anyone with M20K has been pump cavitation/vapor locked problems

[gabez]

it seems my m20K 231 '84 is susceptible to pump cavitation at around 12K DA. it usually drops from ~12 GPH to 6-7 GPH and it goes away when I quickly use the boost pump. last time happened at 10.5 but given the OAT the engine was seeing 12K DA. It seems that leaving the cowl flaps in trail mode helped. I actually did a test where I had them in trail mode and the plane was fine, I then closed it and it happened. 
 

I had the entire fuel system rebuilt and overhauled, usually I see 24 GPH on climb at sea level and it drops a few GPH as I hit higher elevation. Usually climb 35/27

thank you 

Gabe

[PeteMc]

I've never seen anything and I've never used either boost pump in flight. 

 

[takair]

Did this start suddenly?   After work being done?  Would start by looking at potential leaks to the inlet of the pump. Inlet orings and connections. They may not show aggressive fuel leaks of any. Also. If work was done, check for plumbing, especially to inlet of pump. Look for any and multiple 90 degree fittings. Aircraft are especially prone to this if fuel flow sender or hoses are new and someone decides they want nicer routing by adding 90 Fittings in place of straight runs which take up room, especially in a K. Pictures of the engine compartment might be helpful in this case. 

[gabez]

5 hours ago, takair said:

Did this start suddenly?   After work being done?  Would start by looking at potential leaks to the inlet of the pump. Inlet orings and connections. They may not show aggressive fuel leaks of any. Also. If work was done, check for plumbing, especially to inlet of pump. Look for any and multiple 90 degree fittings. Aircraft are especially prone to this if fuel flow sender or hoses are new and someone decides they want nicer routing by adding 90 Fittings in place of straight runs which take up room, especially in a K. Pictures of the engine compartment might be helpful in this case. 

none of the routing or fitting has changed. it doesn't happen unless the DA is 12K

Edited May 14, 2024 by gabez

[hubcap]

I am not an expert, but I have a little over 400 hours in my 231 and have never seen anything like this in my plane. I have never had to use the boost pump in flight.

[Jim F]

I have over 700 hours in my 231 and set the fuel flows on 1000s of engines and I have never needed the boost pump in flight.  I have never had any symptoms that resemble cavitation.  Is this real or instrumentation?  You did not mention EGTs when this happened.  Do you have an engine monitor?  I don't think my engine will stay running at 6gpm.  Does it happen on both tanks?  If you have a restriction in fuel flow that causes the fuel pump to cavitate I would suspect that to happen at full power and max fuel flow.  What engine and addon's do you have?

First, is this instrumentation?  Hard to believe that the cowl flap position could affect anything.  The fuel pump main cooling has to be the fuel going through it.  I would look at the fuel flow sender wiring that might(?) be affected by changes in air flow.

Second, I would have the intake pressurized and look for leaks in the upper deck lines and fuel pump Aneroid.

Third, check all fuel screens.

Forth, have the fuel flows set by someone with experience.  If the fuel flows are off, check the fifth item before setting the fuel flows

Fifth, verify that the fuel return line is not blocked.  This is the line on the top of the fuel pump that send a small portion of the fuel back to the tank and any vapor.  If it's blocked the vapor would be pushed into the engine (long shot).  Not sure how you would get the fuel flow set correctly if this line is blocked.

Jim

 

 

[231LV]

Agree with other 231 drivers....been flying my 231 since 2003 and never used the boost pump in any circumstance...

[gabez]

3 hours ago, Jim F said:

I have over 700 hours in my 231 and set the fuel flows on 1000s of engines and I have never needed the boost pump in flight.  I have never had any symptoms that resemble cavitation.  Is this real or instrumentation?  You did not mention EGTs when this happened.  Do you have an engine monitor?  I don't think my engine will stay running at 6gpm.  Does it happen on both tanks?  If you have a restriction in fuel flow that causes the fuel pump to cavitate I would suspect that to happen at full power and max fuel flow.  What engine and addon's do you have?

First, is this instrumentation?  Hard to believe that the cowl flap position could affect anything.  The fuel pump main cooling has to be the fuel going through it.  I would look at the fuel flow sender wiring that might(?) be affected by changes in air flow.

Second, I would have the intake pressurized and look for leaks in the upper deck lines and fuel pump Aneroid.

Third, check all fuel screens.

Forth, have the fuel flows set by someone with experience.  If the fuel flows are off, check the fifth item before setting the fuel flows

Fifth, verify that the fuel return line is not blocked.  This is the line on the top of the fuel pump that send a small portion of the fuel back to the tank and any vapor.  If it's blocked the vapor would be pushed into the engine (long shot).  Not sure how you would get the fuel flow set correctly if this line is blocked.

