Help me diagnose an engine running problem

[Battson]

Hey, so I have a problem and it's not clear to me what might be causing it. Maybe you can offer some experience?
The engine starts fine, second blade despite the fuel injection... runs fine at 1200 rpm or less, all indications normal.... when I open the throttle above 1200, it splutters and coughs and starts to die. Full throttle kills the engine but pulling back on the throttle quickly revives it. I have tried adjusting the mixture right on the edge of 1200 and nothing happens, maybe the slightest rise in RPM but not sure about that - it's within measurement error...
Any thoughts??

Jonathan

[Battson]

Ok, noticed one problem with the instrumented readings - the fuel flow stays a steady 20L/hr independantly of the throttle.... 450RPM dead idle (needs turning up) through to the 1250 RPM splutter / cough onset - flow rate is always 19-22L/hr flow rate. So, fuel line restriction then, or fuel servo problem. I'll check both.

I also noticed the #1 and #2 cylinders EGT comes up much faster than the rest - #3 - #6 are all the same, but that's probably nothing to do with it.

[Craig Benbow]

What engine Jono? Looks like classic fuel starvation. Can you detail the fuel system. I assume you followed Bobs recommendations for fuel line size etc. Are you flowing fuel from both tanks or just one? Does it make a difference which tank you are pulling from? The injection system returns unused fuel to the tanks does it? The fuel flow seems odd. Should be down to near nothing at idle and only crank right up when you go full power. That's why I asked about the injection system.
The higher temps in #1 and 2 will be due to leaning. Looks like you are really starving it somehow....

[wingnut13]

Sounds like it needs more fuel!

[jaredyates]

If this is a Bearhawk, has it been subjected to a fuel flow test? A successful test would narrow down the nature of a fuel delivery problem to something in the fuel injection portion.

[Mark Moyle]

What engine, fuel injection type/brand? Sounds like the idle circuit works fine. Power circuit/wide open throttle circuit maybe plugged or not getting fuel. Mark M. Sent from my iPad using Tapatalk HD

[Battson]

So - more hunting has taken place - here's what we know now:

The inter-cylinder temperature difference may be significant - the rear cylinders are lower (tail dragger), and hence are receiving more fuel than the front cylinders and running colder (EGT). This suggests the fuel splitter valve in the flow divider may not be operating correctly, but having disassembled it - no problems are evident. It seems to be travelling smoothly, vent isn't blocked. Does anyone know if the valve's piston is direction - i.e. can it be installed rotated or "clocked" the wrong way around and cause a problem??
Until we know that, I needed to keep looking.

The fuel flow from empty tanks with just gravity feed is 70 L/hr at the engine driven pump - boost pump off. With the boost pump on, the flow available to the fuel servo unit is easily above 90-100 L/hr. Here comes the bad news....
The fuel flow to the flow divider when the fuel servo is operating is not correct. I took off the air box and disconnected the fuel hose into the flow divider, routing this hose to a fuel can. I blocked 3 of the 4 metering orifices in the throttle body with my fingers, and blew compressed air carefully over the 4th while the boost pump was running. With no compressed air the idle fuel flow setting was perfect - 18L/hr - but with airflow this should come up to >60L/hr fuel flow with the throttle wide open. The fuel flow rate did not change significantly. This suggests the fuel servo is probably malfunctioning. Bugger!!

[Battson]

Oh - I did check the fine-gauze finger strainer in the servo - no debris or blockage there.

[Mark Goldberg]

Jonathan - as I recall this is a Bob Barrows engine. Have you consulted him? He is by far the most knowledgeable engine guy I know. Mark

[Battson]

Hi Mark,

Yes, I talked to Bob. Bob didn't have any certain diagnosis and referred me to Don Rivera at Airflow Performance (who is hugely experienced with Bendix RSA fuel controllers). The servo was tested before it left Airflow Performance, and the WOT fuel flow tests looked fine. The tests we've conducted so far do not match the results from the shop, but I need to try some more..... because after this point it'll start costing money to get bench-tested or overhauled. Not a great proposition, seeing as this servo has just come straight from an overhaul and bench test!

[wingnut13]

Have you made any headway with your problem? ~Todd

[Battson]

Hi Todd,
Nothing to report yet. I've not had a chance to get back to the hanger during the working week. We suspect a fuel servo problem, although we have assurance from Don that they are pretty bulletproof and it was tested before it left his place. I have learnt something that might just explain the dud test on the servo, but I can test that in 5 minutes this weekend. This would still leave us without a primary cause.
Jonathan

[Battson]

Solved. But before you ask - yes I did a visual check of the lines and a compressed air blow after assembly. These little grommets must have only shaken loose after the engine running at fuel pressure. Bob recommends using a bit of wire to physically check your hose fittings internally, after assembly. I can see they were formed by the assembly process for the Aeroquip 303 fittings. Very thankful that it turned out to be an easy fix, but boy did these little guys take some finding. They were piled up in the flow divider stainless steel inlet fitting.

[Craig Benbow]

Little buggers Jono! That is a good find and way better now than on takeoff from some way too short runway....... Did the barb chop out a bit of rubber as it was forced into the hose ya reckon or some other thing produce those??