightly increased risk of fire.  The engine should start pretty well if the ignition system is set up correctly and the plugs are good.  Once in a great while, a flooded start happens, but shouldn’t become SOP.

[Shadrach]

1 hour ago, McMooney said:

i do exactly as you've been told,

 

hot start, if that doesn't work, flood it.

I think that’s a sub-optimal approach. I have seen flooded engines catch fire at the pump. I’m kidding. Starting an engine simply requires a combustible mix of fuel and air.  I have never had to flood a Lycoming engine to get it to start.

[ArtVandelay]

Does no one check the fuel pressure before starting?

[DC_Brasil]

17 hours ago, MikeOH said:

For anything but a cold stat my technique is to leave the throttle where it was at shutdown (1100 rpm), NO boost pump, mixture ICO, start cranking and WAIT 2 to 3 seconds to see if it lights off...if not, then I SLOWLY start advancing mixture until it starts.  This works for me for anything from a few minutes to a couple of hours after shutdown.

Lycoming IO-360-A3B6D here. 

That's exactly how I do it.

[McMooney]

10 hours ago, Shadrach said:

I think that’s a sub-optimal approach. I have seen flooded engines catch fire at the pump. I’m kidding. Starting an engine simply requires a combustible mix of fuel and air.  I have never had to flood a Lycoming engine to get it to start.

probably true, but, it works within 5 or 6 blades every time.  Guess you could just crank with the pump on till it starts.

[Justin Schmidt]

29 minutes ago, McMooney said:

probably true, but, it works within 5 or 6 blades every time.  Guess you could just crank with the pump on till it starts.

Mine backfires doing that

[Fly Boomer]

On 10/3/2025 at 7:28 PM, Tiny18Driver said:

Does anyone have thoughts on starting a sort-of warm engine that’s most likely in vapor lock? 

This is Maxwell's hot start procedure, but it may suggest an approach:

https://youtu.be/jbRYqS-fRo0

 

[DCarlton]

On 10/3/2025 at 9:21 PM, MikeOH said:

For anything but a cold stat my technique is to leave the throttle where it was at shutdown (1100 rpm), NO boost pump, mixture ICO, start cranking and WAIT 2 to 3 seconds to see if it lights off...if not, then I SLOWLY start advancing mixture until it starts.  This works for me for anything from a few minutes to a couple of hours after shutdown.

I've been doing this too (basically the Maxwell procedure) since the last discussion on MS.  I used it yesterday.  Works great.  Haven't found a need recently to go full throttle with mixture off, then reverse the two when it starts.  I do sometimes advance the mixture too fast though; gotta ease it in.  Funny though; I'm still as puckered up with hot starts as I've always been even though I'm having no issues.  Some sort of hot start PTSD.  

[MikeOH]

1 hour ago, DCarlton said:

Some sort of hot start PTSD

Me, too!

The ‘trick’ where it differs a little from Maxwell’s technique is cranking a few second BEFORE moving the mixture from ICO. This ensures the mixture is lean, first!

[Beard]

I do similar to MikeOH on a hot start. I pull the mixture at 1,000 RPM to shut down and when I restart I slowly push the mixture in (without touching the throttle) while cranking. It typically catches when the mixture is the richest. The is a Youtube video demonstrating this.

[Tiny18Driver]

Thanks everyone for the discussion. I went back and looked at the video again and keyed-in on the fact that the Lycoming basically floods itself after shutdown. So in my case, if I pushed the mixture forward too quickly (don’t remember if I did or not), the motor didn’t fully light-off because it re-flooded. If that is indeed true, my previous “Plan B” to do a normal start 2nd, because I’m assuming the cylinders are ‘dry,’  actually exacerbates the problem since the cylinders are actually crazy flooded by that point.

So now my question becomes: is there some sort of a rule-of-thumb anyone uses in choosing between boosting first (as in a normal cold start), or a hot start after the plane has sat for a period of time after a flight (e.g. out-&-back). I understand a hot motor and an obviously cold motor, is there some clue as to whether there’s still fuel / vapor in the fuel lines and the cylinders, or if it’s all evaporated out of a ‘warm’ motor?

