Engine stumbled two back to back flights

[201er]

After a fuel stop enroute to Florida, 10 gallons in I switched tanks and a few seconds after the switch the engine started to stumble. The fuel pressure gauge dropped so I immediately turned on the boost pump. The stumbling continued and fuel flow was erratic. I enriched the LOP mixture because the engine doesn’t like to relight LOP.

Eventually normal function resumed.

On the previous flight on the way to the fuel stop, I inadvertently deliberately ran a tank dry. Running the tank dry was deliberate but the moment it happened was a bit unexpected. I was about 10 minutes from beginning my descent and expected to switch the tank at that point before it ran completely dry. When there isn’t a lot of gas remaining, I want to arrive with everything being usable.

What made it worse was my passenger flipping out over it which startled me much worse than the tank running dry. I immediately reached for the fuel selector and flipped to the other tank and turned the boost pump on.

Anyway, the surprising thing was the engine stumble after refueling. Well duh, there was still air in the line from running dry on the prior flight. I’ve always been taught not to switch tanks right before taking off or critical phases. And this was a very good reminder why!

So, just a reminder that you can have some engine stumble not only when running a tank dry but also on the subsequent flight when you switch to the previously dry tank. 

[jetdriven]

Yes, anytime you run a tank dry, you may want to switch to that tank after you refill it, and start it up on that one to purge the air from it.

[DCarlton]

19 minutes ago, 201er said:

After a fuel stop enroute to Florida, 10 gallons in I switched tanks and a few seconds after the switch the engine started to stumble. The fuel pressure gauge dropped so I immediately turned on the boost pump. The stumbling continued and fuel flow was erratic. I enriched the LOP mixture because the engine doesn’t like to relight LOP.

Eventually normal function resumed.

On the previous flight on the way to the fuel stop, I inadvertently deliberately ran a tank dry. Running the tank dry was deliberate but the moment it happened was a bit unexpected. I was about 10 minutes from beginning my descent and expected to switch the tank at that point before it ran completely dry. When there isn’t a lot of gas remaining, I want to arrive with everything being usable.

What made it worse was my passenger flipping out over it which startled me much worse than the tank running dry. I immediately reached for the fuel selector and flipped to the other tank and turned the boost pump on.

Anyway, the surprising thing was the engine stumble after refueling. Well duh, there was still air in the line from running dry on the prior flight. I’ve always been taught not to switch tanks right before taking off or critical phases. And this was a very good reminder why!

So, just a reminder that you can have some engine stumble not only when running a tank dry but also on the subsequent flight when you switch to the previously dry tank. 

I’m very thankful I’ve never had a non pilot passenger on board during an inflight anomaly or emergency.  I’m certain I would be far more distracted by the passengers reaction that the emergency itself.  You’re brain is so occupied and busy when something happens you don’t seem to have the bandwidth to panic.  Seems like a form of self protection.  Appreciate you sharing the experience.  So far, I’ve never run a tank dry and don’t want to.  

[PT20J]

The first instance sounds like vapor. Js do get vapor in the fuel lines. Most of the time it doesn't cause a problem but LOP at high density altitude it can cause roughness. You can see it in the fuel pressure fluctuations if you have a digital engine monitor (the factory gauges seem to filter it out by accident or design).

When I used to fly float Beavers Part 135 we only filled the rear tank for long flights and it was considered poor form to leave any gas in it for the next guy that might have a heavy load. So we would run it dry by watching the fuel pressure and when it started to drop you had a few seconds to switch to a tank with fuel. Once I got distracted by a radio call and I can tell you that pax HATE it when the engine quits. 

I know that Deakin advocated it, but I just don't see any reason to run a tank dry in flight. My CiES fuel senders are plenty accurate.

 

[Will.iam]

Actually i find that non pilot passengers handle it way better when you pre brief them what is about to happen and that you are intentionally doing it and it’s normal. Then when it happens it’s a non event. I even say there it it is reach down at a normal speed and turn the selector valve. Whereas the complete surprise followed by yelling a cuss word while jumping in your seat and snatching the switch over i find generally puts them in hysterical mode with even pants wetting and swearing never to fly with you again. 

