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Engine stumbling - analysis of engine data

These “chrome” valve guides are not chrome plated inside, they are manufactured from a high chrome content phosphor bronze.

My guess would be that the muck which finds its way from the exhaust manifold up the side of the valve stem is less likely to stick to a hard and highly polished chrome surface.

Administrator
Shoreham EGKA, United Kingdom

Peter wrote:

The pre-“C” cylinders had the valve sticking problem. The chrome valve guides more or less solved it, c. year 2000.

What is the engineering theory behind chrome valve guides mitigating (to some extent) stuck valves?

United States

There seems to be the assumption here that the influence of dirt in the injection system will be uniform & limier, in my experience they are NOT very often that way, if they can’t get through the injector they will tumble and move around under fuel flow, pressure & vibration and the result is an irraticly variable fuel flow.

This can make troubleshooting very difficult to say the least and I am left wondering if some of these engine monitoring systems are not a bit TFC and send us troubleshooting when all that has happened is the engine has swallowed a bit of ice and coughed as it has run its. Way through the system.

Aviathor wrote:

Unfortunately the cylinder has already been replaced because of a “burned valve”. I have not seen the cylinder, nor spoken to the mechanic. I cannot however imagine the valve was actually burned. At least there is no engine data to support that. If there was a problem with a valve, it must have been a sticking valve, not a burned one.

Can you PM the shop’s name please ?

FAA A&P/IA
LFPN

Yes; plus cylinder removal is not without risk. Too many examples of cylinders coming right off right after being “fitted”, or engines seizing up because somebody turned the prop while the crankcase through bolts were loose. Not every mechanic knows the right procedure, and if somebody moves the prop while he is off having a tea… Or under-torqued bolts (and bearings slip and block the oil passages) or over-torqued bolts (which snap and the cylinder comes right off). The bolt torque has to be done with the right lube on the bolt threads otherwise the torque wrench is seeing the wrong figure when it limits. Etc.

Administrator
Shoreham EGKA, United Kingdom

Aviathor wrote:

If there was a problem with a valve, it must have been a sticking valve,

I sticking valve can normally be solved / repaired with the cylinder still on the engine. Your correct that one a C cylinder sticking valves are far less likely, though not impossible.

JP-Avionics
EHMZ

Peter wrote:

Even if the valve was sealing really badly, the engine would still run perfectly.

Apart from stumbling twice, the engine ran fine. I believe that after the engine stumbled the second time, the valve on cyl 4 did not shut properly, as indicated by the raise in EGT, but I did not notice anything in the way the engine ran at that point. The stumbling only lasted a second or two, but it was bad enough to catch my attention, especially the second time.

LFPT, LFPN

Peter wrote:

Even if the valve was sealing really badly, the engine would still run perfectly.

I had a severely burned valve in a Mini I owned as a student (incidentally I had to park the car on a hill, because I couldn’t afford a new battery, and the battery was utterly shagged. The whole car was shagged, really).

Once you got up to enough revs, there was no power loss evident. You could drive at 50mph+ all day long without a problem, so I drove it two hours to my parents house, since they had a garage. It ran like a bag of shite at low engine speeds though. When I took the head off, there was a small hole (about 3mm diameter) burned in the side of one of the exhaust valves.

So yes, the seal can be absolutely terrible, and the engine will continue to run (certainly at higher power settings when there’s not enough time for much gas to escape through the bad seal).

Andreas IOM

Even if the valve was sealing really badly, the engine would still run perfectly.

If that was not the case, Continental engines would not work They have appalling compressions, bad enough to render a Lyco engine unairworthy on the spot. Presumably this is due to poor QA on the valve seat machining. But it works because the gas can’t escape in any quantity before it is game over and the piston is heading back down.

So I think the burnt valve is a redherring, although it may be a symptom of something more serious, like a sticky valve, which is what I would put money on.

Administrator
Shoreham EGKA, United Kingdom
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