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Engine riddle for you..... Two dead cyl on left Side?

I have a Lycoming O-320 on my RV 4 that has decided to run on only two cylinders. I would have thought that virtually impossible to happen. Especially both being cold/dead on the (same) left side. It?s a carbureted/aspirated, mid-time engine. Shut off with mags a couple weeks ago to check ground. Exhaust is two pipes w/crossover. No recent maint done/changes before last flight. Fuel sumped.

Flew it in just fine, never skipped a beat, left it outside in average weather for a few days, went to start it up one morning and shook like after normal easy start. Knew I was in uncharted territory by the sound it was making. Tried running it up to 1000-1200 RPM thinking a fowled plug, leaned it, cycled mags with 30-40 drop each side. Shut it down after 1-2 min thinking She never gives me any trouble.

Took it over to maint shop, removed the cowl, started up again with same results and tests for the A&P. No backfiring.
Both left cylinders dead cold, hot on R side. Trouble shooting included the obvious:


Good visual insp, nothing noted

Removed, cleaned, gapped, tested plugs. One failed at 120 PSI. All were normal color, no excessive deposits or wear. Replaced. Nothing out of ordinary noted looking into the cylinders.

Again, started up, tried various mixture settings, etc? Same results

Compressions checked, mid 70s all around.

Valve covers removed, pulled prop through all cyl, valves moved appropriately. So nothing sticking open/closed.

Tested each plug wire, visible spark noted from each one while grounding it out.

Mags checked/opened up. One shaft/bushing with some sloppiness. Removed and sent out for a 500hr insp/check. No visible arc/carbon pattern noted that would maybe indicate a cross firing.

Didn?t pull the air filter (round K&M with wire mesh inside/outside element) So no rats could get in and plug two left intakes (reaching here I know}

Sooooo, I?m open to any suggestions from all the experts out there. I?m stumped.
Feel free to throw out some less obvious trouble shooting suggestions. Just not making any sense to me
Will compile and start over when mags get back in a couple weeks. Personally, I don?t think it?s the mags??.

Thanx all.

Dave
[email protected]
 
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This is a bit of a reach, but do you have primer lines to the left cylinders only? A stuck primer solenoid causing them to run way over rich at idle?

Paige
 
You might want to start looking at intake pipes, there might be cracks a long the flange where they meet the cylinder, also check for blockage in the intake system or in the exhaust, the primer lines being broken at the cylinder would cause a rough running engine but not that excessive, but you never know.
Also as a last 2 checks before cylinder removal inspect the cylinder barrel to cylinder head joint I have seen that develop cracks in the past, while it is rare it does happen, and since the piston is usually past the crack during compression check you would not pick it up during one. If all is good from there I would boroscope the cylinder and see what's going on if you have a good one, but personally I would just remove the jugs at that point to see what's going on with my own eyeball.

Hope this helps

Mitch, A+P IA

If you have any questions feel free to email me directly,

[email protected]
 
good puzzle...

Dave, a very professional problem description. I love a good puzzle. But at this point I have no clue. Can you swap plug leads right to left?
When you pull the prop through slowly by hand, are the appropriate hissing noises where they should be? Do all four cyls feel the same.
Will ponder as I wander...
Roger
 
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After running and shut down, did you check and see if the lower plugs were wet? With compression, and valve movement, then it is either spark, or spark at the wrong time, and fuel.

If the plugs are wet, then narrow to spark. Since you checked for spark, then I would say spark at the right time and place. This would say check the mag caps to see if it is carbon tracked and you just happened to catch it with some absorbed moisture in the insulator that caused dissipation of the ignition energy.

When I have checked everything and there is nothing wrong, but it still does not work, then it is time to look closer.

If the plugs are not wet, then suspect some plugged orifice or tube in the carb that might send fuel to only one side. I don't know this carb that well, maybe RocketBob can help there.

Good puzzle there - actually sounds like a good reward and interesting find when you get it solved. Let us know what you find!
 
Since nothing's mechanically broken it will run given fuel, air, and spark.

"Left outside a few days..."? I'd be looking for air blockage due to a critter invasion.

John Siebold
 
Interesting problem. You know that what you need is fuel, air, and spark, at the right place and right time. You have verified that there is spark and can assume that the timing is OK because two of your cylinders work. You know the fuel is flowing because it is carbed and two cylinders work. So I would start with the assumption at this point that it is either an air problem, or a primer overfueling problem. For the air problem, I would start with looking at the exhaust side then move to the intake. Also, if you start looking at the exhaust and it is dry (no raw fuel in it) then it is not an overfueling problem. If it's not that, well, it wouldn't be the first time I was wrong...:)

Tim
 
I always try and look for the simplest solution to any problem - multiple failures and complicated scenarios are fun to contemplate, but rarely the right right answer. AS the saying goes, when you hear hoofbeats, you probably don't need to be expecting to see zebras. In this case - I am stumped. You might see if your mechanic has a spare magneto to stick in there just to rule out an issue. But it is hard to understand why you'd lose just the left side cylinders, unless you're plug wires are really going to the wrong places....

