What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Exhaust valve stuck? Very rough idle.

tomhanaway

Well Known Member
Lycoming IO-375.
Working through a very rough running No. 1 cylinder at rpm’s below 1000 rpm. EGT cold on 1 until after 1000 rpm when engine runs smoothly.
Engine ran fine for 175 hours. New, consistent issue. Engine hot or cold.

So,
Likely possibilities.
Fuel, spark, exhaust valve, induction leak(?).

Fuel flow test at idle and full throttle setting is equal for all four cylinders. Pulled apart AFP fuel divider anyway and it seems clean.

All new spark plugs (tested in spark plug pressure box), all harness wires ohm tested. Dual P mags sent back to emagair for checkup. Still waiting on those.

No. 1 cylinder is 80/80 on compression test. Borescope shows exhaust valve moving and nice burn pattern on face of valve.
Before I tear apart valves, is it possible to be having a valve issue that would cause rough idle with full compression and moving valves ?

I keep suspecting something in fuel flow at idle but not sure what else to check since flow for all cylinders is identical.

Thanks,
Tom Hanaway
 
For the adventurous

If you are up for it. Two guy job. Cowl off, idle. Spray carb cleaner at the #1 intake gasket (and the intake hose).

Yeah, I've done this, once. Scary.
 
SDS Intake pipe to cylinder head. O-Ring mod works Great.

The intake O-ring mod kit from SDS. No Mo Leaks!
 
Move Sparking Plugs?

Move both spark plugs from #1 to another cylinder, see if the problem follows. Cheap, easy, fast.
 
Intake leaks should cause problems above 1000 RPM also. My first guess would be a blockage in the bottom of the V slot in the spider feeding the #1 cyl. Did you look for debris in the bottom of the V slot's when the spider was apart? problems associated with sticking valves should not be limited to idle. Not a lot in the valve train that would only cause problems below 1000.

Larry
 
Last edited:
Still troubleshooting.
No leaks at any of the intake or exhaust gaskets.
Both intake and exhaust valves are opening/ closing (boroscope).
Wiring on pmags to come back but emagair said they were good. Going to put it all back together when mags arrive. If problem still there, will switch 1and 2 harness to see if problem shifts cylinders.

Really stumped.
 
Is the injector shroud intake clean?

Each injector has a small air hole that draws in some ambient (yes unfiltered) air and this air is supposed to swirl the fuel around as it is being injected into the cylinder so that the fuel flow turns into very small droplets. If the air hole is blocked then the fuel would not atomize very well at low fuel flow/pressure (like at idle). Since the fuel nozzle itself would still be unobstructed the fuel balance test would show normal.
 
Someone asked me status. Here’s where I am so far.
Just got pmags back and putting it all together. Will know tomorrow if fixed.
Spoke to Sal’s cylinder service in Texas.
If it’s not fixed tomorrow, I’m pulling the cylinder and sending it to him.
Could be as little as a bad hydraulic lifter causing a valve to not open/close all the way to worn cam lobe (unlikely, but bad and expensive).
 
Someone asked me status. Here’s where I am so far.
Just got pmags back and putting it all together. Will know tomorrow if fixed.
Spoke to Sal’s cylinder service in Texas.
If it’s not fixed tomorrow, I’m pulling the cylinder and sending it to him.
Could be as little as a bad hydraulic lifter causing a valve to not open/close all the way to worn cam lobe (unlikely, but bad and expensive).

Have you done an injector flow test yet, as someone suggested?
 
>Working through a very rough running No. 1 cylinder at rpm’s below 1000 rpm. EGT cold on 1 until after 1000 rpm when engine runs smoothly.
Engine ran fine for 175 hours. New, consistent issue. Engine hot or cold.

Tom,

Pulling the cylinder is really invasive and that REALLY should be the VERY last resort.

The engine runs smooth above 1000 RPM???

Is the problem the same whether the engine is cold or warm?

Please closely look at the fuel nozzle shroud bleed intake...

Make some clear screen shots of your Savvy EGT charts and share them with me via email (there is also a "share" function on Savvy where you can let someone look at your flight(s).

Ask Savvy to review the charts and recommend advice.

My unsolicited advice is to PLEASE first swap the injector nozzles shrouds from #1 to #3 (you have to remove them to do the cylinder swap anyway) and see if the problem moves to #3. Super simple and not invasive.

Same with the spark plugs.

Remove the rocker covers and use a dial indicator to accurately measure the opening and closing of the valves and compare measurements to #3.

