placido wrote:
But if it was a plug and it was moved shouldn‘t the EGT problem move with the bad plug?
Now I am confused.
You are depicting TWO problems.
My latest comments were related to the former. An EGT 300°F higher on one cylinder cannot be caused by a plug that does not fire. I think you should treat the two issues separately.
It would be really helpful to see the engine data on Savvy Analysis.
Want to be systematic? Let’s enumerate the likely causes and start eliminating.
Rough engine when running on a single mag – Probable causes
With an engine monitor it should be easy to identify the affected cylinder by looking for a falling EGT. That information, together with the mag on which roughness and falling EGT occurs will identify the plug which is not firing. Then you can change the plug, test the lead and finally inspect the mag distributor cap.
Higher than normal EGT on the new cylinder – Probable causes
Any other ideas?
How do we eliminate each of these causes?
Ok Aviathor, understood now that I should treat them separately.
On #2 Probes have been switched and problem remains with cylinder 5. Since they switched the probes I assume the second point has also been taken care of?
Exhaust valve has been inspected no leaks found. Dry tappet clearence all within tolerances.
Induction leak test performed, no leak found.
Fuel flow on all cylinders has been measured. Fuel quantity is identical on all cylinders.
The mag had been rechecked 2 month ago and the timing should be correct.
I guess with all the valve measuring checks having been performed we can also eliminate incorrect valve timing.
I really struggle to find other possible sources for the hight EGT problem.
A fault on a single cylinder can only be attributed to something that only affects that cylinder; that is a component that is unique to that cylinder. Therefore it cannot be valve or ignition timing because it would affect the other cylinders too.
I haven’t read all previous posts; what happened with the scuffed piston and cylinder?
Rough engine when running on a single mag – Probable causes
Do another run-up and mag check. Observe the EGTs during the mag check. Make sure they are all rising. If one cylinder’s EGT falls during mag check, there is an ignition problem on the plug connected to the mag you’re testing.
Higher than normal EGT on the new cylinder – Probable causes
How did you swap the probes? Did you just pull out the probe from cyl N and stick it into cyl M and vice versa so that cylinder N read as cyl M on the instrument and vice versa, or did you take apart some connectors? What I am getting at is that if it is not the probe, it could still be that any probe connected to that port on the instrument would read 300°F more.
The amount of fuel from the injectors was measured
I bet it wasn’t…
This is quite tricky to do. You have to run the fuel into a graduated beaker, and plot the fuel flow versus the total fuel flow, for each injector (or at least for the suspect one, plus one other). But to get a beaker in place you have to bend the stainless steel tubing up a bit, which is an undesirable procedure because you really do not want to kink those thin tubes.
The other thing is to see if the high EGT cylinder also has a high (or otherwise abnormal) CHT. That is the normal in-flight procedure for isolating an EGT probe which is about to get eaten and is reading high.
Peter wrote:
I bet it wasn’t…This is quite tricky to do.
It should be quite simple to verify that without having to measure how much fuel comes out from the injectors: run up the engine to 2000 rpm or whatever RPM is normally used for run-up, and lean to find peak EGT. If one cylinder is significantly leaner than the others, you will see it easily.
Peter wrote:
The other thing is to see if the high EGT cylinder also has a high (or otherwise abnormal) CHT
Placido has TIT. The higher EGT should reflect in a higher TIT. The problem is that he has no baseline because the engine monitor data cannot be downloaded.
There is only one TIT, which is a mixture of all 6 EGTs.
Yes, but if one EGT is 300°F higher than normal, the TIT will be higher.
The absolute EGT temperature is meaningless, if the probe is inserted a bit deeper into the exhaust tubing, the reading will be different. The main use of EGT is to know the relative temperature relative to peak. EGT does not reflect the true combustion temperature nor is it a proxy for the CHT. Does the engine develop full power at full rich setting? Does it run smoothly at full power?
I had a chat with my mechanic today and we have the following situation.
The probes have been swapped from one cylinder to the other but the problem remaind with cylinder 5.
They have performed the mag checks and it clearly shows that the EGT drops exactly on the cylinder 5 when the engine runs rough on one cylinder. The spark plug have been swapped also to eliminate the plug as the problem. No matter what they do the problem always remains with cylinder 5. They are convinced it is an ignition problem so we are back to the very beginning which had been excluded once the magneto has come back from the shop saying that it was all good.
They now believe it is the harness or the ignition distributor (or whatever this is called in English). If it is the harness they said they need to exchange the whole harness. They did check it themselve removing the magneto cover and measuring everything but could not find any issues so now they will talk to the magneto shop to see what else they could check with regards to the ignition.