[hazek]

I was always wondering how does one preserve the most fuel on the fullest tank for that landing but I thought running a tank dry is extra wear if not actually harmful for the fuel pump? Plus a backfire might happen which harmful for the exhaust. Do people really do this routinely?

[Will.iam]

Well to have a backfire or afterfire, you would need raw gas in the exhaust. Running out of gas  by definition there is no gas in the exhaust or cylinder or intake and your sparkplugs are still firing as the prop windmills in the air so when you switch and gas starts coming back in your motor will start running super lean to lean as pressure builds and fuel flow returns to normal if you are lop or through peak to rop what ever your fuel mixture was last set to. The popping you get as the mixture comes back online can be reduced if you pull the throttle to idle before you switch and then push it back in after fuel is there basically what you do every engine shutdown and start.  

[Echo]

It is NEVER cool or funny to scare your passengers. You get surprised by running a tank dry and my trust in you as a pilot is nil. 

[Shadrach]

8 hours ago, hazek said:

I was always wondering how does one preserve the most fuel on the fullest tank for that landing but I thought running a tank dry is extra wear if not actually harmful for the fuel pump? Plus a backfire might happen which harmful for the exhaust. Do people really do this routinely?

How does one get a “backfire” (afterfire) without fuel?  I’ve heard the fuel pump lubrication argument made before and I think it’s highly unlikely. That being said, I try to catch it at the first waiver of the fuel pressure needle.

[Shadrach]

On 8/6/2025 at 9:34 PM, 201er said:

After a fuel stop enroute to Florida, 10 gallons in I switched tanks and a few seconds after the switch the engine started to stumble. The fuel pressure gauge dropped so I immediately turned on the boost pump. The stumbling continued and fuel flow was erratic. I enriched the LOP mixture because the engine doesn’t like to relight LOP.

Eventually normal function resumed.

On the previous flight on the way to the fuel stop, I inadvertently deliberately ran a tank dry. Running the tank dry was deliberate but the moment it happened was a bit unexpected. I was about 10 minutes from beginning my descent and expected to switch the tank at that point before it ran completely dry. When there isn’t a lot of gas remaining, I want to arrive with everything being usable.

What made it worse was my passenger flipping out over it which startled me much worse than the tank running dry. I immediately reached for the fuel selector and flipped to the other tank and turned the boost pump on.

Anyway, the surprising thing was the engine stumble after refueling. Well duh, there was still air in the line from running dry on the prior flight. I’ve always been taught not to switch tanks right before taking off or critical phases. And this was a very good reminder why!

So, just a reminder that you can have some engine stumble not only when running a tank dry but also on the subsequent flight when you switch to the previously dry tank. 

It’s a good idea to check fuel selector continuity before every flight. An added bonus is that you purge any air trapped in the system while you’re on the ground.

[Hank]

7 hours ago, Shadrach said:

It’s a good idea to check fuel selector continuity before every flight. An added bonus is that you purge any air trapped in the system while you’re on the ground.

Yep. Crank the engine, then switch tanks.

[hazek]

14 hours ago, Shadrach said:

How does one get a “backfire” (afterfire) without fuel?

When you re-introduce fuel of course. If you do so reflexively without first reducing power and mixture to idle. The startle effect could easily cause one to do so.

[Shadrach]

2 hours ago, hazek said:

When you re-introduce fuel of course. If you do so reflexively without first reducing power and mixture to idle. The startle effect could easily cause one to do so.

No fuel should make it past the combustion chamber into the exhaust if the ignition system is functioning. So I’m not really sure what you’re talking about. I’ve seen carbureted engines “backfire” on start up when an over primed engine has spark introduced. I have seen carbureted and injected engines have “afterfire” after multiple failed start attempts have pumped the exhaust system full of a combustible mixture of fuel and air. What I have never seen is an engine that is pumping air while windmilling at 2500 RPM with hot mags, do either of those things. Perhaps you can elaborate on how Such a thing happens?

[hazek]

I have no clue. I’m just asking questions. But I do believe I heard Martin the Bonanza youtuber say this in his in-flight mag check video. Not sure though.

