GE's CF34 high EGT possible causes, suggestions needed.

[flame_bringer]

Been trouble shooting this problem all day on one of our embraer 190s GE's CF34, engine n.o 1 mysteriously gives an excessive EGT at all power settings.
MCC insisted that the problem is on the avionics side and hence made us swap the thermo couples with another aircraft, engine run C/O and same problem.
They made us slave in a new FADEC computer and still to no avail.
They made us cont check,voltage check and insulation check the Fadec-EGT harness, all was normal.
pnuematics checked for leak, all turbine and compressor stages have been boroscoped by the B1 guys including the combustion chamber and no finding so far all looks normal.
compressor wash was done and still the same.
Personally I doubt that it's the thermo couples or anything to do with the avionics it seems as though the indication is real and it is indeed an indication of engine performance degradation, However I can't seem to think of something else to be the reason since the turbine stages,combustion chamber,compressor stages and the whole of the gas path looked fine upon boroscoping, and there is no leak in the pneumatics.


Any suggestions in this regard from experienced people?
Thanks in advance

[top-lame]

Am not familiar with this engine...but,
A thermocouple "cant" produce a higher emf than expected for a given temp.
Q; Was the temp rise sudden or progessive?
Was the rise after an engine change.(check for double wiring reversals)

Since the system is not "loaded" with a meter type indicator, but with a high impedance....then it may have a parasitic emf caused by a series wiring splice which has corroded. (legitimate or illigitimate/unofficial repair)

...I once encountered a splice in the wing root which was corroded and did cause premature fire warnings. (new splice fixed it)

Cant help feeling your up for an engine change..hope above helps.

[itsresidualmate]

I take it N1 and N2 assemblies rotate freely by hand without any undue drag?

Any difference in the fuel flow between both engines?

Any debris on chip detectors? Possibly do an analysis of engine oil, see if anything is breaking down in bearings? Pull oil filters?

Do those motors not have bleed flaps that bleed excess HP air into the fan duct? Duct stuck open?

[Lightning5]

Unsure ref CF34 but I know from 757 R.B. experience if you get the probe connections in a incorrect order when being stacked at a junction this could give a spurrious indication. its been some time so forgive in my memory fails!!

[spannersatcx]

Have a look at any temperature sensors involved, i.e. Compressor inlet temp sensor etc, they affect scheduling and can cause a shift as the engine thinks it's hotter or colder than it actually is. Are there any temperature correction plugs if these are wrong it can affect the engine indication.

[Fargoo]

How excessive? 20 deg, 40 deg, 100 deg???

[N1 Vibes]

Flame Bringer - your question asks about "high/excessive EGT":

1. Has the engine experienced an EGT exceedance?

2. Is it experiencing an increased EGT trend (i.e. the EGT has been increasing over successive flights/days/weeks when you analyse your Eng Condition Monitoring)?

3. By how much is it "excessive EGT at all power settings"?

You might have problems with:

- Bleeds (a/c or engine) - ECS sys check/component changes
- EGT indication - this could be a short from something under the cowlwhen you close the cowls, but OK when you bell out the system with the cowls open - check for anything loose on the cowl inner skin
- Deterioration (if it is a long term trend) - boroscope
- Deterioration (if it is a short term trend) - possible FOD - boroscope
- Basic wiring short - just that it could be hiding in all those lovely connectors (which I guess are all sealed)

- Just noticed you are in Bahrain - what about silica (sand) contamination of the turbine hardware? - Boroscope to see if the hardware is covered in glass

If you could give a bit more info...

Good Luck,

N1 Vibes

[Papa2Charlie]

Hi,

As mentioned by a few of the other posters, we could do with a little bit more information on the problem. Anyways here's my two cents :

1. Compare the N1, N2 and FF of both engines. If the values are similar between the engines, then you most likely have an indication issue and this would force you down the route of the EGT T/S tasks in the FIM.

2. If there is a difference in the N1, N2 and FF, then the EGT difference is real and there is most likely something up with the engine. Possible candidates are :

- Bleed systems (handling and cabin). Confirm the correct operation of both systems.
- Core damage (BSI the compressor and turbines). Start with the turbine and then if no findings, move onto the compressor....you'll inevitably find something unwanted (and probably unimportant) in the compressor.
- What age is the engine? Is it coming due for a refurbishment?
- Download the DFDR and get someone in engineering to confirm that the engine is behaving reasonably close to the other (e.g. variable stages are operating to the correct angle etc.).
- Check the tech log and see if there has been any relevant maintenance accomplished in the last few months e.g. EEC swaps, T/C replacements, surges.

All the best,

P2C.

[flame_bringer]

Thank you all for your replies.
We have tried comparing it with the other parameters and everything seemed normal it was just the EGT that was abnormal which is why it seemed rather mysterious.
However upon another compressor wash the problem dissapeared, exceedance value was 30 to 40 C, I'm assuming that there was some impurity inside the engine that the B1 guys failed to see, which was causing the problem.
Anyways Thank you all once more, I appreciate your help.

[Basset hound]

I'll bet it was silica...