Cherokee 140 Intermittent Partial Power

[Mike W.]

Hi Everyone,

I apologize in advance for the long post, but I (and a few A&Ps) am stumped and looking for a troubleshooting starting point. Here is what I am trying to fix:

A few weeks ago I did a new weight and balance. Tanks were emptied and aircraft weighed...all is good. Refilled tanks and flew the aircraft a few days later. While on an IFR flight at 6000 ft. (80 degrees F) with full tanks, I was about 30 minutes into the flight and I switched tanks from right to left (fuel pump on, switch tanks, fuel pump off). About 30-60 seconds later engine started sputtering and I lost about 500-600 RPM. I immediately turned on the fuel pump and switched tanks back to the right tank. 30-60 seconds later, RPM back to normal and engine smooth. Turned the fuel pump off and engine continued to run fine. Landed at my destination (which was about 10 minutes away). On the ground, I switched back to the left tank and ran the engine at full RPM. I could not duplicate it on the ground. Called my A&P and after a discussion we determined that it could likely have been air in the fuel line since that was the first time that tank was used since the weighing. I flew the aircraft back on the right tank (35 minute flight) just to be safe. Again, after landing I switched back to the left tank and could not duplicate the sputtering and loss of RPM...problem solved. Fast forward to this weekend. I was flying locally for about 45 minutes on the right tank (the good one). I was at 1500-2000 ft. and temp was around 90 degrees F (humidity was pretty high). When I applied power to climb, engine started running rough and lost about 500-600 RPM. I did the same thing as previously, switched to the other tank (left) and turned on the fuel pump. 30-60 seconds later, RPM back and engine smooth. I talked to three different A&Ps and they all had different opinions: 1) Engine driven fuel pump, 2) leak in the fuel line allowing air in, 3) fuel blockage somewhere. All of these seem reasonable, except that it is very intermittent. Any recommendations on where to start troubleshooting? Prefer most conservative first

 

[wsuffa]

Are there vents to the fuel tanks? If so, is the vent on that side blocked (insect nest or the like)?

 

[Mike W.]

Are there vents to the fuel tanks? If so, is the vent on that side blocked (insect nest or the like)?

There are vents. I will check them today when I go to the hangar. What concerns me is that both sides have done it now. I have owned this airplane for a while and I have always "trusted" it. Now I am concerned about flying it. Thanks for the suggestion.

 

[Domenick]

Check the fuel filter(s) too.

 

[Mike W.]

Check the fuel filter(s) too.

I'm going to show my lack of knowledge here, but are there more fuel filters other than the gascolator? If so, where are they located? Thanks.

 

[Dan Thomas]

I'm going to show my lack of knowledge here, but are there more fuel filters other than the gascolator? If so, where are they located? Thanks.

There should be a finger strainer in each fuel tank outlet. I's a coarse screen designed to keep larger debris out of the line, stuff like a dead bug or a leaf.

The second filter is in the gascolator (fuel strainer). It's a fine screen, and it's supposed to be cleaned at annual inspections.

The final filter is in the carburetor inlet, and it should be done annually as well but a lot never get looked at at all.

My bet is on a failing engine-driven pump. A bit of air getting into a pump that has leaking check valves could do as you described. How long (hours and years) since it was last overhauled? While we're at it, how old is that boost pump? If it failed you could be in big trouble when the engine-driven pump does this.

 

[timrb]

Except for the sputtering, it sounds like a problem I had once with my 140. Muffler baffles had broken loose and were periodically blocking exhaust flow, causing intermittent power loss. Whack your muffler a few times and listen for loose metal inside.

Tim

 

[nrpetersen]

Also check the muffler visually from up the exhaust pipe. Do it with a flashlight in a darker area so you can see the baffles inside.

 

[Mike W.]

Except for the sputtering, it sounds like a problem I had once with my 140. Muffler baffles had broken loose and were periodically blocking exhaust flow, causing intermittent power loss. Whack your muffler a few times and listen for loose metal inside.

Tim

Thanks. I have a powerflow muffler. This could be the problem...I checked the muffler and the baffle is missing. It is all making sense now. A few flights ago my passenger asked if I heard something hit the bottom of the airplane on landing. I didn't, but he swore he heard something. We taxied in and checked the airplane and did not see anything. However, when we took-off again, we saw a cylindrical object on the runway (the size of a muffler baffle). We let the airport know something was on the runway, but I could have been the source of that. The baffle is definitely missing.

 

[Mike W.]

