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I could surely use some help - trying to figure out this engine problem

patterson

patterson

Well Known Member
Patron
I have a Lycoming with about 50 hours on it that is behaving very badly.
It is cyclical roughness, happening every few minutes. It begins with roughness, followed by lower RPM's, higher EGT's (all) and a feeling as if it is self leaning! Fuel flow goes down by 2-4 GPH too (11+ down to 7 or 8 and EGT's rising to Peak). A few seconds (20-30) and it cures itself. Then in a few moments, it all begins again.

Isolating the mag does not improve or make it run worse or better. Same for the one Lightspeed II/Hall effect I am running. Feels like the engine is about to give up, then it takes a breath and goes on normally for a few minutes. It will run hard and even overspin the CATO prop (it's underpitched right now), then runs very poorly... like I need to find a place to land. Goes from 2750 down to 2450, and then, slowly, back up again. Scary!

Because if "feels" like a fuel issue, I traced everything in the (new) AFP 150 for loose fittings or fouled filters.... no joy. Cleaned everything and tightened fuel fittings and no change in the way it runs. Takes a few minutes on the way to altitude before it starts in, then never corrects. Same issues regardless of altitude or RPM. Could the mag be working intermittently? Hall Effect? If so, you would think isolating them would make it quit altogether, but it doesn't change anything except the EGT's go way up, and then I get nervous and switch back to both.

Any thoughts you may have are much appreciated.
Ron
RV-4
 
I am neither a mechanic nor an expert, but it sounds to me like it could have something to do with a fuel tank venting problem
 
fuel

How much fuel in the tanks when this is happening?

What engine? Injected or carb?

Will the engine do this consistently during a ground run?
 
Last edited:
Do a fuel flow test on both tanks by disconnectiong the fuel line from the carb or injector servo and use the boost pump to run fuel into a gas can.

You should get about 25 gph.
 
Fuel leak air not fuel

I had a similar issue with missing and chased electrical untill it just could not be electrical. Make a fuel hose clear tygon and run it from you red cube to the spider if you are fuel injected. Run the engine up to 1500 and check for air bubbles. It will bubble at idle but at 1500 rpm there should be none. If you see bubbles it is sucking air into the fuel. I traced my issue to the fuel valve pipe threads. Did not leak fuel but sucked air. Pulled the fittings off and sealed with permatex 2. Problem solved. Hope this helps.
 
I agree with the assessments that it sounds either like it is sucking air or is a clogged vent. Does it happen on either fuel tank? If so, then a clogged vent is less likely, but still possible.
 
Engine Problem

One more troubleshooting thing to do. Assuming that the engine is running on the mechanical fuel pump only, next time it happens do nothing but turn on the electric fuel pump. If the "self leaning" stops, then you would have confirmed that you have fuel line sucking air, or an obstruction/flow restriction causing the fuel in the line flashing to vapor/bubbles, thus reducing the flow to the engine.
 
Clogged Vents

On wet tank (type of fuel tank design), a clogged fuel vent will most likely cause skin damage / collapse before the engine dies. Doesn't tank much negative pressure inside the fuel tank before the thin wing skins concave.

If this is a carburetor'd engine, I'd suspect issues with the float / needle sticking if there is adequate fuel pressure on the fuel pressure gauge.
 
Check for play in the mixture cable - verify that you cannot move the mixture arm manually with the knob locked.

Is the fuel system design per AFP's recommendations? I.e., pump and filter locations, etc.

Did the problem start abruptly?

Also, call Don at Airflow Performance. He may want to check out the servo.
 
flynwest said:
I had a similar issue with missing and chased electrical untill it just could not be electrical. Make a fuel hose clear tygon and run it from you red cube to the spider if you are fuel injected. Run the engine up to 1500 and check for air bubbles. It will bubble at idle but at 1500 rpm there should be none. If you see bubbles it is sucking air into the fuel. I traced my issue to the fuel valve pipe threads. Did not leak fuel but sucked air. Pulled the fittings off and sealed with permatex 2. Problem solved. Hope this helps.
Click to expand...

I have done this. The only thing I can add is that it is no fun standing behind a prop spinning at 1500 rpm. Obviously, tie down and chock, and it wouldn't hurt to have a trusted buddy at the controls. Scary!:eek:
 
patterson said:
I have a Lycoming with about 50 hours on it that is behaving very badly.
It is cyclical roughness, happening every few minutes. It begins with roughness, followed by lower RPM's, higher EGT's (all) and a feeling as if it is self leaning! Fuel flow goes down by 2-4 GPH too (11+ down to 7 or 8 and EGT's rising to Peak). A few seconds (20-30) and it cures itself. Then in a few moments, it all begins again.