Please do an automotive type compression check on all four cylinders. Remove one spark plug from all cylinders and open the throttle to WOT and then use a car type compression tester. This is more accurate and faster to determine...IF...a problem exists than the airplane differential pressure check at 80 PSI. Warm up the engine and then pop the spark plugs out and do the compression test while the engine is still warm. The differential leak type check will help determine where the air is going when the cylinder is at TDC on the compression stroke.

If the rocker cover is off of cylinder #1 and you can safely watch the valves opening and closing when the engine is cranked over then you can see if any valves are hanging up.

If the compression check shows a bad cylinder #1 then remove the rockers and inspect for bent/cracked parts. Check the pushrod straightness and length and compare to #3.

If the cam lobe had "flattened" then there would be metal in the oil filter and the sump screen. If you find no metal then you don't need to worry about that.
 
...
Please do an automotive type compression check on all four cylinders. Remove one spark plug from all cylinders and open the throttle to WOT and then use a car type compression tester. This is more accurate and faster to determine...IF...a problem exists than the airplane differential pressure check at 80 PSI. Warm up the engine and then pop the spark plugs out and do the compression test while the engine is still warm. The differential leak type check will help determine where the air is going when the cylinder is at TDC on the compression stroke.
...
I've bought and played a bit with the automotive testers and they seem to work pretty well but I have not yet seen any guidance on what pressure we should see, or how we should perform the test. Do you know of any standardized ways of using an automotive style compression tester on a lycoming?
 
Great trend tracker each year. Dragsters do it after each run. First time you are looking for a weak jug versus the others. 130ish on the good, maybe 70ish on an outlier.

Look for changes year to year.
Takes about 6 seconds of cranking per jug to max out the gauge.

A great battery capacity trend checker also. A bit of starter over use, so let it cool and don't abuse it if your battery gets weak.

You'll feel like the guy on Youtube Project Farm!
 
Last edited:
Special thanks to everyone, especially Bill H.
Went to try engine again after reinstalling p mags. Weird, the roughness started shifting from No. 1 cylinder to 2, then 4.
What the heck?
Discussed it with my friend at the airport. He did an “oh, by the way” type question about whether I had fueled up at my local airport. Hmmmm. Actually no, I’d fueled up at a small, unattended strip at the top of a mountain. No one around, etc. plane first acted up onlanding 1/2 hour later.

Pulled samples from both wing tanks. First tank filled from unattended airport, the fuel was extremely dark blue ( as if fuel had evaporated off, leaving only dye). After about 10 ozs. It went to a normal light blue. Also, some dark particulate in fuel (guessing parts of interior of hose).

So, we’re heading down a new path this weekend. Draining tanks, flushing them and fuel lines, changing fuel filter, flushing all the way to injectors, etc.
Will see how it goes after that.
Onset of problem, in connection with topping off with fuel from a sparsely populated airport, certainly leads me in this direction.
 
>Do you know of any standardized ways of using an automotive style compression tester on a lycoming?

-Do this when the engine is warm

-Dont stand near the prop when you are cranking the starter

-Make sure the throttle is WOT, mags off, and one spark plug is out of each cylinder

-Connect up the pressure gauge to one cylinder and make sure the gauge cannot move into the propeller arc.

-Clear prop and hit the starter button and you should count 6 compression events and then let off the starter.

-Record the cylinder numbers and pressures in your supplemental engine logbook

-A 7.3:1 compression engine will show about 125-ish PSI and a 10.5:1 compression engine will show 150-ish PSI. All cylinders should read about 10 percent of each other.

In Toms case if all cylinders show the same PSI then you know the air is flowing in and out correctly and that the cylinders, valves, head casting, piston casting, cam and lifters are all doing their thing correctly and that the rough running is fuel or spark.

Smart money is betting on fuel because with two ignition systems and two spark plugs per cylinder it should run smooth on just one system. It runs smooth at high RPM so it is not mechanical. I am betting on the fuel nozzle shroud is clogged or the the flow divider "V" is clogged...

Could it be sticking exhaust valves??? If it runs rough when it is first started and is cold and then once it warms up the engine runs smooth then it could be...except...if the engine runs rough at low RPM whether it is cold or warm, first flight of the day or after a number of runs... then we are right back to fuel.

Bets are on...Cho-Me-Da-Monay!!!!
 
TA DA.
We have a working engine. It was a batch of bad fuel. We just went down different rabbit holes because of the focus on cylinder 1. It only became clear as other cylinders, that had been running fine, starting to show symptoms.

Also, in an early attempt to fix the idle, I dyslexically turned the idle adjustment the wrong way so idle was way too lean.
Thinking about what changes I made brought me back to idle mixture. Once reset, the engine is happily idling at 750.

Thanks to everyone for their thoughts and help.
 
Back
Top