Vapor Lock in Bravo?

[carqwik]

After a stop to pick up a Mooney driver dropping his plane off at the MSC at KCHD, took off to return to DVT (via the long way around Phoenix to the east....).  At some point shortly after takeoff...and I think it was with the power reduction to cruise climb power...got a weird shudder in the airplane.  Engine instruments all looked normal, gear/flaps fully retracted, etc.  Lasted maybe 5-10 seconds then it did it a second time for about 2 seconds.  Engine ran just fine after that.

My only thought is that it was some kind of vapor lock (hot in Phoenix...maybe 95 degrees on departure with an already warm engine)...never seen this before in the plane but not even sure what it was.  Anyone else ever seen this in a Bravo?

[MB65E]

It might the fuel pressure dropping when the electric fuel pump is turned off.

Or I should ask with full power does the boost pump come on automatically as it should? Strang stuff happes to the  lycoming fuel systems above 100° (115° etc) You were probably running too rich for the higher density altitude you experienced. 

-Matt

 

[jcollier]

2 hours ago, carqwik said:

After a stop to pick up a Mooney driver dropping his plane off at the MSC at KCHD, took off to return to DVT (via the long way around Phoenix to the east....).  At some point shortly after takeoff...and I think it was with the power reduction to cruise climb power...got a weird shudder in the airplane.  Engine instruments all looked normal, gear/flaps fully retracted, etc.  Lasted maybe 5-10 seconds then it did it a second time for about 2 seconds.  Engine ran just fine after that.

My only thought is that it was some kind of vapor lock (hot in Phoenix...maybe 95 degrees on departure with an already warm engine)...never seen this before in the plane but not even sure what it was.  Anyone else ever seen this in a Bravo?

I have had the exact same thing happen a couple of times.  Once was a couple of weeks ago in climb around 3500 ft.  Just a quick 2-3 sec shutter.  Everything was completely normal, other than the shutter.  I had already backed off the boost pump in the climb.  I had also checked they fuel via the drains, during pre-flight.  

The other time this happened was at 15,500 ft a month or so back.  Had been in cruise for probably an hour and suddenly a shutter that lasted for probably 3-5 sec.  Everything was completely normal prior and after the event.  All gauges were normal.  Someone mentioned possibly water in the fuel or prop needs balancing...but it is very sudden, and it's gone.  Makes me a little nervous....still haven't figured it out....

-Joshua

[ArtVandelay]

Did y'all download engine monitor data to study, sometimes graphical representation shows anomalies...

[carusoam]

Any JPI data would be helpful...

Since we are suspecting a fuel issue related to temperature /vapor pressure...

any data related to the engine, OAT, FF, FP, Temps under the cowl including TIT and CDT...

Anything that looks at warmth under the cowl that would give us a hint to how warm the fuel may be getting on it's way to the injection system.

See If the fuel pressure oscillates vs. steady numbers.  FF may give some quirky data if bubbles are forming before the sensor.

What altitude did this occur at?

Who would we approach in the engine and fuel,system world to ask this question to?

The guys that balance fuel injectors may be a really good resource for this question....  Even though they have IO550s in their lab.

The one thing you want to do is determine if it is only a vapor issue vs.. Something that could be causing a fuel blockage or some other issue that may be more serious.

Keep in mind, I'm only a PP, not a mechanic.

Best regards,

-a-

[Awful_Charlie]

I'd never had thought this possible before if using 100LL, but last weekend after having been parked for a couple of hours in over 40C (and a large part of my tanks are painted black), I couldn't get a stable idle, and during taxi when I tried to open the throttle it stumbled badly unless I used the electric pump or opened the throttle much more than I wanted. To make it harder for the mechanical fuel pump, it was down to the last 10 gallons in he selected tank. The giveaway was the fuel pressure fluctuating down to barely readable.

After refuelling with fuel from an underground tank, all back to normal!

Edited August 16, 2016 by Awful_Charlie

[FoxMike]

During summer ops, when I stop at the self serve for 10 minutes or so the engine will heat soak making the engine hard to start.  Once it starts just a few seconds of boost pump will clear the vapor and it will run smoothly to the hangar.  When I stop for lunch (maybe an hour or so) I have more trouble getting the engine to start but the vapor will clear with a shot of boost.  I would not recommend continuous use of boost pump as real rich mixture causes a rough engine.  I would recommend ground ops with engine leaned as much as possible.  I head for the mountains for lunch fairly often in the summer and the DA at startup is 10-12K.  Starting difficulties and idling about the same as at home DA around 7-8K.  Patience is required for best chance of starting.  Do not crank the engine for very long as it is very hard on starter and battery.  If the engine will not start in 10-15 seconds, I stop and wait a few minutes to allow the starter to cool.  

