Menu Sign In Contact FAQ
Banner
Welcome to our forums

Engine stumbling - analysis of engine data

A_and_C wrote:

I think I would be looking for first dirt in the injectors and second water in the fuel, you may have picked up some dirty fuel.

After discussing with the mechanics before doing run-up tests and deciding whether I could continue to Paris, these were the hypothesis that were held as most likely causes.

However, given that a few flight hours later a mechanic decided to change a cylinder due to a “burned valve”, the cylinder changed happened to be #4 which is the one that exhibits a funny EGT pattern, I rule out a clogged injector or water. Furthermore, if there was water contamination, all 4 cylinders would have been affected.

PetitCessnaVoyageur wrote:

Do you know if the cylinder are C-marked, which would allow us to forget the sticking valve ?

I do not know. But judging from other threads, it does not completely resolve the sticking valve problems. I do have a picture of the cylinders somewhere, but not readily available.

Frankly my vote is on the stuck exhaust valve. The EGT graph does not support the burned valve theory. The engine first stumbled because the valve was stuck open and no compression or combustion took place in the cylinder, hence the EGT for #4 dropped, but the valve quickly “recovered”. The second time around the exhaust valve was stuck open for longer. When it finally closed enough for combustion to take place, it did not close enough to entirely contain the combustion gases until the end of the combustion cycle, therefore some gases escaped during combustion, hence the increased EGT

For what it is worth, cylinder #4 is also the richest of the 4 cylinders (the last to peak) although it is the hottest in terms of CHT.

Although the symptoms in my case were less severe (the cylinder only shut down momentarily), my experience bears a resemblance with this post which also provides the steps to cure the problem, well illustrated with pictures of each step. @achimha has previously reported having done something similar here.

LFPT, LFPN

terbang wrote:

Did I get it right, #4 is already replaced now? Anyway, I would do a lean sweep and an inflight mag check. If the problem still persists, this may give some clues.

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.

Last Edited by Aviathor at 20 Oct 13:48
LFPT, LFPN

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

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

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

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

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:

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

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.

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?

Sign in to add your message

Back to Top