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  #1  
Old 11-20-2015, 01:36 PM
Carl Froehlich's Avatar
Carl Froehlich Carl Froehlich is offline
 
Join Date: Dec 2007
Location: Dogwood Airpark (VA42)
Posts: 3,253
Default When a stuck valve is not a stuck valve

All,

A few weeks ago we had an interesting engine issue on an RV-8A. The specifics are provided for your information and engine file. Here are the details:
? Engine is a standard compression IO-360, 180hp parallel valve with AirFlow Performance fuel injection
? Total engine time since new is 800 hours (over 13 years)
? Running dual pMags for the last 450 hours (dual Lightspeeds before that which were removed for repeated failures)

The engine started to show signs of a stuck exhaust valve:
? Rough idle on initial start up (classic indication) but the problem did not clear when the engine was warm (at idle) ? not a classic stuck valve indication.
? #4 EGT way low ? as if the cylinder were simply dead
? Exhaust pipe ?popping? at idle
? The engine ran fine at cruise and full power

From these clues the decision was to ream #4 exhaust valve guide. After removing that valve spring we found the valve to be free moving ? not at all stuck. On further inspection we found the valve, the valve stem and exhaust port to be pristine (engine spends 90% of it time running LOP cruise). We reassembled the engine and on start up found the problem still present.

At this point we went back to basics: Fuel/Air/Spark

We did the normal timing checks, spark plug examinations, intake tubes clear, fuel injectors clean. We freed and separated all the ignition lines to see if we had cross talk or arcing to ground and verified all ignition wires good ? no joy. We swapped the spark plug leads for #3 and #4 cylinder hoping we could move the problem from #4 to #3 (The pMag is a wasted spark coil, so #1 and #2 cylinders, like #3 and #4 cylinders spark on each revolution). The problem did move but it moved from #4 to #2 cylinder. At that point we figured we had a weird ignition issue and called Brad at pMag. We swapped the #3 and #4 leads back but the problem remained on #2 cylinder. Brad was patient, but he came to the same conclusion we did to lock in on the stuck valve. The offending cylinder (first #4 but now #2) was dead (low EGT) when running the both pMags, left only and right only. Brad concluded that a multiple failure of both pMags that moved from one cylinder to another was unlikely if not impossible.

The other data points:
? Leaning at fast idle all cylinders other than #2 displayed the proper EGT rise. #2 went down even further
? Doing the ignition check all cylinders other than #2 displayed the proper EGT rise. #2 went down even further

From this we moved on to fuel. Talking with Don at AirFlow Performance he told us to do a fuel flow test before anything else. The flow test was disconnecting the spider lines at each fuel injector and collecting fuel from the spider in a shot glass. The throttle was at idle and mixture about half way in. Boost pump on and collection for 25 seconds. Result showed #2 line was half the flow as the other three. From Don we also learned:
? The spider has a spring loaded diaphragm that is closed at idle, open at power. If the diaphragm is open the spider is just a open cylinder and fuel flows out the connected lines to the fuel injectors. The nozzle in each injector determines how much of the distributed fuel goes to that cylinder (on this engine the cylinders have been balanced to within 0.1gph EGT peak between cylinders).
? If the diaphragm is shut, fuel exits the spider via a .007" orifice, one for each line.

The flow test shows that at idle #2 cylinder is getting a lot less fuel than the rest. This explains why leaning at idle #2 EGT went down when the other three went up. #2 was already much too lean. Extrapolating this data we then imagined a piece of crud in the spider that originally was blocking fuel to cylinder #4, then moved to block fuel to cylinder #2. This is the only logical theory as to why the problem moved from #4 to #2.

Pulling the spider off and sending it to AirFlow Performance, Don reported finding some ?crud? in it. Of note, a few weeks earlier during conditional inspection the entire injection system was sent to Don for bench testing and such as it was over 10 years old. Getting the spider back and installing it the engine is now again purring like a kitten.

Carl
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  #2  
Old 11-20-2015, 10:45 PM
longline longline is offline
 
Join Date: Sep 2014
Location: silverdale, WA
Posts: 208
Default Nice investigative report

It is interesting that the spider ended up the culprit. What sort of filtration did you have in the system?
This is the reason that I am installing a more effective fuel filter after the fuel pumps. Pre-filter before pumps, to preclude tank mung from causing pump issues, and a 40 micron filter to protect the injection unit.
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  #3  
Old 11-21-2015, 07:25 AM
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DanH DanH is offline
 
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Location: 08A
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Default

Quote:
Originally Posted by Carl Froehlich View Post
• The spider has a spring loaded diaphragm that is closed at idle, open at power. If the diaphragm is open the spider is just a open cylinder and fuel flows out the connected lines to the fuel injectors. The nozzle in each injector determines how much of the distributed fuel goes to that cylinder (on this engine the cylinders have been balanced to within 0.1gph EGT peak between cylinders).
• If the diaphragm is shut, fuel exits the spider via a .007" orifice, one for each line.
Close.

The diaphragm doesn't open and close, but rather, raises a sleeve in response to increasing fuel pressure. The spring provides the opposing force. The sleeve forms one component of a valve. Fuel flows into the top of the divider body, down through the center of the sleeve, and into a precisely machined divider insert, which forms the other part of the valve.