When I owned a T210 I found that a rag soaked in cold water applied to the fuel divider (reaching through nose bowl) and placing rag for a few moments on the fuel line from the firewall to pump helped clear the vapor lock.  You could reach the fuel lines through the cowl flap.  This will likely turn a new shirt to an old shirt so reserve this for dire situations.

[DVA]

Vapor lock comes in varying degrees, so a single technique to purge the fuel lines of “gas air” won’t necessarily work. I break it down by short heat soak (about 10 min or less) and long soak (about 30-60min) and yes, there is that charlie foxtrot area in the middle ~15-30 minutes where anything can happen.

This discussion is for a fuel injected engine.  

During a short heat soak the likely culprit will be the upper fuel lines to the distribution device and the injector lines to the cylinders. Note, these fuel lines often sit atop the hot engine, and since heat rises, the relatively small volume of liquid fuel in these lines atomizes quickly and becomes a vapor (gas air). To fire a mixture off in the combustion chamber, there must be an atomized fuel mist suspended in the surrounding air. If the fuel is too atomized as in a vapor, the fuel density won’t be sufficient for firing, hence a hard start, actually due to a overly lean mixture.

To clear a short soak, you need to pressurize the upper fuel lines with just enough fuel to push out the vapor, and not too much that you flood the intake ports.  This is where most pilots get in trouble with hot starts; a hot engine needs far less fuel to start than a cold engine. The theory is to begin the starting sequence for a short heat soak with NO fuel flow and then ADD fuel slowly until it fires off.  Most often, we do it backwards and that makes things exponentially worse.

During a long heat soak, the entire fuel system comes in to play from the fuel tank feed lines to the fuel pumps, to the pump chamber, to the feeder lines, on up. It generally takes longer for these components to heat up and begin to vaporize after sitting than do the upper fuel lines. The clearing technique here is different than a short soak, as you want to (have to) push the vapor out of the pump circuit and that takes time; sometimes a lot of time.

To clear a long soak, you need to purge the entire fuel circuit of vapor, not just the lines going to the injectors. In a long soak, there is a lack of liquid fuel in the pump circuit (because it got hot and vaporized away) ... and a fuel pump, while good at pumping a liquid, is very inefficient at pumping vapor. So you crank and crank and crank and nothing happens because nothing is happening - no fuel is flowing - because the fuel pump is essentially pumping air. In cases of a long soak, using just techniques that will effectively clear a short heat soak will do little to clear the pump circuit, causing a hard start.

Knowing a little about why it’s hard to start a hot engine often makes it easier to find a solution. 

If this works for anyone send money; I have airplane payments to make and my wife and mistresses want jewelry. 

Short Soak:

1. Throttle cracked to the point where it would need to be to have about 1000-1200RPM if the engine were running
2. Boost pump OFF - (Do not use the boost pump or primer at all)
3. Mixture Full Rich for about 5 seconds then Idle Cut Off
4. Begin cranking the engine, wait a 2 seconds then slowly (over 5-10 seconds) move the mixture toward rich. Don’t exceed recommended cranking time.
5. As soon as the engine starts, keep the mixture at about that point, adjust it and throttle for smooth operation. You should ALWAYS run the engine as lean as possible on the ground.

Rationale: The throttle is cracked open so that when the engine fires, there is proper air flow for the fuel that is being slowly added by the mixture control. The Mixture is open fully for a few seconds first to allow any built up vapor pressure to purge out, then its closed; this gives liquid fuel a clear path down the lines. The mixture is kept closed until the cranking so that you have complete control of how much fuel to add to get the engine lit off - which likely will be different every time - this technique also significantly reduces the changes that you will flood the engine. There is NO boost pump used because the mechanical pump should be able to provide fuel at a rate that keeps excess fuel low, unless you have a hot soak condition... You will know if you have a hot soak event because the above technique will not work after two tries. Summary: There is liquid fuel available at the fuel pump but there is vapor in the injector lines. The vapor does not ignite easily and it blocks liquid fuel from flowing past the vapor area (vapor-lock) causing a hard to start condition. You have to relieve the pressure of the vapor and then slowly add liquid fuel to the lines and the injectors so that the mixture of fuel and air is correct for ignition in a hot cylinder.

