Carl Froehlich
Well Known Member
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
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