EGT help needed

[Frequent Flyer]

Hi Guys, as you all know, i bought Tjoeks Cobra 2 weeks ago. Now, I'm trying to sort out the EGT's, they give verry high readings (700-800degC) but the H20 temp (between 60-75degC) shows normal, and the CHT probes (i know they dont give a exact temp reading but can still be indicative) gives me a 90 - 100degC reading (which was the same as i got on my previous plane). Tjoek said that the probes were installed and the guy that did the instalation cut the wires to short The Needle jet is set correctly (3rd gap from the top)- i.o.w not running to lean. The guage seems to work fine - egt drop and rise normally during a descend / climb) Anny input

[Morph]

What do your plugs look like? They should be a nice tan/brown colour. If they are burning white or grey then you are definately running lean and a lean running engine also shows high egt's

I do know that it is not recommended to cut the leads shorter on the EGT probes.

Your EGT's should be 500 to 620 deg C, with a max of 650 deg C

[Frequent Flyer]

Thanks Greg, the plugs are a nice brown colour and as i said, al the other vitals looks good. Do you think i should replace the probes then On my previous plane the egt's showed between 580 - 600 which was normal with a 5-10degC difference between them..

[Morph]

What brand of EGT probes and gauge are they? If MGL call Ranier at MGL and ask his advise. They could be set up for the wrong type of sensor. 021 855 1383

Otherwise, try and see if you can borrow probes from a friend to test. Just be careful trying to remove them. Because the exhaust undergoes such high temps the probes can be stuck and could snap off if you try to remove them. This happenend to me and I had to take the whole exhaust into an engineering works to get the broken probe removed

[Frequent Flyer]

Ordered new probes from aviation engines today. will replace them and let you know. spoke to alex and he said it will not make a difference if the egt probes wire is cut shorter. i'm a bit confused about the lenght of the egt probe wire, can it really make a difference? i know that when you extend them you can use normal 2 core electrical wire and it will still give you the correct temps if i'm not mistaken. In any case, i was also told to throw out the egt guage as its not necessary and by the time you see someting's going wrong on the egt, it's already to late. but i like monitoring my engine temps more than anything else - to me it's more important than climbing @ x ft/min and getting x mph @ x rpm with higher temps.

[Mogas]

Are the probes fitted closer to the cylinders than normal? Rotax has a published distance probes should be from the cylinders. If the positioning of the probes is not correct, the output on the guage is not accurate.
I fear this may be a waste of cash buying new ones. If all other indications are normal; colour of plugs etc. I would just live with it.
Once again, it is worth mentioning that the value of monitoring EGTs is in detecting a change from normal. Actual numbers are not that important.

[Frequent Flyer]

egt probes are of the screw in type and the exhaust manifold has pre-drilled holes. And yes i totaly agree, the values are checked for any change or deviation from the normal - same reason i also watch the cht's - not for the actual value but rather a sudden change thereoff. but in this case the readings are almost off the scale 700-800degC in the cruise. Thanks for al the input so far, will be installing the new probes, a new prop (changing from a warp to a nc) and will also check the 'jetting' this weekend.

[Miskiet]

You get K type and J type thermocouples (probes) amongst a few others. They are used for different temperature ranges/applications. The instrument should have a set-up for changing to either (Most probes we use are K -type) - I would guess yours is set up for J-type and that is why you have got the off-set temps. Alternatively you can contact the manufacturer and find out what the correct type of probe is for your instrument.

Cutting the wires shorter should not make a difference as long as both connections are at the same temperature - Be careful if you cut them (the wires are very fine copper strand - difficult to make good connections Thats why they don't recommend cutting it)

[Duck Rogers]

You get K type and J type thermocouples (probes) amongst a few others. They are used for different temperature ranges/applications. The instrument should have a set-up for changing to either (Most probes we use are K -type) - I would guess yours is set up for J-type and that is why you have got the off-set temps. Alternatively you can contact the manufacturer and find out what the correct type of probe is for your instrument.

Cutting the wires shorter should not make a difference as long as both connections are at the same temperature - Be careful if you cut them (the wires are very fine copper strand - difficult to make good connections Thats why they don't recommend cutting it)

And there is the correct answer..........
And if anyone so wishes, a comprehensive tutorial on EGT thermocouples can be posted to explain the whole temperature thing

[Miskiet]

You can do it Duck - I'm rusty.....

[Frequent Flyer]

that will be great mr duckie - shed some light on the topic for us

[Morph]

http://en.wikipedia.org/wiki/Thermocouple

[Wargames]

Thanks Morph,

At last I'm understanding where they get there reading from. In short a device reading tempreture is nothing else than a calibrated volt meter. Cool.

[Frequent Flyer]

Ok, got new probes, opened up everything and the old installation looks ok - EGT Probe wire runs along seat frame, then connects to a different kind of electrical wire (more or less the same thickness), then continues on to the panel where somewhere it chenges to yest another kind of electrical wire but this time much thinner. so i decided not to install the new probes until i get some more help. I also checked the main jet size - 158 and the needle jet is a 11G2. i also decided to raie the needle jet one notch (2nd from bottom to bottom) to see wether it has the desired effect of enrichening the mixture and thus lowering the egt temp? am i understanding this correctly?
The answer - it did but the reading i get now is between 600 & 700degC and not 700-800degC, most important - it showed a drop in egt temp so the guage is at least working correctly. As stateded in one of the posts above, should i let it be and just monitor a change in the temps and the difference between the two or what should i do

[Mogas]

Drop the needle?
Sounds like you raised it which is right for richer. Better check and make sure Stefan.
BTW, raising the needle enriches the mid-range, does diddley squat for idle or max RPM mixture. To change idle or max mixture means change jets.
I dunno what the correct jet sizes are offhand but Rotax publishes a chart for the correct jets for your engine and operating altitude and temperature. Often one finds 2 stroke motors that were origionally jetted for somewhere else.
Short story: I knew a clever chap once who thought it was smart to change his jets for better range where we were at a field altitude of 5500 ft. He got away with it for quite a while, often chortling how fuel efficient his trike was and how clean his plugs were. Refused to listen to conventional wisdom. Some months later we went on a flyaway to the Zambezi valley, about 500ft agl and nice and HOT. One holed piston and trike in the river. Oh, and he thought EGT guages were unimportant.
Then I suggest you pull the plugs after an hour or so and check the colour, should be a nice brown/tan (normal) not whitish (too lean) or sooty black (too rich).
The best way to do this plug check is to shut it down from mid range RPM (switch off mags) not after a a long idle as this could give you a false reading.
Its easy to get right, not much to go wrong once its right.
Good luck.
M