So, I made it down to pick my plane up from having a cylinder overhualed yesterday and, about 30-40 minutes out from landing, I had an issue. My fuel flow slowing starting to decrease and I'd see the CHT on my new cylinder starting to increase. So, I would increase the mixture to maintain control of the cylinder temps. Well, this continued until I had full mixture and the max fuel flow I was getting was around 14 gph. Normal should be more like 23 gph. Last fall at annual we adjusted the fuel flow and it was dead on when we finished. At takeoff from Montrose, everything functioned normally and readings were normal. For the cylinder breakin, I was running full RPM and cowl flaps open (due to the warmer temps and to help keep cylinder temps in check). I had followed the breakin procedure of 75% BHP for the first hour, then alternating between 65% and 75% every 20 minutes thereafter. Total flight time was 3.4 hours and I was in the alternating phase when this occurred. Luckily, when I reached full rich and the CHT was starting to increase, it was time to descend and I was able to keep it from continuing to climb by reducing power for the descent.

I met with my mechanic this morning and all the settings on the fuel pump hadn't moved. No issue with the mixture control was apparent. He is thinking that it could be the engine driven fuel pump, but he's scratching his head because he's never heard of one failing. And there there is the question of whether or not something like that can "soft" fail...or if it's an all or nothing. I pulled the data and EGTs remained normal througout that time period. It doesn't appear to be a distribution issue, but a flow issue. Both the analog and digital fuel flow reflected roughly the same when I noticed the 14 gph. A static run on the ground maxed out around 13 gph with full rich.

So, any thoughts? I'm really bummed because my mechanic is headed out of town for 2 weeks and, unless I can come up with another option, it looks like I might be grounded until mid July! Sigh...

Clogged fuel filter/strainer or pump screen?

Maybe some debris in the flow divider, or the line running to it?

If all cylinders are the same, it's unlikely to be a clogged injector.

Grassstrippilot wrote:So, I made it down to pick my plane up from having a cylinder overhualed yesterday and, about 30-40 minutes out from landing, I had an issue. My fuel flow slowing starting to decrease and I'd see the CHT on my new cylinder starting to increase. So, I would increase the mixture to maintain control of the cylinder temps. Well, this continued until I had full mixture and the max fuel flow I was getting was around 14 gph. Normal should be more like 23 gph. Last fall at annual we adjusted the fuel flow and it was dead on when we finished. At takeoff from Montrose, everything functioned normally and readings were normal. For the cylinder breakin, I was running full RPM and cowl flaps open (due to the warmer temps and to help keep cylinder temps in check). I had followed the breakin procedure of 75% BHP for the first hour, then alternating between 65% and 75% every 20 minutes thereafter. Total flight time was 3.4 hours and I was in the alternating phase when this occurred. Luckily, when I reached full rich and the CHT was starting to increase, it was time to descend and I was able to keep it from continuing to climb by reducing power for the descent.

I met with my mechanic this morning and all the settings on the fuel pump hadn't moved. No issue with the mixture control was apparent. He is thinking that it could be the engine driven fuel pump, but he's scratching his head because he's never heard of one failing. And there there is the question of whether or not something like that can "soft" fail...or if it's an all or nothing. I pulled the data and EGTs remained normal througout that time period. It doesn't appear to be a distribution issue, but a flow issue. Both the analog and digital fuel flow reflected roughly the same when I noticed the 14 gph. A static run on the ground maxed out around 13 gph with full rich.

So, any thoughts? I'm really bummed because my mechanic is headed out of town for 2 weeks and, unless I can come up with another option, it looks like I might be grounded until mid July! Sigh...

Dealt with this issue just today on the 185.

Fuel flow gauge started reading lower on my last flight.
I could get the EGT to raise up a little by leaning but it would only richen to about 50 rich of peak.
If I ran the electric pump the flow would indicate higher and EGT would cool another 100 degrees!
I tried adjusting the high side of the engine driven pump but that only barely changed the flow.


It ended up being a dirty screen.
The one that screws into your fuel control unit.

Cleaned it and the flows came back and i had to readjust the high side to get pressures back down to specified numbers.

If it was the EDP failing wouldn't activation the boost pump bring the flow back up?
Sounds like a restriction to me as well....