A couple of weeks ago, Louise reported a rough running engine and she headed straight home from the practice area. Cylinder #1 was showing cold EGT's and CHT's. Mag check showed the same performance on either or both mags. I was in another plane, talking to her on the radio, and I was really expecting that this higher-time engine had sucked a valve or something. Fortunately, she had no trouble getting home, and also fortunately, I happen to be evaluating a new borescope from ATS (a serious, kick-butt borescope...), so we pulled one plug and snaked the camera in there.

I was much relieved to see a normal cylinder, piston, and valves! But what surprised me most was that both plugs - platinum, fine-wires - were fouled with a little bridge of what looked like lead. It was just between the center conductor and the fine wire - everything else was clean. we pulled all the plugs, and everything else was clean - so whatever happened affected both plugs on the same cylinder.

I wouldn't have given very high odds on the probability of that happening....but there you go.
 
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Back to primer lines. I know from personal experience that if the line has come off the engine, that cylinder will suck so much air that it's too lean to fire. But two cylinders at once?
 
Back to primer lines. I know from personal experience that if the line has come off the engine, that cylinder will suck so much air that it's too lean to fire. But two cylinders at once?[/QUOTE]

BUT...those 2 primer lines probably go to a common "T". A break beyond the "T" would affect both cylinders.
 
RE: Two dead cylinders.

I like the idea of a broken primer line to that side of the engine.
Also, almost---grounding out the ignition leads one at a time and pulling the prop through to check for a spark is not an adequate test of the ignition leads.
They must be checked with a High Tension Ignition Lead Tester.
Just because it will "spark" when held close to ground; does not mean it will fire when under compression load across the spark plug gap--when the engine is running or attempting to start.
 
Chugg a Lugg

Mags are my best guess but you mentioned that mags were checked out & ultimately had the 500hr inspection performed. Was this the 500hr breeze over or an actual 500hr overhaul? (I've had both done to me in the past).

Coincendentally the Australian Govt released a Mag Airworthiness Bulletin a couple days ago: http://www.casa.gov.au/wcmswr/_assets/main/airworth/awb/74/005.pdf
Everybody should download this document, excellent at explaining all the various possible mag defects.
 
Thanx all !!

Great suggestions, and learning some good stuff here.

Unfortunately, will have to wait another week or two for mag rebuilds to arrive back. Dont think any wires could be switched. a lot of work and wires are short, only go to respective cyl. Plugs were cleaned, tested. Will look at using tester if avail. Thought maybe the left bundle could be shorting out passing thru baffles or something, but spark present and appears close in timimg during hand propping.

Primer line to each cyl on left, one on right. Will look harder at this, isolate primer pump. Will kill the engine if I hit it while running. Maybe stuck "On". Very quiet, so cant hear it. Come to think of it there was fuel on ground, which is unusual for this plane. Fuel running out of bottom of cowl, not exhaust.

Be tough for a critter to get up there past closed carb heat flap, thru screens on filter, block intakes. Will rule it out though.

Can a carb malfunction in some way as to only allow fuel/air mix into R cyl intakes? Dont have full understanding of airplane carbs, but just seems unlikely?? Will look closer at intake system, connections.

Well, once again, thanx for suggestions. I'll compile, print out for A/P, work down the list. Will let you know who the winner is in a cpl weeks.

Dave
 
Great suggestions, and learning some good stuff here.

Unfortunately, will have to wait another week or two for mag rebuilds to arrive back. Dont think any wires could be switched. a lot of work and wires are short, only go to respective cyl. Plugs were cleaned, tested. Will look at using tester if avail. Thought maybe the left bundle could be shorting out passing thru baffles or something, but spark present and appears close in timimg during hand propping.

Primer line to each cyl on left, one on right. Will look harder at this, isolate primer pump. Will kill the engine if I hit it while running. Maybe stuck "On". Very quiet, so cant hear it. Come to think of it there was fuel on ground, which is unusual for this plane. Fuel running out of bottom of cowl, not exhaust.

Be tough for a critter to get up there past closed carb heat flap, thru screens on filter, block intakes. Will rule it out though.

Can a carb malfunction in some way as to only allow fuel/air mix into R cyl intakes? Dont have full understanding of airplane carbs, but just seems unlikely?? Will look closer at intake system, connections.