[Ragsf15e]

2 hours ago, Tiny18Driver said:

Thanks everyone for the discussion. I went back and looked at the video again and keyed-in on the fact that the Lycoming basically floods itself after shutdown. So in my case, if I pushed the mixture forward too quickly (don’t remember if I did or not), the motor didn’t fully light-off because it re-flooded. If that is indeed true, my previous “Plan B” to do a normal start 2nd, because I’m assuming the cylinders are ‘dry,’  actually exacerbates the problem since the cylinders are actually crazy flooded by that point.

So now my question becomes: is there some sort of a rule-of-thumb anyone uses in choosing between boosting first (as in a normal cold start), or a hot start after the plane has sat for a period of time after a flight (e.g. out-&-back). I understand a hot motor and an obviously cold motor, is there some clue as to whether there’s still fuel / vapor in the fuel lines and the cylinders, or if it’s all evaporated out of a ‘warm’ motor?

If it’s warm, treat it as hot, don’t add fuel.  If it’s warm never even fires at all, then you can prime it.  Always try the hot start first if it’s even a little warm, because if you add fuel as in a normal start, you can’t go back.

[MikeOH]

2 hours ago, Tiny18Driver said:

Thanks everyone for the discussion. I went back and looked at the video again and keyed-in on the fact that the Lycoming basically floods itself after shutdown. So in my case, if I pushed the mixture forward too quickly (don’t remember if I did or not), the motor didn’t fully light-off because it re-flooded. If that is indeed true, my previous “Plan B” to do a normal start 2nd, because I’m assuming the cylinders are ‘dry,’  actually exacerbates the problem since the cylinders are actually crazy flooded by that point.

So now my question becomes: is there some sort of a rule-of-thumb anyone uses in choosing between boosting first (as in a normal cold start), or a hot start after the plane has sat for a period of time after a flight (e.g. out-&-back). I understand a hot motor and an obviously cold motor, is there some clue as to whether there’s still fuel / vapor in the fuel lines and the cylinders, or if it’s all evaporated out of a ‘warm’ motor?

This is why you need to crank for a bit BEFORE you advance the mixture AT ALL!  That is the 'clue' to if there is fuel/vapor remaining.  If there is still fuel/vapor from the last shutdown the engine will fire.  If it does NOT, then SLOWLY move the mixture and introduce fuel; it will fire when you get to the proper fuel/air ratio.

The ONLY reason to run the boost pump/prime is if the engine is stone cold and you need to get fuel in there.  Otherwise, you're going to flood it.

This advice is for fuel injected Lycoming IO-360s ONLY!  I have no experience with carbs or Continentals

[Shadrach]

7 hours ago, MikeOH said:

The ONLY reason to run the boost pump/prime is if the engine is stone cold and you need to get fuel in there.  Otherwise, you're going to flood it.

Not necessarily. If the engine fires but doesn’t start (hot or cold) a shot of prime will facilitate easier starting on the next attempt. 
 

This is not rocket science nor is it Voodoo. Hot engines prime themselves when heat boils the fuel in the injector lines pushing it into the manifold. If that fuel is combusted or pushed into the exhaust before the engine starts, it will need to be replaced to provide a combustible mixture for the next attempt.  There are several ways to accomplish this, but I think people would do best just to simply think about what’s happening underneath the cowl and act accordingly.

[Paul Thomas]

36 minutes ago, Shadrach said:

Not necessarily. If the engine fires but doesn’t start (hot or cold) a shot of prime will facilitate easier starting on the next attempt. 
 

This is not rocket science nor is it Voodoo. Hot engines prime themselves when heat boils the fuel in the injector lines pushing it into the manifold. If that fuel is combusted or pushed into the exhaust before the engine starts, it will need to be replaced to provide a combustible mixture for the next attempt.  There are several ways to accomplish this, but I think people would do best just to simply think about what’s happening underneath the cowl and act accordingly.

I suspect you can picture that in your mind and see how the fuel travels through the system enabling you to think through what needs to be done. Some people do not have that capability and it's not a knock on them; people process information differently. Some people need to rely on the if/then logic and need to follow the checklist.