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O-360 Backfiring and Low CHT

Ironflight

Ironflight

VAF Moderator / Line Boy
Mentor
OK, here's a puzzler for the peanut gallery....lookign for ideas!

Louise aborted an evening flight today after noting considerable backfiring at low RPM's, accompanied by both a very low CHT AND EGT on #2 Jug. I ran it and confirmed the situation. AS I ran it up, the temps came up, but stayed power than the other cylinders. Mag check was OK - a bit rough, but no significant differential drop. I believe that I have noted a slight increase in backfiring as Louise taxies in from her daily commute the past couple of weeks, so the problem may have been progressive.

We pulled the cowl - compression check was good, no difference between it and the other cylinders - 72 or 74 over 80, not terribly hot. Upper and lower plugs were very clean. Mag timing is perfect. Checked all the plug wires and Magneto caps - there were fine. Pulled the rocker box cover, and nothing is amiss under there. No apparent leaks (exhaust, oil, or other) from the jug. About the only thing I haven't done is replace the intake coupling tube/hose - might be an intake leak, and the hoses are all pretty old. I tightened up the hose clamps, but don't have any spares to test by changing them out.

Before I try and find some intake couplings, anyone else have any ideas?

Lycoming O-360-A1A, stock carb, mags, nothing unique.

Paul
 
I'd suspect the intake tubes lacking any other obvious issue. A leak would screw up the mixture on that jug and maybe lead to the problem.

Steal one off your other plane and check it out.

Some Lycoming expert may have a better idea....
 
Intake leak

I'm voting for an intake leak, one cylinder only and the symptoms indicate lean.

John Clark ATP, CFI
FAA FAAST Team Member
EAA Flight Advisor
RV8 N18U "Sunshine"
KSBA
 
Could be a sticky valve my friend. They stick, they unstick, then stick again. Not sayin' that's it for sure, but if you don't find something else, maybe try a rope trick. Unspring the valves & operate the valves by hand. If they are leaded, you'll be able to tell. If they were bent or burnt, you wouldn't get compression.
 
I vote for the intake leak. I saw the same thing, only difference, the leak was not on the odd ball cylinder, it was on another one.
 
An intake leak...

...or valve sticking...

From Sacramento Sky ranch...

14. Sticky valves. Occurs most frequently in the hot summer months. Frequently occurs on the first start of the day when the engine is cold. Remove rocker gasket. Turn engine backwards until valve is fully closed. Using both thumbs, depress valve springs. Valve should open and close without any feel of dragging. Valve movement should be smooth.

Time to perform the SB 338 valve wobble test...:)

http://www.lycoming.textron.com/support/publications/service-bulletins/pdfs/SB388C.pdf

Has it been hot in TX recently...:D
 
Paul, I agree that most backfiring at LOW rpm's are the result of sticking valves. Intake leaks usually are recognized by higher CHT's at higher rpm's.

Vic
 
Yes and no...

David-aviator said:
SI-1485 might help in determining if the wobble test is necessary.
http://www.lycoming.textron.com/support/publications/service-instructions/pdfs/SI1485A.pdf

FWIW, my nearly new engine backfires on occasion at idle power in the traffic pattern. I've about concluded it is normal, but who knows? Engine seems to run fine otherwise.

dd
Click to expand...

...my Tiger O-360 engine needed a TOH at 600 hr - on Lycoming rebuild with new cylinders with the hi-chrome guides - so the 1000 hr first check recommendation is not very foolproof.

If you have the correct Lycoming tool, it is pretty non-invasive if you check the guides only and don't clean the lifters. The valve springs do not have to be removed with the correct tool - but if you have the AC Spruce tool, then the valve springs do need to be removed...
 
Great ideas folks - I hadn't thought about sticky valves (even though those used to be discussed all the time in the Grumman world, as Gil well knows) - I'll look at that this morning.

Paul
 
Normal

David-aviator said:
FWIW, my nearly new engine backfires on occasion at idle power in the traffic pattern. I've about concluded it is normal, but who knows? Engine seems to run fine otherwise.

dd
Click to expand...

Not to worry Dave - this is a very well known (and normal) behavior for Lycomings on RV's with un-muffled exhausts.....numerous explanations having to do with unburned fuel in the exhaust. If you don't like it, then you get to enjoy the game of smooth power reductions and carrying just a little power all the way around.

