Egt probe issues... Help!?!?

GEM930 · #1 06-09-2010, 01:25 PM

My egt probes are acting strange and i'm about to replace them all. Any ideas would be really appriciated. First a a little history.. I replaced the carb and single drive dual mag setup on my o320 with an electronic fuel injection electronic ignition system from realworld solutions. After a few growing pains
it seems to be working great. I'm at the point of fine tuning things now and really need my egt probes to be working correctly. Unfortunately they are providing strange readings. To further complicate the issues I replace my steam gauges with an mgl voyager. I have no reason to believe the voyager is the source of my strange readings as the problem, at least, kinda seems to follow the probes. Anyway ... Since the upgrade my #1 egt is usually low, usually 400 degrees lower than the #4. Typically 1- 990, 2- around 1200, 3- around 1300, and 4- around 1400. At high idle #1 stays around 600, with all the rest around 1000. Now is where is gets confusing. I swapped 1 and 3 probes and then both started reading low. The original #1 probe, now in the #3 cylinder was still the lowest, but the #3 probe, now in the #1 cylinder started to read Almost as low. Swapped them back and when back to how it was before. Swapped the leads at the mgl unit(cylinder 1 &2) and the # 1 started reading low..... Ok ...no problem with the mgl unit... Must be a probe... Swapped back and replaced #1 probe. Still reads low and now I have a splice in the line!!! Put original probe back and placed all probes in the end of my head gun. All probes seemed to be seated the same distance in and in the same positions. Now I get a spread of 150 degrees from the highest to the lowest 850-1003)!!! And...... the #1 probe is not the lowest.... It's in the middle!!! I'm about to scrap all four and buy new ones. These have 340 hours on them and I think they are Falcon brand. Any ideas be for go crazy!!! Also I have swapped Injectors and checked for air leaks by switching to the backup efi system (one large throttle body injector instead of four port injectors) and the strange reading still persist. Please forgive any typos.. Long post for an I-phone!

GEM930 · #2 06-09-2010, 01:32 PM

Sorry about the paste in the middle.... high maintance girlfriend and big thumbs on the iPhone.... Could a moderator remove it for me??? I guess egt issues as not all my problems!

GEM930 · #3 06-09-2010, 01:37 PM

Probes are Westach and yes girlfriend is still mad at me!

Brantel · #4 06-09-2010, 01:40 PM

Women and airplanes are like oil and water 98% of the time....

PCHunt · #5 06-09-2010, 01:40 PM

You can edit your own post. Look for an edit button.

corbinace · #6 06-09-2010, 02:19 PM

Is there any chance of poor connections or excess resistance?

I am referring to the thermocouple leads not your relationship.

GEM930 · #7 06-09-2010, 04:35 PM

No on the plane.... I checked everything several times....
Probably on thr girlfriend, but that is a post for another time and another forum!

I think I'll leave the random paste in... I'm not proud... And the witty responses may be worth a laugh!

mfshook62 · #8 06-09-2010, 05:12 PM

Thermocouples work by producing a voltage at the junction of 2 metals, and this voltage is a function of the temperature. Higher temp = higher voltage. The junction is usually made from chromel-alumel alloys, which produce a linear voltage over a wide heat range from very cold to red hot. Its not good to splice other metals in series with these 2 metals, because you end up with another metal junction. One way to check your thermocouples is check the voltage at room temperature and then place them in boiling water and check the voltage again with a digital meter. If the thermocouples are working the voltages should all be very close at room temp and consistently higher in boiling water. You should also check the resistance of the lines leading to your meter, they should all read the same, since voltage drops with resistance, per Ohm's Law.

Have fun.

elippse · #9 06-10-2010, 09:41 AM

In order for the temperatures to be correct, whatever kind of connection you do on one lead you must also do exactly the same on the other lead so that any dis-similar metal voltage generation takes place on both wires of the pair from the thermocouple; this forms what is called a "common mode" voltage. You also should not gang all of the returns together except, if called for, and only at the display box, otherwise you could have slight voltage drop in a single wire return which throws off all of the signals. That means don't ground the return side of the pair, as it must go to a cold-junction compensating circuit. If you use a switch to select a signal, you must switch both wires, not just one. The voltages on these wires is in the millivolts, thousandths of a volt, and great care should be taken in the layout and connections!

rocketbob · #10 06-10-2010, 09:50 AM

Are your probes are of the grounded or ungrounded variety? If they are of the grounded variety, you may be having problems due to the junction being in contact with the engine. Ungrounded probes are isolated from the engine via a ceramic insulator inside the probe. All it takes is a few microvolts to cause an error of several hundred degrees.