Isolating the mag does not improve or make it run worse or better. Same for the one Lightspeed II/Hall effect I am running. Feels like the engine is about to give up, then it takes a breath and goes on normally for a few minutes. It will run hard and even overspin the CATO prop (it's underpitched right now), then runs very poorly... like I need to find a place to land. Goes from 2750 down to 2450, and then, slowly, back up again. Scary!

Because if "feels" like a fuel issue, I traced everything in the (new) AFP 150 for loose fittings or fouled filters.... no joy. Cleaned everything and tightened fuel fittings and no change in the way it runs. Takes a few minutes on the way to altitude before it starts in, then never corrects. Same issues regardless of altitude or RPM. Could the mag be working intermittently? Hall Effect? If so, you would think isolating them would make it quit altogether, but it doesn't change anything except the EGT's go way up, and then I get nervous and switch back to both.

Any thoughts you may have are much appreciated.
Ron
RV-4
Click to expand...

The AFP150 may have to go back to Don for a bench check.
The fuel vents need be checked for obstruction.
The drop in fuel flow and increase in CHT confirms it is a fuel flow problem. I wouldn't fly it again without having the AFP150 checked.

The only other issue could be fuel vaporization somewhere before a pump. It won't vaporize downstream from a pump at 25-30 psi and flowing but it could be vaporizing just before a pump. This could be an issue forward of the firewall going to engine pump, but it should not be a factor if the electric pump is aft of the firewall and operating.

Are both pumps on when it happens?
 
good ideas

Thanks for the troubleshooting ideas.

Yes, it did start abruptly... about 10 hours after install of FI.

It happens on either tank so probably not a fuel line vent.
Don at AFP thinks it could be sucking air. I will try the Tygon tube to see if I can see bubbling.

I'll check the mixture arm for tightness at the servo or play in the cable.

I've re-tightened all the fittings, but never considered they might not fully seal and could suck air but not leak gas. Is this very common?

Tried boost pump many times... no change

Ron
 
Air leak

I don't know how common this is. I'm the only one anybody I know has had it happen to. The fittings on the fuel valve are blue anodized so I could not see any fuel residue. Once I got the valve out I could rub a little blue dye at the pipe thread on the valve. Removed them and resealed them problem solved. They were originally sealed with PTFE lasted 700+ hours before failure. I used Permatex 2 this time. Oh yeh take a picture of the position of the fittings before you take it apart. Ask me how I know.
 
I had a similar issue. No fuel leak but a rough running engine on occassion. In trying to isolate where the issue could be. I installed a fuel line from a gas can and affixed it to the top of the motor mount so it could gravity feed to the mechanical pump on the engine. Engine ran great so i knew it was in the fuel lines somewhere. In my RV8 the fuel filter is vertical and on the gear tower. the top nut on the fuel line coming out of the filter was not 100% sealed and would suck air breaking the vacuum in the fuel line. Sealing this connection took care of the issue.
ZGood luck finding the issue. I know how gut wrenching it can be when something is not right.
 
I had a similar issue, though it didn't have as big of an RPM drop. It turned out to be a spark plug that had a cracked insulator around the sparking probe. I was pretty lucky it didn't break apart and damage a valve. We found it with a very close visual inspection of each spark plug. Although, this might not be your issue since it happens on both mags. But you could also have multiple issues creating a compounded problem ... it's worth a check.
 
JonJay said:
I have done this. The only thing I can add is that it is no fun standing behind a prop spinning at 1500 rpm. Obviously, tie down and chock, and it wouldn't hurt to have a trusted buddy at the controls. Scary!:eek:
Click to expand...

Thats why we have GoPro cameras.
 
Sucking air

I had air intrusion with electric fuel pump on and no fuel leak. Wasn't so bad it caused engine symptoms. Discovered it after attaching tygon tube on outlet of AFI fuel servo. Problem solved by putting that super sticky fuel lube goop on all the fittings between the selector valve and inlet to AFP filter.
 
If the problem only manifests while in the air, check your quick drain on your gascolator if you have one. I had one that would let air in at high power settings (on take off). I have heard of one other that leaked air at higher airspeed partially due to ram air flow(similar to pitot pressure) over a less than ideal drain tube exit angle / location.
 
Add these to the fittings and re-test... any gaps letting air but not fuel leak will be microscopic.

http://www.aircraftspruce.com/catalog/hapages/del37fittings.php

04-05002.jpg
 
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