Long Soak:

1. Throttle Closed
2. Mixture Full Rich for about 5 seconds then Idle Cut Off (Be very sure it is fully at idle cut off)
3. Boost pump on (or on low if two speed) for 30-60 seconds. (yes, a full half a minute to a minute)
4. Boost pump OFF
5. **Throttle cracked to the point where it would need to be to have about 1000-1200RPM if the engine were running
6. Begin cranking the engine, wait a 2 seconds then slowly (over 5-10 seconds) move the mixture toward rich. Don’t exceed recommended cranking time.
7. As soon as the engine starts, keep the mixture at about that point, adjust it and throttle for smooth operation. You should ALWAYS run the engine as lean as possible on the ground.

Rationale: The Mixture is open fully for a few seconds first to allow any built up vapor pressure to purge out, then its closed; this gives liquid fuel a clear path down the lines. You want to be very sure that the Mixture is fully at idle cutoff because we do not want any fuel to get past the metering circuit. Running the boost pump with the mixture closed will pressurize the fuel circuit and circulate some liquid fuel which will help cool things down and reduce additional vaporization. Excess vapor will be expelled through a vent port and the mechanical pump and the lines leading to the metering circuit will become fresh with cooler fuel. This takes time, and you have no worry of flooding the engine because the mixture is at idle cutoff. You finish by following the same procedure as a short soak. Summary: The mechanical fuel pump has to have the engine cranking to do its job. It would take too long and be too hard on the starter to use this pump to purge vapor. (Hence why I’ve seen pilots cranking the engine for absurdly and dangerous amounts of time). The electric pump is parallel* with the mechanical pump and has the ability to run quickly and more efficiently to do the job of purging the vapor, but its not an easy job - it takes a lot of time therefore you need to run the pump for at least 30-60 seconds. Once the vapor is purged, liquid fuel can flow past the metering circuit, and now you have the situation of a short soak to deal with, see above.

So why not just use a Hot Soak procedure every time?

You could, but after you understand why the damn thing won’t start and you think about it, you can use the technique that works the best.  If I land the plane, shut down hot, and then go for a restart in less than 10 minutes or so (all thing considered equal), I know that my problem is not at the pump, its at the top lines, so why waste time.

Disclaimer: Follow your POH unless you fully understand the pro’s and con’s of using other methods and other’s advice.

DVA

*As Don Kaye correctly pointed out, the electric fuel pump (on the M20M) is physically in “series" with the mechanical pump as shown on the schematic. The point is that neither of the two are dependent on one another and both can participate alone or together, in parallel, to provide fuel flow and pressure. 

**PaulM Points out that I omitted a step to crack the throttle in the Long Soak procedure. It is now there as a new #5

[donkaye]

On August 16, 2016 at 9:13 AM, DVA said:

The electric pump is parallel with the mechanical pump and has the ability to run quickly and more efficiently to do the job of purging the vapor, but its not an easy job

In the Bravo I believe they are in SERIES.

[DVA]

11 hours ago, donkaye said:

In the Bravo I believe they are in SERIES.

Hi Don,

Thanks you for pointing that out. If you reference the fuel schematic the electric pump is indeed shown in series with the mechanical pump. I chose my words poorly - “parallel with” - when I tried to illustrate that the electric fuel pump is independent of mechanical pump, yet they share and influence they same fuel circuit.

[PaulM]

Page 3-10 in the M20M operations manual:

1. FUEL VAPOR SUPPRESSION (Fluctuating Fuel Pressure)
2. Fuel Boost Pump . . ON
3. Fuel Pressure . MONITOR
4. Fuel Boost Pump . . ' OFF : (If condition'still exists, REPEAT PROCEDURE).

So the factory says its possible to have vapor issues in flight, and to use the boost pump if you see it. 

 

As for DVA's instructions, parts of those are for Continental engines (which have the fuel return line) not Lycoming engines that only have the single delivery line.   

I use the advice from David @ All american on the Bravo:  I couldn't find a written copy (they told me when I picked up the plane)   but here is the same advice on an E. http://www.mooneypilots.com/mapalog/HOT STARTS.htm

Always shut down at 1100rpm (This sets the throttle correctly for the next start)

Cold start:  Use standard procedure with boost timed for the CHT (If you were using a block heater in the winter you don't need 20 seconds of boost)

Short Soak: (refuel, quick passenger turn etc) 

Throttle set (at shutdown)

Mixture cut-off

Crank, and when it catches smoothly increase the mixture to your ground lean position rather than full rich.

