FUEL pressure fluctuation / fuel flow change

Stephen Shore

Stephen Shore

Pre-takeoff checklist
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sshore
1995 Commander 114B with a Lycoming IO-540 with 2016 TTH since new on the engine....

In cruise I see fuel pressure at a steady 25 psi and fuel flow set at 10.8 gph (running LOP and have GAMIs). Sporadically the fuel pressure will momentarily drop down to near 15 psi and fuel flow increases to 11.2 gph. Fuel pressure quickly recovers and gph goes back down to 10.8 gph.

This happens so quickly that there is no noticeable change in engine - sound or RPM.

Question - why would fuel flow increase with a reduction in fuel pressure?
 
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Just a couple of questions to clarify what I'm reading.

What's the oil pressure have to do with this. You state oil pressure, but it's not part of the problem, but part of your title? Did I miss something?

On the oil pressure side, isn't 25# a bit shy? Overhaul in your immediate future anyway?

Sounds like an injector is malfunctioning, and "opening up" a bit, causing flow to increase, and as such, pressure will decrease.

Disclaimer: I'm not an aircraft maintenance professional. I only play one on T.V.
 
Stephen Shore said:
1995 Commander 114B with a Lycoming IO-540 with 2016 TTH since new on the engine....

In cruise I see oil pressure at a steady 25 psi and fuel flow set at 10.8 gph (running LOP and have GAMIs). Sporadically the fuel pressure will momentarily drop down to near 15 psi and fuel flow increases to 11.2 gph. Fuel pressure quickly recovers and gph goes back down to 10.8 gph.

This happens so quickly that there is no noticeable change in engine - sound or RPM.

Question - why would fuel flow increase with a reduction in fuel pressure?
Click to expand...

Drop in fuel pressure when the flow is increased is expected behavior. But you should be seeing some change in engine behavior, unless there is a fuel leak somewhere.
 
Sorry about the confusion - I mistyped. Should be FUEL pressure fluctuation. NOT oil pressure.

I am trying to figure out how to edit the post title...
 
Rgbeard said:
Just a couple of questions to clarify what I'm reading.

What's the oil pressure have to do with this. You state oil pressure, but it's not part of the problem, but part of your title? Did I miss something?

On the oil pressure side, isn't 25# a bit shy? Overhaul in your immediate future anyway?

Sounds like an injector is malfunctioning, and "opening up" a bit, causing flow to increase, and as such, pressure will decrease.

Disclaimer: I'm not an aircraft maintenance professional. I only play one on T.V.
Click to expand...

This actually makes sense. My #2 cylinder CHT runs 20 degrees hotter (365 F) and EGT runs cooler, which indicates a clogged injector / lean cylinder to me.

I will check that....
 
Fuel psi notwithstanding, 25 psi on the oil is way to low.

The fuel issue could be a pump leaking internally and fuel being vented overboard.
 
To edit title open thread. Upper right side thread tools. Click on that to edit title..:)
 
AKBill said:
To edit title open thread. Upper right side thread tools. Click on that to edit title..:)
Click to expand...
For some reason cannot edit title. I was able to edit the text in the body of the post.

I apologize for the confusion. No issues with OIL pressure!
 
Ok, go to the top of the thread or the first post you made. Look to the right side top of the screen. See "Thread tools", click that and a drop down will appear "Edit Title". Click on that and edit as you like..
 
If no external leakage, I'm thinking fuel injector..
edit: or fuel regulator/relief
 
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AKBill said:
Ok, go to the top of the thread or the first post you made. Look to the right side top of the screen. See "Thread tools", click that and a drop down will appear "Edit Title". Click on that and edit as you like..
Click to expand...
Done and thank you!
 
Looks like it was a slightly clogged injector. I ended up taking all the injectors out and cleaning them per GAMI instructions. On the test flight, I had much improved consistent CHT's across all cylinders, with the exception of #5 which is at the back by the oil cooler. That cylinder runs cooler because it is in the air flow for the oil cooler.

Anyway 5 good consistent readings across all 5 cylinders (with #5 cylinder running about 20-30 degrees cooler).

Fuel pressure held very steady at 27 psi and I even gained a little RPM increase on my takeoff roll.

Thanks for all the help and I apologize again for the confusing post on the first run.
 
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