What's new
Van's Air Force

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

Lose 2 Cylinders at Idle

Jesse

Jesse

Well Known Member
I have a plane here with a Lycoming O-360 that has been down for a panel upgrade. When we start the engine, Cylinders 1 & 4 drop out at Idle. At power (pretty much anything over 1,000RPM) the EGT's and CHT's come up, but at idle, they drop back out. It flew fine and made good power. The engine has 2-300 hours.

We checked for stuck valves and they are fine, not stuck or even a little sticky.

We can test new plugs, but the chance of having both plugs on two different cylinders dropping out the same is very low.

We have looked for intake leaks, and haven't completely ruled that out, but everything looks great on those cylinders.

Leaning out at idle doesn't change anything, but the RPM does increase significantly.

Any other thoughts, things to try?
 
Last edited:
My engine did the same on one cylinder. The flow divider had piece of junk in it, causing the idle fuel flow to be restricted. At higher fuel flow/RPM, the plunger inside raises the internal valve, allowing normal fuel flow.
 
Jesse said:
I have a plane here with a Lycoming O-360 that has been down for a panel upgrade. When we start the engine, Cylinders 1 & 4 drop out at Idle. At power (pretty much anything over 1,000RPM) the EGT's and CHT's come up, but at idle, they drop back out. It flew fine and made good power. The engine has 2-300 hours.

We checked for stuck valves and they are fine, not stuck or even a little sticky.

We can test new plugs, but the chance of having both plugs on two different cylinders dropping out the same is very low.

We have looked for intake leaks, and haven't completely ruled that out, but everything looks great on those cylinders.

Leaning out at idle doesn't change anything, but the RPM does increase significantly.

Any other thoughts, things to try?
Click to expand...
Missing intake port pipe plugs, leaking primer, leaking stand by vac system, really has intake leak written all over it :)
Good Luck,
Mahlon
 
Raymo said:
My engine did the same on one cylinder. The flow divider had piece of junk in it, causing the idle fuel flow to be restricted. At higher fuel flow/RPM, the plunger inside raises the internal valve, allowing normal fuel flow.
Click to expand...

Not fuel injected, so I think we can rule that out.
 
mahlon_r said:
Missing intake port pipe plugs, leaking primer, leaking stand by vac system, really has intake leak written all over it :)
Good Luck,
Mahlon
Click to expand...

That's exactly what I have been thinking. We'll check all of those things. Thanks!
 
Basics

Jesse,
I am by no means a engine guy....
But basics
Air fuel spark.
Air would just be intake spider....and gaskets. Leaks cause poor running engine, especially when idling because the ratio of leak to combustion air is high. if all looks good, most likely not your smoking gun
Fuel...carb is carb...and if spider is ok, then fuel should be ok as well.
Ignition: assuming mags and aviation plugs, you did not say age/condition etc. Two mags wired normally would have a mix of wires from both mags so both mags failing is improbable. Plugs could be bad on those two cylinders and an easy thing to check/clean/test/ohm.
Primer---is the primer only on 1 and 4?
Good Luck
 
Leak Down test?

I am a firm believer in them, much better than compression. I spent a couple years on a top fuel motorcycle race team, and one of the first things we did after every run was a leak down test. We would know instantly how much work we would be needing to do to be ready for the next round and where the issue was if it was mechanical.
 
I had an O-360 lose one of the pipe plugs in a primer port on the way to OSH about 15 years ago. Was running fine until I pulled power to descend for a fuel stop, then ran horribly. Bringing power back up, ran fine.

Now, losing two, at the same time.....
 
We found the problem we are pretty sure. Just about to run a test.

This is what happened. The primer (electric) was sticking and staying on. The primer goes to cylinders 1, 2 and 4. Cylinder 3 has the manifold pressure pickup in the primer port. It turns out that cylinder 2 had a leaky primer fitting, so just cylinders 1 and 4 were getting full time primer, which must have just made them run rich at high power, but so rich at idle that they stopped firing at all (or were firing but the EGT's were just hugely low because they were hugely rich. Kind of the opposite problem as we were thinking, but similar symptoms. We fixed the sticky primer and fixed the leaky primer line into cylinder 2 and are about to test.

Thanks for the ideas, guys.
 
Jesse - you have a lot of experience, yet you still put things out there when you find issues. That really helps all of us learn. Thanks for posting.
 
You might consider doing away with the primer system altogether.

a couple shots with the throttle pump before starting and the carbed O-360 is good.

I have never had a primer on my O-360 Lyc.

JMHO
 
caryr said:
You might consider doing away with the primer system altogether.

a couple shots with the throttle pump before starting and the carbed O-360 is good.

I have never had a primer on my O-360 Lyc.

JMHO
Click to expand...

This is generally considered a bad thing to do because a back-fire can light the fuel in the carb and any fuel that has leaked out will start a fire.

If you must, do it only while the engine is turning over to keep the fuel moving into the cylinders. If a fire does start, keep cranking until it goes out or someone gets to you with a fire extinguisher.
 
Raymo said:
This is generally considered a bad thing to do because a back-fire can light the fuel in the carb and any fuel that has leaked out will start a fire.

If you must, do it only while the engine is turning over to keep the fuel moving into the cylinders. If a fire does start, keep cranking until it goes out or someone gets to you with a fire extinguisher.
Click to expand...

I have been doing this for years - I push while I crank the engine - once in summer and twice in winter. I don't over prime it. I feel that removing the priming system removes a lot of failure modes, weight, complexity etc. The fire risk is real if you flood the engine. I know if incidents where that happened resulting in a lot of fire damage. So you have to be sure not to flood it. 1 squirt while cranking won't flood it in my experience.

Everybody has their own ideas on how to manage risk and they are based on assumptions and personal prejudices. I don't expect everyone to agree and we all have to do what we are comfortable with. I love taking stuff OFF the airplane whenever I can. That's just the way I think.
 
You must log in or register to reply here.
Back
Top