You know there is an RV in the tight pattern when you hear the popping!

Paul
 
Exhaust or intake? If it is in the intake, it is very likely a sticking valve. If in the exhaust, it could be a lean cylinder (intake leak) that is not properly firing, allowing unburned fuel into the exhaust, the same way FI engines do at high vacuum settings.
 
Paul, I agree that most backfiring at LOW rpm's are the result of sticking valves. Intake leaks usually are recognized by higher CHT's at higher rpm's.

Vic
 
Check your valve lash. The Arrow at my flight school recently started acting up and we found that the stem keepers were shot and it had similar symptoms.
 
Induction leaks will cause the engine to idle at a higher RPM depending on the size of the leak, and as RPM comes up any misfiring will usually go away. A sticky exhaust valve will cause a rough running engine regardless of RPM until the offending cylinder warms up. Intake valves very rarely stick because they simply are not exposed to hot exhaust gasses. I would turn the engine over by hand when cold to be sure you have compression on all four cylinders and if that checks out ok listen for a leaky valve. Have someone turn the prop while you have your ear on the exhaust pipe.
 
OK folks, lots of good ideas. AS I said up front, compression was good on all cylinders (both by the gauge and the good old "comparative armstrong method"), and all else seemed fine. Ran it again this morning, and it did the same thing. I called an IA buddy who has a hangar full of tools, and we decided to check the valves - sure enough, the intake valve felt like it has something gumming it up. The exhaust valve is like butter. We started in to the "Indian Rope trick", and got the valve out, the guide reamed, and the stem cleaned. At that point, we talked about how to close it up, and he had to leave.

So.....several hours later, we are still trying to get the dang valve back in to it's guide! We've quit for the night, or we'd be at risk of burning down the hangar in frustration. At this point, we'll take any and all tricks that folks use to get the valve back in to the guide. We've been working with "grabber fingers", hooks made out of coat hangers, dental floss, etc. Close, but no cigar at this point.

Paul
 
Just saw a demo of...

Ironflight said:
.....
So.....several hours later, we are still trying to get the dang valve back in to it's guide! We've quit for the night, or we'd be at risk of burning down the hangar in frustration. At this point, we'll take any and all tricks that folks use to get the valve back in to the guide. We've been working with "grabber fingers", hooks made out of coat hangers, dental floss, etc. Close, but no cigar at this point.

Paul
Click to expand...

...of this at Dayton a few weeks ago.

Bill Scott used one of these IIRC (not HF - but similar)

http://www.harborfreight.com/24-inch-2-in-1-pickup-tool-94162.html

It can be done....:)

I do remember him saying that you definitely should not use the piston in any way to help the process. He used the grabber through one port and a strong light in the other. Once it lined up, it just slid into place...:cool:

Keep on trying...
 
az_gila said:
...of this at Dayton a few weeks ago.

Bill Scott used one of these IIRC (not HF - but similar)

http://www.harborfreight.com/24-inch-2-in-1-pickup-tool-94162.html

It can be done....:)

I do remember him saying that you definitely should not use the piston in any way to help the process. He used the grabber through one port and a strong light in the other. Once it lined up, it just slid into place...:cool:

Keep on trying...
Click to expand...

That's almost the same Grabber I've used, mine has 4 tongs on it which make it a little easier to use.

The trick is.... lots of profanity and you gotta hold your tounge just right :D
 
Yup - bought a similar one at Lowe's today - and th one we got has an LED in the end of it, which is really handy in the cylinder when you are trying to grab the valve. The problem I have found with the four-finger one so far is that when you grab the cylindrical valve stem, the "opposing fingers" are grabbing diagonally across the shaft, and the grip isn't all that secure. If I look closely at the Lycoming SI-1425, it almost looks like they are using a two-fingered tool. I might have to go to the various and sundry local tool outlets and put together a collection of variations......

I'm holding off on the profanity until Louise starts - that'll give me tacit permission to contribute!
 
Fixed!