Jim

 

 

Hi Jim thanks for the note. the plane is being serviced at United in KWVI, not sure if you are familiar. Never had an issue at take off I am getting strong 24 GPH until I back the power to 35inchs. CHT are not more than 350 on the hottest cylinder during climb and I keep my TIT between 1400 and 1450. at cruise usually between 10-12K feet I set the plane at 30inch 2400 and 12.3 GPH, temp on the CHT are 320 ish and TIT about 1480 or so. pretty consistently. 

I don't believe is the fuel sensor there is a clear loss of power, I do have a G2 I would be happy to send you my last data if open to look at them. 

the problem doesn't happen consistently I had several flights at 10-12K feet with no issues. on my last one it happened and then it went away with the boost pump. I do get some more flow fluctuation at higher altitudes than if I am at let's say 5-6K feet. 

I have talked to savvy aviation about this, their take was that on the new mooney there is a clear indication on the POH to turn on the boost pump at 12K DA while in the older planes they didn't do such a good job on the POH and it just say to turn on the boost pump if there is fuel flow fluctuation. They also said to always used the boost pump is TIT goes over 1450 in climb

I used to climb at 33/26 but recently switch to full power 40/37 till about 8K then I back it to 35/37 the engine seems fine and runs cool tho perhaps the pump is not getting enough fuel and vapor locks.....not sure

 

thank you 

 

from Savvy:

Mike forwarded me your message. I am Paul Kortopates, Savvy’s Mooney specialist as well as a Mooney CFI and a Director in the Mooney Safety Foundation that does the Mooney PPP’s.

 

What your describing is signs of vapor lock and, if so, your data should show the TIT creeping up without any change in the controls. Vapor separation in the fuel means your not getting the full normal amount of fuel but air instead leading to roughness as a minimum. But it can result in power loss if not corrected by the boost pump. But as long as the boost pump is correcting it, it pretty much has to be vapor lock, which is actually pretty common in turbo’s going over 12K’ DA.

 

The early K POH’s don’t adequately cover vapor suppression as well as the more modern POH due but all turbo manufacturers these days warn about possibility of vapor lock anytime your climbing above 12K’ density attitude. Activation of the low boost pump should be sufficient to rectify this.

In all the newer Mooney turbo POH’s they warn to turn on the low boost pump in climb as your going past 12K DA or whenever TIT > 1450F (full rich).

In contrast since Cirrus recognized many pilots forget about this need in their Turbo’s they recommend switching on the boost pump prior to takeoff and keeping it on throughout the climb. In fact they call for the low end fuel setup to be done with the boost pump on.

The Mooney Bravo (Lycoming) does this automatically with the power full throttle, automatically turning on the boost pump while its at full throttle..

The Acclaims go back to doing it manually.

Edited May 15, 2024 by gabez

[PeteMc]

Did I miss if you said you actually saw a fuel fluctuation or you thought it was a fuel starvation issue?  

Do you have pressurized mags?  You can get some misfiring if the mags are not pressurized, but I would think it would happen at any higher altitude, not just at 12K.   But I learned about this issue when I first got my K many years ago, so I don't remember exactly at what altitude range was the issue.  And I had my mags pressurized within a few flights, so I never actually experienced the issue.   

 

[gabez]

5 hours ago, PeteMc said:

Did I miss if you said you actually saw a fuel fluctuation or you thought it was a fuel starvation issue?  

Do you have pressurized mags?  You can get some misfiring if the mags are not pressurized, but I would think it would happen at any higher altitude, not just at 12K.   But I learned about this issue when I first got my K many years ago, so I don't remember exactly at what altitude range was the issue.  And I had my mags pressurized within a few flights, so I never actually experienced the issue.   

 

I do see fuel fluctuations. yes my mags are pressurized I don't hear misfires or big changes in CHT. Also my mags have been recently rebuilt (6 months) 

[Jim F]

10 hours ago, gabez said:

Hi Jim thanks for the note. the plane is being serviced at United in KWVI, not sure if you are familiar. Never had an issue at take off I am getting strong 24 GPH until I back the power to 35inchs. CHT are not more than 350 on the hottest cylinder during climb and I keep my TIT between 1400 and 1450. at cruise usually between 10-12K feet I set the plane at 30inch 2400 and 12.3 GPH, temp on the CHT are 320 ish and TIT about 1480 or so. pretty consistently. 

I don't believe is the fuel sensor there is a clear loss of power, I do have a G2 I would be happy to send you my last data if open to look at them. 

the problem doesn't happen consistently I had several flights at 10-12K feet with no issues. on my last one it happened and then it went away with the boost pump. I do get some more flow fluctuation at higher altitudes than if I am at let's say 5-6K feet. 

