EGT's too cold and too hot - help needed

[gwilson]

I have a 1978 Tiger AA5B with anengine mystery that I hope someone can help me with.

In March, I had my annual inspection with no surprises. The mechanic suggested that I replace the spark plugs so I gave the go-ahead and asked that he use fine-wire plugs since that what the plane had for the past several years with no issues. On my first flight after the annual, I noticed that the EGT in cylinder #3 was running higher than normal. I went back to the mechanic but he could find nothing wrong. Since the difference was not too severe, I did not worry too much about it. Last week, I started having a rough mag check so I went back the mechanic. He cleaned all the plugs and the mag check seems better (although not perfect)....but now my EGT problem is much worse! Cylinder #1 reads lower than usual and cylinder #3 reads higher than ever. The difference between the two EGT's exceeds 200 degrees so it triggers the alarm on my engine monitor.

Somewhat frustrated, I went to another mechanic. The new mechanic found a crack in the intake pipe leading into cylinder #3 so I was ecstatic that the problem had been resolved! A new tube was installed and I picked up the plane for the flight home. However, the EGT problem is unchanged!

So, here's my plan:
Step 1 - I'm going to swap the probes between cylinder #1 and #3 to eliminate a probe issue
Step 2 - I'm going to swap the plugs from the left side to the right side to eliminate plug issues

Does this sound like a good approach? Am I missing another obvious possibility? It's odd that the problem started with the installation of new plugs and got much worse when these same plugs were cleaned! Could a plug be causing this and eluding two mechanics?

Any ideas or suggestions?

Thanks!

Greg

 

[GravityHater]

yup sounds good to me, eliminate the messenger as the cause of the bad message; swap egt probes.

 

[Berkut]

Could a plug be causing this and eluding two mechanics?

Possibly. How are your mag checks now that the induction leak is fixed? Still rough, but not as rough?

 

[gwilson]

Yes - mag checks are much better... the engine sounds great. The EGT's are the only problems.

 

[erj-145mech]

Only do one thing at a time, otherwise, you won't know what made the difference in your performance. Swap probes, check it out. Swap plugs, check it out. etc

 

[Berkut]

I wouldn't swap the probes first myself. Try swapping a couple of plugs and see if the problem follows them. One weak plug can cause that cylinder to have a high EGT.

 

[charlesmelot]

Swapping probes and plugs is OK but what really counts in terms of EGT is the rich/lean spread (difference between fuell rich and peak) not the raw temperature. If all the spreads were 200 degrees what difference do the raw temps make? Charlie Melot Zephyr Aircraft Engines

 

[Berkut]

I agree that absolute EGT values are of little importance, since relative EGT's are all we care about. But in this case, there was a change in EGT after some ignition parts were replaced.

 

[charlesmelot]

I agree that absolute EGT values are of little importance, since relative EGT's are all we care about. But in this case, there was a change in EGT after some ignition parts were replaced.

Valid point. That's why I agreed with the plan to check it out.

 

[wooferdog]

I think a bad lead in the harness could be the culprit.