Boost pump removal

[Gubni]

I sent my M20K continental engine off for rebuild and I was told in 2019 there was an AC or maybe SB that said the boost pump should be removed permanently. Has anyone else heard of this or has a link? It doesn't make any sense to me.

Edited July 6, 2021 by Gubni

[N231BN]

Perhaps you are referring to CSB19-01A that removes the diverter valve for the priming system.

[Gubni]

22 minutes ago, N231BN said:

Perhaps you are referring to CSB19-01A that removes the diverter valve for the priming system.

I don't think so. He told me he was going to put plugs in where it was.

[larryb]

How do you plan to prime and start the engine?

[N231BN]

I don't think so. He told me he was going to put plugs in where it was.

That's correct, you remove the primer nozzles as well and just use the injectors for priming. The prime switch remains which just runs the boost pump on high.

Search for CSB19-01A and see for yourself.

[EricJ]

44 minutes ago, Gubni said:

I send my continental engine off for rebuild and I was told in 2019 there was an AC or maybe SB that said the boost pump should be removed permanently. Has anyone else heard of this or has a link? It doesn't make any sense to me.

The TCDS shows which electric fuel pump is required equipment on K models.   There is no case in the TCDS for a K model where an electric pump is not required.   If you remove it the aircraft will no longer be compliant to the TCDS (that's bad).

[Gubni]

56 minutes ago, N231BN said:

That's correct, you remove the primer nozzles as well and just use the injectors for priming. The prime switch remains which just runs the boost pump on high.

Search for CSB19-01A and see for yourself.

How do you run the boost pump on high once it's removed?

[carusoam]

The priming system of the M20K… is the most complex of all Mooneys…

It has been updated over the years…

So… if you have the opportunity to update the one you have… it should be a pretty clear route to follow…

There shouldn’t be any confusion or surprises…

Removing the boost pump indicates confusion…

One of the best MSCs has done something similar when removing an M20C’s carb heat system…

 

See if you can clearly state what needs to be done for your M20K to get it from the old state to the new state…

PP thoughts only, not a mechanic…

Best regards,

-a-

[PT20J]

9 minutes ago, Gubni said:

How do you run the boost pump on high once it's removed?

Looks like @N231BN gave you the correct answer. You should read this before asking any more questions.

CSB19-01A.pdf

[CharlesHuddleston]

I love this site. Just learned something that will apply to my engine. Printed it out, and will give it to my AP at the end of the month during my annual. Not sure this has been addressed yet!

[carusoam]

Skip, you are THE Man with the plan!

I suspect, @Gubni heard his mechanic describing this, but sounded like removing the pump…

Inviting @M20Doc to stop by when able…

Best regards,

-a-

[Guest]

Skip and others have it correct, it’s the priming nozzles and diverted valve which are removed.

Clarence

[N231BN]

Looks like [mention=12631]N231BN[/mention] gave you the correct answer. You should read this before asking any more questions.
CSB19-01A.pdf

Thanks for posting the link, my phone wasn't behaving last night.

[jlunseth]

Well, here’s the question about that. Using the existing system the primer fuel is dumped into the intake manifold, not the cylinders. This revised system, without the diverter, will put the fuel directly into the cylinders, and that is fine, it works. 

Typical warm weather priming is 6-8 seconds per the POH (I am going from memory here). However, cold weather priming, requires 12-15 seconds of prime at 20 dF and 20-24 seconds at 0. It is true, you need alot of fuel if you are to attempt a cold start at, say, 20 dF.

The CB warns about hydrolock, which would happen if you dump a lot of fuel directly into the cylinders instead of into the intake manifold.

So how long can you hold the button down without risking hydrolock?

It appears from the CB that a cold weather start should not be attempted below 20 dF, and that is pretty good advice, the engine is very hard to start if you are ever in that situation and without pre-heat. But it does not directly warn against an attempt at colder than 20 and it does not say at what duration you are going to be risking hydrolock. It also appears there is a special inspection required if you do manage to hydrolock. 

So why would you want to do this? If you want to prime directly into the cylinders, before this change, you can just use the boost pump button instead of the primer button.

Not an irrelevant issue up here in the cold north.

[Warren]

There are drains in each cylinder that must be inspected for version and potentially replaced when removing the priming circuit.  Theoretically these drains and the sniffle valve should provide some protection from over-priming.  The newer drains have a higher level and will allow more fuel to puddle in the intake.

I think moving to priming through the injectors is a better practice.  The prime circuit dumps fuel into the intake manifold on each side past the wye.  When heavily primed I suspect most of the excess fuel puddles and runs into cylinders 5 and 6.  The risk of hydrolock when distributing the prime fuel to all six cylinders should be much lower.

Once you have enough fuel in the engine that it has pooled and starts running out the drains, more fuel will not help.  No idea how to determine this level other than experiment and look for fuel in the sniffle valve drain.

The other advantage is that the injectors are fully primed and will start to provide additional fuel immediately when the engine is cranked.

In cold weather you might try low boost while cranking to add a little fuel and maybe a little extra vaporization surface area.  Caveat:  I have never tried to start an engine without preheat anywhere near the temperatures being discussed.