Long Soak: (lunch)

Throttle set (at shutdown)

Mixture full rich

Boost pump for 2-3 seconds (just to get pressure at the sensor)

Mixture to ground lean:

crank & be ready to advance the throttle if it doesn't fire.   (assume flooded) 

If the time has been > 1 hour and this doesn't work.. assume cold dry engine & follow the cold start procedure.. 

Notes on DVA's post:

On August 16, 2016 at 0:13 PM, DVA said:

Short Soak:

1. Throttle cracked to the point where it would need to be to have about 1000-1200RPM if the engine were running
2. Boost pump OFF - (Do not use the boost pump or primer at all)
3. Mixture Full Rich for about 5 seconds then Idle Cut Off
4. Begin cranking the engine, wait a 2 seconds then slowly (over 5-10 seconds) move the mixture toward rich. Don’t exceed recommended cranking time.
5. As soon as the engine starts, keep the mixture at about that point, adjust it and throttle for smooth operation. You should ALWAYS run the engine as lean as possible on the ground.

1&2 good

3 What does this do?...  Releasing trapped vapor?  Even if it does it will just put more fuel into the intake which is assumed to be too rich already. 

There is no fuel pressure and no air pressure.  the assumption is that there is enough vaporized fuel already in the plenum that we don't want any more until the engine fires.  I have never done step #3.. seems like a placebo to me, won't hurt, doesn't make a difference. 

I'll give #3 a try next time..  and see if I perceive any difference. 

4&5 fine. 

On August 16, 2016 at 0:13 PM, DVA said:

Long Soak:

1. Throttle Closed
2. Mixture Full Rich for about 5 seconds then Idle Cut Off (Be very sure it is fully at idle cut off)
3. Boost pump on (or on low if two speed) for 30-60 seconds. (yes, a full half a minute to a minute)
4. Boost pump OFF
5. Begin cranking the engine, wait a 2 seconds then slowly (over 5-10 seconds) move the mixture toward rich. Don’t exceed recommended cranking time.
6. As soon as the engine starts, keep the mixture at about that point, adjust it and throttle for smooth operation. You should ALWAYS run the engine as lean as possible on the ground.

Rationale: The Mixture is open fully for a few seconds first to allow any built up vapor pressure to purge out, then its closed; this gives liquid fuel a clear path down the lines. You want to be very sure that the Mixture is fully at idle cutoff because we do not want any fuel to get past the metering circuit. Running the boost pump with the mixture closed will pressurize the fuel circuit and circulate some liquid fuel which will help cool things down and reduce additional vaporization. Excess vapor will be expelled through a vent port and the mechanical pump and the lines leading to the metering circuit will become fresh with cooler fuel. This takes time, and you have no worry of flooding the engine because the mixture is at idle cutoff. You finish by following the same procedure as a short soak.

#1.. this assumes idle amount of air.. generally the bravo starts better with the 1100RPM setting. 

#2  again, not sure about the pressure release

So the problem is #3... This is correct for continental engines with the return fuel line.. but not lycomings.. there is no vent port, running the boost pump with the mixture closed does nothing. The only place to get that vapor out is via the injectors & the plenum..  

the manual procedure is to hit the boost on full rich for 1-3 seconds.. that gets liquid fuel up to the flow sensor  and at worst overprimes the engine... hence being ready for a flooded start throttle movement.

I think this was discussed a while back:

 

[DVA]

53 minutes ago, PaulM said:

3 What does this do?...  Releasing trapped vapor?  Even if it does it will just put more fuel into the intake which is assumed to be too rich already. 

PaulM, thanks for responding. This was actually a Lycoming engineer’s suggestion I heard years ago, and one that was also mentioned again at a forum in KOSH this year. Theory was that it helps to release any built-up pressures in the upper lines, I never tried to see if omitting that step helped, hurt, or changed anything. I disagree that it will put more fuel into the intake, as there is no fuel pressure at all at that point in the sequence. I also think there is a misconception that in a heat soaked engine that we are working against an overly rich condition, I believe it is just the opposite - vapor is less dense than liquid fuel - its a lean mixture and lack of the **right amount** of consistently flowing fuel that makes it hard to start. That can be part of great debate.

1 hour ago, PaulM said:

As for DVA's instructions, parts of those are for Continental engines (which have the fuel return line) not Lycoming engines that only have the single delivery line.

They are only DVA’s instructions if they work! They are a combination of other people’s techniques that I have simply repeated, and that make engineering sense to me - and, work for me on my Bravo and other airlines with Continentals as well.