Also I have never heard of a partial failure on the engine driven pump.
I'm sure it can happen though.
Even total failure is pretty rare, there is a little piece of square stock that drives the pump but shears off and totally stops the pump if it binds up.

Thanks for the thoughts guys. I really appreciate it. I'm going to find the fuel control screen and pull/clean it. I'll probably pull the gascolator too. That's a real knuckle buster! Then I'll go run it and see what happens. If it's still an issue, then we will hook the gauges to the pump and see what its output is and if that is the source of the problem.

TangoFox wrote:Also I have never heard of a partial failure on the engine driven pump.
I'm sure it can happen though.
Even total failure is pretty rare, there is a little piece of square stock that drives the pump but shears off and totally stops the pump if it binds up.

That's what everyone is saying...that they just don't hear of them failing, let alone a partial failure. Thanks for the info.

Hey Trent,

Is #1 the screen I'm looking for? Where is the fuel control unit mounted?

Hard to be sure of the whole situation from your description, but:

If you were really running 14 GPH instead of 23, your EGTs should have been sky high?

Transducer issues?

Not necessarily. Normal cruise at 100ish degrees ROP is 12.5-13.5 gph, altitude dependent of course. (I don't normally run ROP but know this from doing my Gami lean test previously.). The only reason I had been running 15-16 gph for the majority of the flight was to accommodate the new cylinder that was being broken in and running a bit warmer as they do.

However, if everything was normal, with full rich mixture, the flow should have been in the 23+ range...but that's not where it would normally be ran.

Alright, another thought: If the problem was occurring despite the mixture control servo trying to regulate the flow, based on the fuel and air pressure, does that mean the problem exists inside or downstream of the servo? (assuming you're getting adequate fuel pressure?) If the fuel flow (and pressure) into the servo was too low, then you'd be able to see that?

That I don't have an answer to. We should know about the pump's pressure on Thursday. Meanwhile, I'll eliminate the easier things like screens tomorrow hopefully.

That's the screen tango fox was talking about. Also check the swaged pin on the mixture arm. I have seen them sheared and the arm rotating on the shaft.

Thanks! Good to know. Hopefully I'll be able to make some progress tomorrow. It would be a real coup to be able to resolve this. As of now the heart of my summer flying is hosed.

Trent, it looks like I'll owe you a drink!

I made it to the airport this morning and first off sumped the gascolator. No sign of any flaoties or anything. Then I moved on to find the fuel control unit. Luckily it was in a pretty accessible location. When I screwed it out, a whole bunch of junk came with it. I have no idea what it is. It reminds me of lint in appearance and texture. At first when I saw it, I thought it was paper/cardboard...like maybe a paper sheath around the fine mesh screen. I think it looked like that from being pressed against the screen. With it out, I briefly ran the electric pump in hi to see if anything else came out but the rag was clean. I cleaned the screen and reinserted the it into the FCU. Two static run ups to full power showed normal fuel flows. So, I took off and flew for 15 minutes over the field. Everything acted normally.

So, the question is what was it and where did it come from? Years of accumulation of little pieces? This wasn't on the annual to-do list, but it is now! I'll fly it again tomorrow just to make sure everything is staying normal. I'll also pull the screen to put on a new gasket and check it for any other debris. With any luck, hopefully this issue has been put to rest.

Again, thanks for all the input. Great community and resource we have here!

Grassstrippilot wrote:This wasn't on the annual to-do list, but it is now!

This should on the 100hr checklist for any fuel injected Lyc / TCM engine?
Maybe question your maintenance guy about that....

I hadn't seen that model of strainer before, the Bendix ones I am used to are about 3x longer.

Yeah, I don't know why it was missed. I know they check them on their bonanza.

I don't think it had been off for a long time. The way it was safety wired on couldn't have been done without big pieces being off the plane.

You are definitely asking the right question, "where did it come from?"

It would be good to know when the filter was last cleaned, so you know how fast it's occurred. If your fuel flow dropped off quickly, it's possibly it's accumulating quite fast....?

That filter is much finer than the gascolator, so hopefully it's just a long, long process of accumulation slowly from crud and dust getting in when you're refueling. Does your aircraft have another filter before the boost pump?

Cory, didn't you recently have your fuel pump overhauled? Maybe the contaminants got in then?