[kortopates]

1 hour ago, jlunseth said:

Well, here’s the question about that. Using the existing system the primer fuel is dumped into the intake manifold, not the cylinders. This revised system, without the diverter, will put the fuel directly into the cylinders, and that is fine, it works. 
Typical warm weather priming is 6-8 seconds per the POH (I am going from memory here). However, cold weather priming, requires 12-15 seconds of prime at 20 dF and 20-24 seconds at 0. It is true, you need alot of fuel if you are to attempt a cold start at, say, 20 dF.
The CB warns about hydrolock, which would happen if you dump a lot of fuel directly into the cylinders instead of into the intake manifold.
So how long can you hold the button down without risking hydrolock?
It appears from the CB that a cold weather start should not be attempted below 20 dF, and that is pretty good advice, the engine is very hard to start if you are ever in that situation and without pre-heat. But it does not directly warn against an attempt at colder than 20 and it does not say at what duration you are going to be risking hydrolock. It also appears there is a special inspection required if you do manage to hydrolock. 
So why would you want to do this? If you want to prime directly into the cylinders, before this change, you can just use the boost pump button instead of the primer button.
Not an irrelevant issue up here in the cold north.

No question, the entire reason the diverter valve was originally installed was for very cold starts. Per the TCM engineer that spec’d the engine for Mooney for the original K they used the diverter valve only because Mooney required very cold start capability (i forget numbers). They were obviously thinking exactly about folks like yourself.
But it’s a pretty redundant system outside of cold weather. But in cold weather it’s much safer because it does help contain the priming fuel and especially helps with vaporization of the fuel. Because of this I always advocated for using the system for priming versus just directly into the cyl.
But the big bore Mooney folks don’t know what we’re talking about as they never had the Diverter valve and always primed directly into the cylinder.

But the risk of hydraulic lock is small. Did you notice the Cylinder drain plugs in the SB too? Those drains, above the intake valve, control the maximum amount of fuel that goes to the cyl when priming before cranking; any fuel that fills to the top of the drain will simply drain out onto the floor through the cyl drains - assuming none are plugged so make sure the system is well maintained.
The new cyl drains are taller too, to allow for more fuel on colder starts helping make up for loss of the diverter.

So you should be at minimal risk of hydraulic lock from priming before startup. Once you start cranking though the fuel will go into the cylinder without restriction and if the cylinder isn’t firing it’ll be going out the exhaust too. But you still don’t want to get carried away over priming before cranking as one of cyl’s intake valve could be open preventing the cyl drain from draining excess fuel away.


Sent from my iPhone using Tapatalk

Edited July 6, 2021 by kortopates

[Guest]

Some Piper Seneca airframes had the same priming system.  Thousands of Cessna airframes powered by Continental engines prime through the fuel injector nozzles.

To clarify, the fuel goes into the intake port, if the intake valve is open and there is sufficient fuel it might enter the combustion chamber as a liquid.  I’ve never heard of one getting a hydraulic lock.

Clarence

[jlunseth]

Something has gone wrong if you have to try a restart under 40 degrees. The power cord to the pre-heater got disconnected, or the line kid never plugged in after I called and asked. But it does happen. I have started as cold as 10 dF. It’s pretty hard on the starter, but it can be done. I might just stick with the system I have for now.

[PT20J]

I believe a Continental IO-360 cylinder has a displacement of 60 cu. in. and a compression ratio (CR) of 8.5:1.

CR = (Vd+Vc)/Vc, where Vd is the displaced volume and Vc is the clearance volume (the remaining volume of the cylinder when the piston is at TDC)

Solving, Vc = 8.05 cu. in. or 4.46 fl. oz.

So, it would take more than half a cup of gasoline to hydraulic lock a cylinder. Seems pretty unlikely from any reasonable priming.

Skip

[Aerodon]

On 7/5/2021 at 6:08 PM, Gubni said:

I don't think so. He told me he was going to put plugs in where it was.

I'm just doing this on my Seneca.  Once you remove the old priming pipes, you have to put two plugs in the manifold where you remove the primer nozzles.  I had aero hose shop make up 4 new hoses, two for each engine.  (the JPI FF transducer goes in the middle of two hoses).

I removed all the diverter valve wiring all the way back to the nacelle and i removed the diode in the circuit.  Good opportunity to inspect all the wiring behind the firewall.

And fortunately, we have the correct injectors.

 

Aerodon

 

 

 

 

[jlunseth]

1 hour ago, PT20J said:

I believe a Continental IO-360 cylinder has a displacement of 60 cu. in. and a compression ratio (CR) of 8.5:1.

CR = (Vd+Vc)/Vc, where Vd is the displaced volume and Vc is the clearance volume (the remaining volume of the cylinder when the piston is at TDC)

Solving, Vc = 8.05 cu. in. or 4.46 fl. oz.

So, it would take more than half a cup of gasoline to hydraulic lock a cylinder. Seems pretty unlikely from any reasonable priming.

Skip

Well, that’s the problem Skip. The very cold weather recommendation is a lot of priming. More than most people would consider to be reasonable.

[PT20J]

46 minutes ago, jlunseth said:

Well, that’s the problem Skip. The very cold weather recommendation is a lot of priming. More than most people would consider to be reasonable.

Well if you prime enough to get half a cup in one cylinder, that would equate to 3 cups total which is 1-1/2 pints. I would say that at that point it is so flooded that it would not start anyway