The running of the primer pump on either engine seems to be a big part of the success when a long heat soak is occurring, at least in my experience. While the fuel circuit is different from Lycoming to Continental, the act of pressurizing, cooling and causing turbulence in the fuel will help to liquify it regardless of design. You point out a valid fact that the Lycoming is not vented in the same fashion as the Continental, thanks for the illustrating that.

1 hour ago, PaulM said:

#1.. this assumes idle amount of air.. generally the bravo starts better with the 1100RPM setting.

Good catch! I have that RPM setting in the Short Soak, and I missed adding it to the Long Soak - I have edited the post. Thank you.

Here’s the bottom line: If there was a single way to make starting easier, we’d all know it! Use what works for you, don’t be afraid to try what works for others.

Best regards,
Dave

[donkaye]

All the above has been interesting reading--but--

My Lycoming start procedures that have worked for me for the past 24 years:

First start of the day:

1. Throttle cracked

2. Mixture rich

3. Boost pump on for 7-8 seconds

4. Wait for about 10 seconds for fuel to atomize

5. Crank.

Engine starts on 2nd or 3rd blade.

Medium delay or delay of less than an hour:

1. Throttle cracked

2. Mixture rich

3. Boost pump on for 3 seconds to flood the engine

4. Mixture lean

5. Crank

6. Gradually push in the mixture control

7. Engine starts at midpoint when the fuel/ air mixture becomes just right.

These procedures make sense to me without a whole lot of thought.  Flood the engine.  Engine starts when fuel/air mixture it just right.

 

[DVA]

There are so many reasons why there are so many techniques that work for one person and not for another.

Idle mixture adjustment, spark timing, integrity of the induction system, spark plug condition, cam wear, compression, ring seal, temperature, altitiude, consistency of the operator, other things ... ad nauseam. They are all different on every airplane and with every pilot.

Airplane ICE are dirt simple compared to automotive, motorcycle engines, etc. ... even newer lawn mower engines have more modern technologies applied. If we had the sophistication of other industries applied to our aircraft engines, this thread would be boring. We’d touch a button, it would start every time, hot cold or in between. That said, “simple" also has its benefits.

The bottom line is we are not just pilots, we are asked to fly our airplanes AND adjust our engines operating parameters. Wouldn’t it be fun to pull off the country road on to the highway and have to grab a red knob and adjust your Honda’s EGT to peak, unless you needed to pass someone?

 “Honey!! Why did you leave the magnetos on with the car in the garage - I’ve told you a hundred times that it might start on its own and roll into my Mooney parts cabinet"

:-)

[carqwik]

Ditto on what Don does...my procedure is basically the same although first start of day priming time is a function of OAT....more cold, more prime (per POH).  Hot/warm start procedure is identical.  Works like a charm....

[FlyDave]

For my Bravo, on a short turn/high heat soak I find that opening the throttle about 1.5 turns (to about 1300-1500 RPM), mixture full rich and no boost pump before or during start, my engine starts after about 6 blades. One thing I've found when I do this is I have to let the engine run at whatever RPM it gets to (1300 - 1500) and don't touch the throttle OR mixture - for about 20 seconds or it will die.

On a longer duration shutdown a 3 second prime and full rich starts it in 3-5 seconds.

Cold, as Carqwik states depends on OAT and, if mixture is guestimated correctly it starts in 1-2 blades.....amazing!

[carqwik]

During annual, discovered that the electric fuel pump was leaking.  Now thinking that the "shudder" was occurring soon after throttle retard to 34" MP which turns off the boost pump.  Perhaps the shutdown of the boost pump - due to a leak in it - was causing some fuel flow interruption to the engine momentarily?  Thoughts?

[carusoam]

On the ground, when you turn on the electric pump, does it stream onto the ground?

My O's boost pump decide to take a holiday.  It was really noticeable to people walking by.  Interesting way to meet other people.

Best regards,

-a-

[Piloto]

Momentary engine surge could be due to many things. I would check the following.

1. Fuel injector line connections.

2. Spark plug wires chaffing against engine or magneto.

3. Magneto P lead chafing against metal.

4. Spark plug fouling, or defective plug. I had a similar problem, switched to Tempest and problem gone.

José

[carqwik]

It was a shudder, not a surge.  I put Tempest massives on last year (maybe two years ago?)...

Guess we'll see if it comes back post annual.  Btw, electric fuel pump O/H costs almost as much as new unit...

[JohnB]

Great discussion guys... another reason why I really like this board! Keeps you thinking with some great tips and topics for Mooney related discussions over beverages!