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  Hot Starting Your Continental IO-470/520/550



Have you Mastered the Hot Start of your big bore TCM engine?


Do you dread refueling in the summer and then having to get the engine started with everything in the engine bay heat soaked?


Well the thing that is really heat soaked that is the cause of the difficulties is the engine driven fuel pump! Yup, that puppy is really hot and guess what happens when it starts "pumping" the fuel? Yup, the fuel gets massive hot and vaporizes and there you go cranking 'till your battery is nearly dead. Been there - done that!


Here is a procedure adopted from Beech List experts, that has worked for me 99% of the time:


1. Front of throttle lever to the second white line (above the letter "T" in Baron, in Bonanzas about 1/4 throttle)


2. Mixture to ICO (Idle Cut Off)


3. Boost Pump to HIGH for 45 seconds or more. The hotter it is outside the longer you should run the boost pump. Put your seat belt on, get your frequencies out and stuff like that while the boost pump is running. The first time you do this have someone outside to be sure you are not throwing fuel out one of your vent lines. Your fuel flow gauge should remain at zero or very close to zero. A climbing pressure reading indicates that your mixture control cable or servo is not completely shutting off the fuel flow. (If fuel spews out onto the ground, your fuel system is in need of service, because the fuel servo is not remaining completely closed! Maybe you need to look at

TCM SB 01-1 and check for a seal leak in your fuel pump? See additional details below)


4. Shut off Boost Pump


5. Mixture to Full Rich


6. Give a Boost Pump shot to bring fuel pressure up on the gage


7. Shut Boost Off and go to Mixture ICO


8. Crank engine


9. At first couple of "pops" move mixture to full rich in a smooth medium speed motion


10. Release Starter as you get the mixture to Full Rich and the engine begins to fire on it's own.

This works for me the first time on 99% of the hot starts that I have encountered. If you get a balked start, that is, couple o pops and mixture to full but no start, Boost again at full rich, then take mixture to ICO and crank again at Step #8.


If one engine is just being a crank and you balk a second time, go to the other engine (Bonanza drivers do not have this option, sorry) and do the same thing, increasing Step #3 Boost to about a minute at ICO. My personal preference is to have the alternator off line until the engine starts, then bring that alternator on line.



If you are in ICO (Idle cut off) and 3-4 GPH is showing on your FF Gauge, read these comments from Mike Busch:


If you are seeing 3-4 GPH fuel flow with the mixture in ICO and the boost pump on, the problem cannot possibly be with the engine-driven fuel pump (what you're calling the injection pump, although that's not really correct terminology) or with the flow divider valve (what you're calling the fuel manifold). The problem can only be one place: in the fuel control unit. From your description, it sounds like that is the only component in the system that HASN'T been repaired!

The engine-driven fuel pump contains a bypass valve that allows fuel pressure from the boost pump to go to the fuel control unit even if the engine-driven pump fails completely. (If it didn't, you couldn't prime the engine.)

The fuel control unit contains two valves, one operated by the mixture control and the other operated by the throttle. The mixture control valve is a proportioning valve that determines how much of the fuel from the fuel pump goes to the cylinders (via the flow divider valve), and how much is returned to the fuel pump vapor tower (and then returned to the tank). If the mixture is full-rich, 100% of the fuel goes to the flow divider and 0% is returned. If the mixture is at ICO, very nearly 0% of the fuel goes to the flow divider and very nearly 100% is returned.

The flow divider contains a valve that shuts off the fuel flow cleanly and completely when the fuel pressure coming from the fuel control unit drops below a preset threshold (either 2.5 PSI or 4.5 PSI depending on the part number of the flow divider). So even if the fuel control unit doesn't totally shut off the fuel flow at ICO, fuel flow stops as long as the fuel control unit mixture valve reduces pressure below the flow divider cutoff threshold.

It sure sounds to me like the fuel control unit mixture control valve is not shutting off the flow properly. If you were seeing just 1 GPH or so at ICO, then I'd believe that the mixture control valve was good enough and that the fault was in the flow divider. But if you're seeing 3-4 GPH at ICO, then that's almost certainly above the flow divider cutoff threshold so the fault has to be with the fuel control unit.

The fuel control unit should pass almost no fuel to the flow divider if the mixture valve is at ICO and the throttle valve is at the idle stop (closed). Obviously, yours is passing lots of fuel. I'd be astonished if that's not where the fault lies here.


Here is a narrative on the science of hot starting by Baron 58 owner DdB:

Everyone has their favorite procedure. Many are anecdotal and work purely by accident.