Thanks. I have a powerflow muffler. This could be the problem...I checked the muffler and the baffle is missing. It is all making sense now. A few flights ago my passenger asked if I heard something hit the bottom of the airplane on landing. I didn't, but he swore he heard something. We taxied in and checked the airplane and did not see anything. However, when we took-off again, we saw a cylindrical object on the runway (the size of a muffler baffle). We let the airport know something was on the runway, but I could have been the source of that. The baffle is definitely missing.

Okay, looks like the baffle is not likely the issue. Talked with the engineer at Power Flow Systems and they said that a missing baffle should not cause this kind of issue. Looks like I am back to fuel system troubleshooting. Thanks for all the replies.

 

[edessa]

Who drained the tanks for the W&B and how did they do it? Did they put in the same fuel that was removed? Where did they store the fuel that was removed?
If it's only happening on one tank I dont think it would be anything that is common to both tanks. Start at the tank and work your self to the fuel selector. Find someone with a borescope and look in the tank at the screen. Remove the access panel next to the gear and look at the fuel line making sure its not damaged (flex and hardline). Check all connections as you move down the line. How is your your fuel selector (seals)? Any leaks behind the side panel? How do the detents feel?

 

[nrpetersen]

Don't discount the failed muffler baffle so quickly. Depending on how it fails, it can effectively block the exhaust port enough to throttle or even kill the engine. The only thing is it would be unusual might be that there was more than one such event that was similar. The same can be said for fuel contamination I suppose, too.

 

[mondtster]

Don't discount the failed muffler baffle so quickly. Depending on how it fails, it can effectively block the exhaust port enough to throttle or even kill the engine.

The power flow baffles are basically like a glass pack muffler; straight through with a perforated core and fiberglass wrapping. Plus, they’re pretty far downstream of the exhaust port. I would be very surprised to hear of a power flow baffle that would create a problem, although I would not say it is impossible.

 

[Dan Thomas]

Don't discount the failed muffler baffle so quickly. Depending on how it fails, it can effectively block the exhaust port enough to throttle or even kill the engine. The only thing is it would be unusual might be that there was more than one such event that was similar. The same can be said for fuel contamination I suppose, too.

The problem happened the first time when he switched tanks. The second time it happened when he added power and went away when he switched tanks. In both cases he turned the boost pump on.

That tells me that the engine-driven pump is pooched. If it's a muffler problem I'd better turn in my mechanic's license.

 

[Jim Carpenter]

Sure seems like engine driven fuel pump is high on the suspect list.
Please don't take this as an insult....but, let's start with simple stuff, in the OP, you mentioned fairly hot ambient temperatures. What altitude are you operating at (Edit:, Oops, I see you mentioned 1500-2000), and did you have the mixture leaned properly for conditions?

 

[edessa]

Why would it be the engine driven pump when it only happens when the OP is on the left tank?

 

[bflynn]

I agree, this sounds like it's isolated to the left tank. It has to be somewhere between the fuel selector valve and the tank.

Some hypothesis come to mind, but they're guesses

• when the fuel was drained, something got in the tank and is blocking the fuel inlet
• Said thing was sucked into the fuel inlet and is lodged in the fuel line heading to the fuel selector valve
• Vent system blockage. My memory is that PA-28s have a single vent with cross connect and that the vent is on the right tank. When you switch to the right, if there's no way for the left tank to get air, the vacuum would starve the line.
• I cannot come up with a rational issue affecting the fuel selector valve.

Of these, a venting issue is my first thought. Perhaps a mud dauber in the cross connect or left tank vent.

 

[Dan Thomas]

The OP said:
Fast forward to this weekend. I was flying locally for about 45 minutes on the right tank (the good one). I was at 1500-2000 ft. and temp was around 90 degrees F (humidity was pretty high). When I applied power to climb, engine started running rough and lost about 500-600 RPM. I did the same thing as previously, switched to the other tank (left) and turned on the fuel pump. 30-60 seconds later, RPM back and engine smooth.

He applied power to climb while flying on the RIGHT tank, the "good" one. The engine acted up. He switched to the left tank, the one you guys call the "bad" one, and turned on the boost pump, and the engine recovered. On the BAD tank.

Now tell me that it's a muffler problem. Or a blockage in the left line somewhere.

 

[Domenick]

...Vent system blockage. My memory is that PA-28s have a single vent with cross connect and that the vent is on the right tank...

Just as a point of information, my Warrior (PA-28-161) has a vent for each tank. I think all PA-28s have fuel tank vents on both sides. The parts manual covering the Hersey Bar models, 140, 150, 160, 180, 235, R180, and R200, all show two vents with no model or serial number notes. All the taper wing P-28s have dual vents.

The mechanical fuel pump also has a vent line.