[Jackk]

How did you think the pax was going to react? lol

 

gotta read the room a little better

[blaine beaven]

1 hour ago, hazek said:

I have no clue. I’m just asking questions. But I do believe I heard Martin the Bonanza youtuber say this in his in-flight mag check video. Not sure though.

An in-flight mag check is a different beast than running a tank dry. Imagine that you were unaware you were flying with a dead mag, and you switch the other one off in cruise. The engine goes dead - but not from lack of fuel. So all of the mechanical bits are still turning, and fuel is still pumping through the cylinders into the exhaust. 
 

If you turn the mags back on at that moment, expect a big backfire because you’ll ignite any residual fuel in the exhaust. The proper technique in that circumstance is pull mixture to ICO (to stop the flow of fuel, allow what is in the exhaust to get blow out), turn your mags back on, and slowly reintroduce fuel with the mixture until it starts firing. 
 

Hope that helps. 

[Jackk]

Unless you have a reason to suspect an issue in flight, the less changes (throttle, mags, etc) you make to the engine the more likely it will be that you’ll still have an engine.

 

Could I very much most likely restart an engine from a lack of fuel and or spark, sure, but why would I?

 

”is there something wrong with it?”

”no”

”DONT TOUCH IT”

 

[Shadrach]

7 hours ago, blaine beaven said:

An in-flight mag check is a different beast than running a tank dry. Imagine that you were unaware you were flying with a dead mag, and you switch the other one off in cruise. The engine goes dead - but not from lack of fuel. So all of the mechanical bits are still turning, and fuel is still pumping through the cylinders into the exhaust. 
 

If you turn the mags back on at that moment, expect a big backfire because you’ll ignite any residual fuel in the exhaust. The proper technique in that circumstance is pull mixture to ICO (to stop the flow of fuel, allow what is in the exhaust to get blow out), turn your mags back on, and slowly reintroduce fuel with the mixture until it starts firing. 
 

Hope that helps. 

I know it’s pedantic to say but the scenario you describe is “afterfire”.

Backfire = improper ignition of fuel/air mixture outside of the combustion chamber on the intake side.

Afterfire = improper ignition of fuel/air mixture outside of the combustion chamber on the exhaust side.

[Shadrach]

8 hours ago, hazek said:

I have no clue. I’m just asking questions. But I do believe I heard Martin the Bonanza youtuber say this in his in-flight mag check video. Not sure though.

Yes, removing spark from a running engine is a good way to fill your muffler with a combustible mixture fuel and air. Removing fuel from a running engine?…not so much.

[hazek]

But when the fuel is reintroduced while the spark is on, does it start to burn immediately and none of it is too lean to burn and therefore fills the exhaust? Again, I have no clue, just thinking this through.

[hazek]

7 hours ago, Shadrach said:

Backfire = improper ignition of fuel/air mixture outside of the combustion chamber on the intake side.

Would you mind explaining how this happens? I didn’t realize I didn’t understand backfire correctly.

[Shadrach]

40 minutes ago, hazek said:

But when the fuel is reintroduced while the spark is on, does it start to burn immediately and none of it is too lean to burn and therefore fills the exhaust? Again, I have no clue, just thinking this through.

I have only seen afterfire occur from a lean mixture in the case of a closed throttle combined with an intake leak.

[Shadrach]

40 minutes ago, hazek said:

Would you mind explaining how this happens? I didn’t realize I didn’t understand backfire correctly.

Sure. This is typically a startup phenomenon and it requires two things:

1) a combustible mixture in the intake system

and

2) an improper combustion event inside the combustion chamber. 

#2 occurs when the spark plug successfully ignites a mixture that is so rich or so lean that it burns so slowly that combustion is still occurring in the system when the intake valve opens for the next cycle. The flame front then propagates backwards, passed the intake valve and into the intake. This is how startup engine fires occur in the winter time.

[hazek]

3 hours ago, Shadrach said:

Sure. This is typically a startup phenomenon

Got it. Makes sense now.

[hazek]

3 hours ago, Shadrach said:

I have only seen afterfire occur from a lean mixture in the case of a closed throttle combined with an intake leak.

If you run your tank dry and the engine stumbles and you switch tanks throttle is open though?