The issue with hot starts involves the fact that avgas has a very high vapor pressure* (= low evaporation temperature). Looking at a typical distillation curve for 1 atm avgas will show that it starts to evaporate around 100F and is 50% evaporated around 200F (some components of avgas, such as isopentane and butane, will practically boil in your hand).

After shutdown, absent convection, the engine driven fuel pump may get very hot (via conduction) as the engine thermally equalizes in the act of dissipating heat. In common terms, the pump becomes "heat soaked", and may reach temps well in excess of 200F. When avgas hits that hot fuel pump it instantly vaporizes, locking the pump at the inlet. The vanes spin in vain (ha!) and no fuel makes it to the injectors. No fuel, obviously no start.

At lower atmospheric pressures, like we have here in Denver, the evaporation rate at a given temp increases, which makes the hot start problem worse. (Related to this is the phenomenon some TN guys experience when rapidly climbing to altitude with warm fuel and high fuel flows.)

The solution is to first cool down the engine driven pump before starting. Someone at Beech must have recognized this, because my POH (IO520CB) includes:

If the engine is hot, and the ambient temperature is 90F or above, place mixture control in IDLE CUT OFF, switch aux fuel pump to HIGH for 30 to 60 seconds, then OFF. Return mixture control to FULL RICH.

When that step is added, the electric fuel pump--sitting in the wheel well of my Baron and not heat soaked---pumps (relatively) cool fuel from the tanks through the engine driven pump and back to the tanks. This reduces the temperature of the heat soaked fuel pump to where fuel does not vaporize upon contact and can thus be pumped to the injectors.

After that it is prime and start as normal.

What is *not* in my POH, but *is* in Andrew's POH (B36TC/TSIO520-UB) is the act of sweeping the throttle from idle to open while the starter is engaged. I have found this to be a beneficial addition, as doing so varies the fuel/air ratio and, when it is right, the engine will fire.

The two steps together, i.e., cool the pump and sweep the throttle, is the procedure touted by Deakin, et. al, for hot starts. They may have originally applied that only to the TAT Turbo, I dunno, but the science is the same for all our IO engines.

*The very high vapor pressure of avgas was, in part, necessitated by the early carbureted engines. It is somewhat ironic that the same high vapor pressure that allows carburetors to work, is also an enabler for carburetor icing..

'80 BE58


Here are some additional comments on hot starting from Continental engine gurus


George Braly & Walter Atkinson in BeechTalk.



Hot Starts by George Braly (11/2017):

Your perception as to the cause of the hot starting issues being fuel vapor in the fuel lines that needs to be purged - - is an "understanding" that is shared by about 90% of the pilots.

Unfortunately, it turns out to not be the cause nor even related to the cause. 

The cause is this:

A) Shut down hot;

B) Engine driven fuel pump heat soaks from the accessory case (which is 175 to 200F).

C) internal portions of fuel pump get HOT (measured at about 150 degrees 20 minutes after a hot day shut down.) 

D) Pilot "purges" the fuel lines of vapor (yes, the fuel in there may well have boiled off).

E) Pilot hits the starter. Engine turns. HOT fuel pump turns. HOT( hot) Fuel is "sucked" uphill from the fuel tank. Hot fuel has a very high (highly non linear with temperature) absolute vapor pressure.

F) HOT fuel under a suction vacuum lifting it from the fuel tank - - hits the HOTTER vanes at the inlet to the slowing spinning (HOT) fuel pump - - and that fuel flashes to vapor AT THE INLET TO THE FUEL PUMP.

So the scenario one often sees on the ramp is this:

Pilot BuzzzzzzzzzzzzzzzWhirlllllllllllllllll primes the engine.

Pilot cranks the engine and it starts.

Engine runs just long enough for the HOT fuel in the tanks to then again hit the inlet to the fuel pump - - where it promptly turns to vapor.  

Positive displacement pump now tries to pump VAPOR - - not liquid.  

Engine quits due to fuel starvation. 


Hot Starting Comments from Walter Atkinson (11/2017)

Scientific starting technique answer:

1) Mixture Idle cutoff

2) Run aux pump for 60 seconds to cool off engine driven fuel pump.

3) Mixture full rich, aux pump to peak FF

4) Crank starter and move throttle forward slowly, being prepared to retard it when it starts. (It WILL start when the F:A mixture is right--every time. Period.)

If the engine is cold, omit steps 1 & 2.  

Now you have a starting technique that always works, no matter what the engine condition--hot or cold. If it's exceptionally hot, run the boost pump for 90 seconds.


Hopefully, this info compilation will be helpful to your Continental Hot Starts!