Picture worth 1000 words; here's the anodized aluminum divider body, upside down, bottom cover removed, machined insert and sleeve in place:



Zoom in on the insert. Each metering slot and hole is ported to an individual injector line. Here the end of the sleeve is where it would be when operating at a cruise fuel flow. The large hole at the top of each slot is uncovered, so at this flow the division of fuel quantity between individual cylinders is a function of injector insert diameter.



At idle, the fuel pressure is very low, so the diaphragm does not pick up the sleeve very much. The end of the sleeve would cover the large hole and almost all of the slot length. The uncovered slot length is thus a variable orifice; increasing fuel pressure raises the sleeve, uncovering more slot length, and allowing more fuel flow to each cylinder. It is the precise width of the machined slot that determines accurate division of fuel at lower flows. I think the slots are 0.007" wide; have to ask Don to be sure.

The flow rate at which the division function transitions between the flow divider and the injector inserts depends on injector insert size. Small restrictor inserts take control of flow division at a lower cruise fuel flow, while larger restrictors result in division being a divider slot function until some higher cruise fuel flow.

Returning to the run problem, yes, if a piece of crud manages to block a slot, fuel delivery to that cylinder would reduced at low fuel flow. As reported, it would run fine at high fuel flow; the big hole would be in use.

Usually a loose bit of crud would be flushed out through a big hole at cruise or WOT, and if smaller than the insert's nominal diameter (0.028" is typical), it would pass through the cylinder and out the exhaust.

To get in there in the first place, a solid particle would have to pass through three screens with a AFP system; 125 micron prior to the pump, 75 micron at the servo inlet, and 75 micron at the flow divider. 75 microns is roughly 0.003". Given the screen and slot dimensions, Carl's crud was probably paste-like, perhaps some soft sealant or fuel lube...strictly a guess on my part.
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Last edited by DanH : 11-21-2015 at 08:27 AM.
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  #4  
Old 11-21-2015, 09:07 AM
Carl Froehlich's Avatar
Carl Froehlich Carl Froehlich is offline
 
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Default

I agree that the piece of crud did not arrive with the fuel. Instead I rank this as a "maintenance induced problem" when the fuel system was pulled to send in for inspection and bench testing. The fuel filter has a 40 micron element.

Dan - thanks for the nice photos and deep dive into the spider!

Carl
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  #5  
Old 11-21-2015, 09:52 AM
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JordanGrant JordanGrant is offline
 
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Location: Virginia
Posts: 324
Default Why the bench test?

Carl,
Thanks so much for sharing this - I learned a lot this morning. DanH's pictures were very educational, too!

I am curious about something, kind of philosophical. Why did you send the AFP system in for bench testing in the first place? I, too, have an AFP system that has been running (perfectly) for 10 yrs and about 650 hrs, so I'm wondering if you had prior symptoms/reasons for sending the system in to Don.

Cheers!
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  #6  
Old 11-21-2015, 10:10 AM
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1001001 1001001 is offline
 
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Quote:
Originally Posted by JordanGrant View Post
Carl,
Thanks so much for sharing this - I learned a lot this morning. DanH's pictures were very educational, too!
Agreed! I love reading this kind of technical info; it really helps to understand things I haven't had a chance to get my hands on yet. If anyone wasn't convinced of the importance of maintaining fuel filters, these pictures ought to do the trick.

One question: what prevents the fuel pressure from equilibrating on the top of the slider? Is the slider sprung downward? What if the spring breaks? (OK, three questions)
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  #7  
Old 11-21-2015, 10:44 AM
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DanH DanH is offline
 
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Quote:
Originally Posted by 1001001 View Post
One question: what prevents the fuel pressure from equilibrating on the top of the slider? Is the slider sprung downward? What if the spring breaks? (OK, three questions)
Fuel enters under the diaphragm, spring is above it. Spring failure would not affect engine power at mid to high fuel flows, but it may run poorly at low fuel flows due to imprecise fuel division between cylinders.

Illustration courtesy of Kitplanes magazine.

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Last edited by DanH : 03-30-2016 at 06:02 AM.
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  #8  
Old 11-21-2015, 10:46 AM
Carl Froehlich's Avatar
Carl Froehlich Carl Froehlich is offline
 
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Default

Jordan - The AFP injection system was flawless over the 10 years of operation. After a couple of years of flying, the new owner of the plane was just going the extra mile by sending in the system for inspection and bench testing. Don reported the system tested just fine. Note - the injectors are removed and cleaned at each conditional inspection as are filters (per the AFP maintenance instructions).

On the spider questions:
- If the spring breaks I'd guess you would be hard pressed to get the engine to idle as you would have a very rich condition.
- As the spider is not a closed system, pressure equalizes by fuel flowing out to the injectors. This is why at power the fuel nozzles control the amount of fuel to each cylinder, not the spider.

Carl
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  #9  
Old 11-21-2015, 12:24 PM
sblack sblack is offline
 
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Default

Awesome posts Dan and others. I am so much less ignorant than I was yesterday! I have always wondered what was in there. Thank you.

This is why I am so happy to pay my dues every year. Best deal on the internet.
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  #10  
Old 11-21-2015, 03:19 PM
rhill rhill is offline
 
Join Date: Feb 2006
Location: Valley Forge, Pa
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Default

"From these clues the decision was to ream #4 exhaust valve guide. After removing that valve spring we found the valve to be free moving ? not at all stuck. On further inspection we found the valve, the valve stem and exhaust port to be pristine (engine spends 90% of it time running LOP cruise). We reassembled the engine and on start up found the problem still present."


Hi Carl,
May I ask what oil are and have you been using?
Thanks
RHill
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