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  #1  
Old 02-14-2018, 11:05 AM
Toobuilder's Avatar
Toobuilder Toobuilder is offline
 
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Question One Cylinder Suddenly Runs Cool?

IO-360 (200HP - Bendix) in the RV-8. Engine was misbehaving with a little intermittent misfire on a recent cross country, so I installed new plugs (gapped and Ohm checked) and removed the Pmags for a visual inspection and it is timed correctly. (jumper in, slightly retarded). Did a compression test and all are strong, as usual.

Test flight saw an improvement to its normal, turbine like smoothness.

Last cross country saw #4 cylinder a little cooler than usual - both EGT and CHT. Going to my normal LOP cruise, #4 continued its trend of acting too cool by being the last to peak by a LONG way. My first to peak was 100 LOP before #4 finally peaked. The "GAMI Spread" was probably .8 GPH. LOP ignition stress test was normal and smooth. Resetting all to ROP then LOP shows the issue is consistent and repeatable. Fuel flow is within the bounds of normal.

I have not accomplished a fuel flow test (the bottle method) yet, but that's my next course of action.

If the cylinder was acting lean, it would be easy to suspect a partly clogged injector, but #4 is strongly indicating a rich condition, isolated to this one cylinder. It's possible that the other 3 have junk in them and are all slightly lean, but this seems very unlikely.

Any thoughts before I dive into the bottle test?
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WARNING! Incorrect design and/or fabrication of aircraft and/or components may result in injury or death. Information presented in this post is based on my own experience - Reader has sole responsibility for determining accuracy or suitability for use.

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1940 Taylorcraft BL-65
1984 L39C

Last edited by Toobuilder : 02-14-2018 at 11:10 AM.
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  #2  
Old 02-14-2018, 12:35 PM
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Carl Froehlich Carl Froehlich is offline
 
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Taking your data at face value, and assuming all well with the ignition, then:
- #4 has too much fuel, or
- #4 has too little air

A few thoughts:
- Verify this is not an igntion issue by swapping the leads on each pMag for #3 and #4. The pMag is a wasted spark so #3 and #4 alway fire at the same time.
- Make sure you did not suck in a rag or something into the #4 intack tube.
- Do the fuel flow test (bottles) a few times to make sure you have repeatable results. Also clean the fuel injector screen just in case. If your test show #4 is in fact getting too much fuel, I?d suspect the spider.

Carl
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  #3  
Old 02-14-2018, 08:35 PM
brad walton brad walton is offline
 
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I had an exhaust valve that was sticking intermittently that exhibited these symptoms. Usually compression would close the valve. But occasionally it would fail to fully close.
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  #4  
Old 02-15-2018, 01:30 AM
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ColoRv ColoRv is offline
 
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Mine acted much like that once. I have an EI Commander that quickly pointed out a Pmag issue. Timing was going wonky. Brad fixed it. Could be worth a look.
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  #5  
Old 02-15-2018, 09:57 AM
Lars Lars is offline
 
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Assuming nothing else checks out, reduced valve lift can cause that condition.

Less lift = less airflow. Injector still squirts same amount of fuel as always (perhaps reduced slightly because the sum total airflow of all airflow through the servo is now reduced, but not enough to matter here). Thus relatively rich compared to the other cylinders. Also means that, because the cylinder isn't making as much power as the others, it's running relatively cool, and because it's getting relatively more fuel for the airflow, it'll peak later than the others.

I had that happen, in my case it was a partially collapsed intake roller lifter on a first generation Superior roller cam equipped engine that caused the problem. Getting a final diagnosis was an interesting educational journey. A flat cam lobe could cause the same thing.
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Last edited by Lars : 02-15-2018 at 10:00 AM.
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  #6  
Old 02-15-2018, 11:33 AM
lr172 lr172 is offline
 
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Don't know much about the Pmag, but assume they use a wasted spark style coil. If that is the case, problems often arrive in pairs and any timing issues would be seen across at least two cylinder. The second set of symptoms stated also don't present like ignition.

For the suddenly rich condition, I would be looking at the spider (seems unlikely to over flow rather than under flow an individual cylinder, but stranger things have happened) and any cause for reduced air flow on the intake, such as an intake blockage on #4, collapsed lifter, sticking intake valve (can sometimes limit valve travel instead of sticking open), bent pushrod from sticking valve, etc. A worn cam lobe seems unlikely given the rapid onset.

A flow test may help you narrow down the two optoins - fuel flow or air flow. Your problem could be either. However, you need to do the flow test at the pressure seen at cruise power settings.

Larry
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Last edited by lr172 : 02-15-2018 at 11:43 AM.
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  #7  
Old 02-15-2018, 01:33 PM
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Toobuilder Toobuilder is offline
 
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thanks everyone. I'm liking the low valve lift angle. Had not thought of that. The overall cruise performance (speed) seems unaffected however, so a loss in airflow would have to be very slight.
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WARNING! Incorrect design and/or fabrication of aircraft and/or components may result in injury or death. Information presented in this post is based on my own experience - Reader has sole responsibility for determining accuracy or suitability for use.

Michael Robinson
______________
Harmon Rocket II -SDS EFI
RV-8 - SDS CPI
1940 Taylorcraft BL-65
1984 L39C
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  #8  
Old 02-15-2018, 11:46 PM
lr172 lr172 is offline
 
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Also check the plunger assembly. If you get any debris in there, it can block the check ball from seating and the lifter can't stay extended. This would reduce the valves lift. This would be consistent with the sudden onset as well. With the engine warm, pull the rocker cover and try to compress the lifter via the rocker. It should be firm. If you can compress it, you need to check that lifter
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Last edited by lr172 : 02-15-2018 at 11:50 PM.
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  #9  
Old 02-16-2018, 10:03 AM
EXflyer EXflyer is offline
 
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Quote:
Originally Posted by Toobuilder View Post
thanks everyone. I'm liking the low valve lift angle. Had not thought of that. The overall cruise performance (speed) seems unaffected however, so a loss in airflow would have to be very slight.
Why not swap injectors as its showing too rich checking valve lift takes more time to do. Also it would not change anything over time while flying as its a constant not a variable.

Just a thought.
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