OK, so sometimes, it's good to leave the workshop late at night, and give it a go again in the morning. I did a little re-reading of SI-1425, and realized that they had taken of the exhaust system and/or intake tubes. Looking at a dead cylinder I had in the shop (OK, I turned it in to a lamp!), I realized that this would give us a much better chance at lining up the valve. The bonus is that there is no easier cylinder than #2 to remove the intake tube from- took about 2 minutes.

With that removed, Louise and I set about to reinstall the valve. Maybe going to church first was the right thing to do, because it only took us about an hour to get it back in the guide using a skinny magnet and a set of forceps through the intake port. Forget the dental floss and thread - it just got in the way. The pick-up fingers were useful for getting the valve roughly in position (through the top spark plug hole), but didn't help much after that.

Once we had it back in the guide, the rest was easy! First, I checked to see if the stickiness had gone away, and yup - smooth as silk. We had to rummage up an intake gasket, but once that was done, Louise did a run-up, and Mikey was a smooth dude once again. A test flight confirmed the fix. The intake hoses are still pretty crispy, and we've ordered a set to make sure we have a good seal on all four cylinders - they are pretty cheap, as engine parts go.

All-in-all, a good learning experience for both myself an Louise - I had read about the "rope trick" many times, but had never done it. Tedious, but it beats, pulling the baffles and cylinder.

Thanks for the discussion folks - Mikey rides again!

Paul
 
Well done Paul, I assume you have a Dynon or similar all cylinder monitor.

Its amazing how much you can learn and how much damage can be prevented by early diagnosis.

Years ago people just kept flying and destroying things.

DB:cool:
 
Can't hurt

Some swear by it, others laugh at the idea, but for me it worked and that is using Marvel Mystery oil in the gas. A couple of years ago on the first flight of the day after leveling out at cruise altitude of 2-3000, we live in flat Fl., the engine, 180 HP carb, would stumble a couple of times. Only did it on first flight of day and I suspected a valve starting to stick.
Well, I started using Marvel Mystery oil at the recommend amount added to the gas and have never had the stumble since ???-?
 
Ollie said:
Some swear by it, others laugh at the idea, but for me it worked and that is using Marvel Mystery oil in the gas.
Click to expand...

I'm not laughing, I'm also using MMO in the gas. Awhile back I had a stuck exhaust valve... not just sticky, but really stuck. Took it to a local engine shop and they got it loose; apparently one of the valve guides has juuust enough play in it to allow gunk to build on the valve stem. Oops, stuck valve. The engine shop guys recommended MMO in the fuel as both a cleaning solvent and lube to keep the stems clean, so I run the recommended amount through roughly every other top-off. Perhaps it's just a wives tale, but no more valves sticking... yet.

After this episode I really quite like having a 4 cylinder engine monitor; when the engine started running rough I was able to pinpoint the problem cylinder right away. Sure cuts down on troubleshooting time...
 
MMO

Ollie said:
Some swear by it, others laugh at the idea, but for me it worked and that is using Marvel Mystery oil in the gas. A couple of years ago on the first flight of the day after leveling out at cruise altitude of 2-3000, we live in flat Fl., the engine, 180 HP carb, would stumble a couple of times. Only did it on first flight of day and I suspected a valve starting to stick.
Well, I started using Marvel Mystery oil at the recommend amount added to the gas and have never had the stumble since ???-?
Click to expand...

This discussion comes up every few months. I found this amusing during the last go-around:

http://www.ntsb.gov/ntsb/brief2.asp?ev_id=20020916X01610&ntsbno=NYC02LA181&akey=1

John Clark ATP, CFI
FAA FAAST Team Member
EAA Flight Advisor
RV8 N18U "Sunshine"
KSBA
 
Who would have though...

John Clark said:
This discussion comes up every few months. I found this amusing during the last go-around:

http://www.ntsb.gov/ntsb/brief2.asp?ev_id=20020916X01610&ntsbno=NYC02LA181&akey=1

John Clark ATP, CFI
FAA FAAST Team Member
EAA Flight Advisor
RV8 N18U "Sunshine"
KSBA
Click to expand...

...MMO had lard in it....:D

"...The contents of Marvel Mystery Oil were 74 percent mineral oil, 25 percent stoddard solvent, and 1 percent lard..."

I guess lard doesn't have to be listed on a MSDS - heck, it's even organic...:)

http://www.turtlewax.com/res/msds/MM010-5.pdf
 
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