I have talked to savvy aviation about this, their take was that on the new mooney there is a clear indication on the POH to turn on the boost pump at 12K DA while in the older planes they didn't do such a good job on the POH and it just say to turn on the boost pump if there is fuel flow fluctuation. They also said to always used the boost pump is TIT goes over 1450 in climb

I used to climb at 33/26 but recently switch to full power 40/37 till about 8K then I back it to 35/37 the engine seems fine and runs cool tho perhaps the pump is not getting enough fuel and vapor locks.....not sure

 

thank you 

 

from Savvy:

Mike forwarded me your message. I am Paul Kortopates, Savvy’s Mooney specialist as well as a Mooney CFI and a Director in the Mooney Safety Foundation that does the Mooney PPP’s.

 

What your describing is signs of vapor lock and, if so, your data should show the TIT creeping up without any change in the controls. Vapor separation in the fuel means your not getting the full normal amount of fuel but air instead leading to roughness as a minimum. But it can result in power loss if not corrected by the boost pump. But as long as the boost pump is correcting it, it pretty much has to be vapor lock, which is actually pretty common in turbo’s going over 12K’ DA.

 

The early K POH’s don’t adequately cover vapor suppression as well as the more modern POH due but all turbo manufacturers these days warn about possibility of vapor lock anytime your climbing above 12K’ density attitude. Activation of the low boost pump should be sufficient to rectify this.

In all the newer Mooney turbo POH’s they warn to turn on the low boost pump in climb as your going past 12K DA or whenever TIT > 1450F (full rich).

In contrast since Cirrus recognized many pilots forget about this need in their Turbo’s they recommend switching on the boost pump prior to takeoff and keeping it on throughout the climb. In fact they call for the low end fuel setup to be done with the boost pump on.

The Mooney Bravo (Lycoming) does this automatically with the power full throttle, automatically turning on the boost pump while its at full throttle..

The Acclaims go back to doing it manually.

You're talking to the right guy, Paul knows what he is talking about.  I would add that you still should verify your fuel flows.  I would also think your data would show fuel flow fluctuations 1-2 seconds before your EGT starts to rise because the flow transduce is up stream.  It's odd to me that I have never seen this as i fly high all the time.

[takair]

17 hours ago, gabez said:

none of the routing or fitting has changed. it doesn't happen unless the DA is 12K

Gabe….consider two things.  If a hose fitting starts to leak, it may not be an obvious fuel leak, but could draw air at the pump.  As DA changes and fuel temperature changes (you mentioned cowl flaps), the point at which it draws air might change.  Might be good to have your mechanic look at the condition of the hoses, especially on the inlet side of the pump.  Another thing…is it possible the cowl flap linkage is actually interfering with the fuel line and opening the flap causes….not temperature change but physical.  Been a long time since I worked on a K, but my minds eye can see some areas I would want to look at.  12gph to 6 is significant…adding boost pump provides some head pressure to the engine pump, but it may be masking something and it may get worse to the point of a fuel leak…which is especially bad on a turbo.

[Pinecone]

Technically vapor lock comes from the pressure on the suction side of the pump to get below the boiling point of the fuel causing bubbles of fuel vapor to form.  This can be due to heat and low pressure.

If there are air leaks, that isn't really vapor loc but there is a similar result, lack of fuel flow.

[gabez]

5 hours ago, Jim F said:

You're talking to the right guy, Paul knows what he is talking about.  I would add that you still should verify your fuel flows.  I would also think your data would show fuel flow fluctuations 1-2 seconds before your EGT starts to rise because the flow transduce is up stream.  It's odd to me that I have never seen this as i fly high all the time.

yeah I have started to built an eye for it, usually I am seeing +/- 0.2-0.5 gallons but if it goes beyond that I know it's about to happen so I turn on the boost pump. 

[gabez]

Well after weeks of digging we found the boost pump had corrosion that had moved downstream and eventually blocked the vapor check valve by the mechanical pump. we took the entire fuel system apart and cleaned it as well as upper deck, injectors, screen on the flow divider, gascolator etc. 

The noisy graft is before and the clean is after

Here are the new grafts as 12K feet indicated and 14K DA. 

 

 

[gabez]

hello, just closing the loop here. after a lot of work we found out the electric boost pump was corroded and was pushing debris down the line into the mech pump. We cleaned everything up and in the process we found the vapor check valve to be clogged which also didn't allow for vapor to the pushed back into the tanks...hence the vapor lock. 

 

Gabe