Well, once again, thanx for suggestions. I'll compile, print out for A/P, work down the list. Will let you know who the winner is in a cpl weeks.

Dave

Dave, there's no way a carb can feed fuel to 2 cylinders and not the other 2. It has a single throat going to the intake manifold where flow is split to the 4 intake pipes. If 2 of the pipes had a mouse nest or serious leak that would screw it up but the carb itself is probably ok.

Since first reading your post I've felt the problem originates with the mags and/or the wires to the plugs. I think you are on the right track pursuing the ignition system.
 
have you removed and visually inspected the hoses that connect the intake
tubes were join to the sump? I am not an aircraft mechanic, but I am an automobile technician. In my experience, mice or rats, will sometimes nibble at rubber hoses but not necessarily totally chew them up. I think a rodent could have chewed a hole in both of those hoses. if the holes were toward each other or on the tops you may not be able to see the damage without removal.
 
fuel on the ground? from cowl?

This is a new clue. Definitely should be investigated for source in any case for safety, but sounding more and more and more like primer lines are the problem.

Ignition just seems very unlikely to me in this case - four plug wires, from two mags, suddenly go bad, while the other four wires, from the same two mags, are OK? Not likely. Even if it is a baffle-chafing problem, that would presumably get only two wires -- the two from the other mag probably go through a different hole.

My money is on the primer plumbing.
 
Yep 2 mags half failing at the same time doesn't make sense. It is more likely induction related. But.....really curious now.
 
Hmmmm

Ignition....not likely based on all above.
Small squirrels in the two left intakes :rolleyes: or primer
 
My guess is fuel fouled spark plugs on those two cylinders. When trying to start an engine that is flooded, the excessive un-atomized fuel is sometimes forced between the center electrode and ceramic insulator by compression. The fuel that is forced between the center electrode and insulator will effectively create a ground path.

A fuel fouled plug will usually fire just fine when checked outside of the cylinder since the plug is not not being subjected to the high resistance of trying to bridge a high pressure air gap. In other words it will fire in atmospheric pressure, but not under cylinder compression pressure since the fuel between the electrode and insulator provides a path to ground that has less resistance than the air gap.

Try installing new plugs in those cylinders, or swap the plugs from the two cylinders that were firing and see if the problem follows.
 
Ignition....not likely based on all above.
Small squirrels in the two left intakes :rolleyes: or primer

If they got in the intakes, wouldn't they have to chew thru the air cleaner? Are the exhausts interconnected for each side, from sitting outside I'd be inclined to check the exhaust for nests.
 
This is interesting and I don't have any sure answer, but could the two bad jugs be firing but very poorly? Before doing any major work, how about disabling the two good pots and trying a start on the poor two. That will tell you if any fuel is getting through and may help in the diagnosis. It certainly seems that the ignition is OK.
 
I'm with the others, this doesn't sound like an ignition issue. Two mags, four wires, or four plugs just don't go bad all at the same time.

That leaves the primer, induction, and exhaust.

The fuel puddle is a new clue. Remove and inspect everthing, starting with the primer.
 
I had a similar problem years ago on an O-320. Out of the blue, I had two of the hydraulic lifters/ tappets on the same side fail due to the internal check valves getting stuck open due to blockage, preventing the valves from opening all the way. I think I had one intake and one exhaust, could have been both intake. The symptoms were two rough running / dead cylinders on the RH side. Since the both lifters were not "pumping up", the valves didn't open properly, thus it passed a compression test easily. I did everything you did until I pulled a valve cover and found the two valves were barely opening. I removed the push rod tubes / rods and attempted to remove any debris from inside the lifter by squirting solvent into the center hole and sucking the debris out with a vacuum. I then re-installed everything except the valve covers and rotated the engine over until oil pressure was indicated. I measured the valve action at the rocker arm and they worked OK after that. Whatever I did, it worked and the engine ran OK afterwards. Your issue may not be related to this at all, but it might be another idea to help you in your troubleshooting.
 
****UPDATE****

Well, winner winner, chicken dinner!

It was a primer pump issue. Had a failure in the primer pump solenoid that was allowing max fuel to be pumped into the L side cylinders, thereby "drowning" them out/over rich. Totally makes sense now.

Sure appreciate everybody's input, definitely helped me sort through things. Start simple and work from there..... learned a lot!

Thanx again!

Dave
 
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Thumbs_21.gif
 
Thanks for the info

I was looking into the primer issue as I am now into the QB fuse and doing my fuel system.

This reinforces my opinion that I think I will use the manual piston primer and keep it simple.

When I phoned Vans they asked why I needed one, of course they rarely see -20 C flying days like we do here in Canada. Nothing like a crisp cold day of flying over a white landscape eh?

Thanks again.:)
 
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