Home » Public Forums » GMCnet » Re: [GMCnet] Non-GMC engine question
| Re: [GMCnet] Non-GMC engine question [message #89090] |
Fri, 18 June 2010 07:24  |
Gary Casey
Messages: 448
Registered: September 2009
Karma: 0
|
Senior Member |
|
|
My apoligies for continuing this somewhat off-topic thread, but I think Ken is "hanging by a thread", so to speak :-(
The high EGT being caused by an air leak is highly unlikely - maybe impossible. As Ken said, the highest EGT previously seen was about 1500 regardless of mixture. Second, compared to the air flow through the essentially open throttle, any "normal" air leak caused by a bad gasket or crack in the intake tube would have to be miniscule. It could make a noticeable change in idle characteristic, but in flight? I don't think so.
The only problem with an EGT probe that I can think of is if the probe shorted to ground. It would still read temperature, but with an error. Some aircraft systems use grounded probes and some ungrounded - I don't remember who uses which.
Compression test? I wouldn't bother. A simple way to perform a surprising accurate compression test is to simply crank the engine with the fuel turned off. Are the compression "lumps" uniform. You'll easily detect a difference between cylinders that way. I do that test occasionally, usually inadvertently during a hot start :-). Any leakage in a cylinder that could even slightly effect performance at a high engine speed will be easily detectable during cranking.
There is a natural limit, but I wouldn't call it a theoretical limit, on EGT. It depends primarily on compression ratio and ignition timing. Running on one plug effectively slows combustion (one flame front instead of two), raising the EGT above the maximum that could be obtained with both firing.
The temperature of one cylinder going up by 100 degrees pretty much correlates to a spark plug that ceases firing. The cylinder will continue to put out about 90% of normal power (may or may not be noticeable) and all other temperatures will remain close to normal. Only the higher EGT will indicate the problem, which Ken reported. Did Ken do a mag check in the air after he saw the problem? I know I would not have done that at night at 3500 feet (why so low? At night?), but there are those more "metallically endowed" than myself that routinely do mag checks in the air. In this case it would have immediately identified the problem if it were indeed an ignition-related failure.
My bets are on either a spark plug going shorted or an ignition wire doing the same. Problem is, it might not show up on the ground, so another, "diagnostic", flight (my wife hates the phrase "test" flight) might be in order.
Good luck,
Gary Casey
N422SG
mcolie wrote on Thu, 17 June 2010 20:19
> Ken Burton wrote on Thu, 17 June 2010 12:54
> > Last night on the way home I noticed one of my airplane cylinders was running 1642 degrees EGT. It got my attention to say the least as I was about 1 hour from home well after dark which limits your landing options in case of an emergency.
> >
> > I have never seen an EGT over 1520 or so. Adding carb heat decreased the reading by about 75 degrees but that was still higher than I have ever seen before. The interesting thing is the CHT was the same on that cylinder as all of the rest which was around 340 degrees. Oil temperature was also in the green.
> >
> > I was running full rich at about 3500 feet with an OAT of 72 degrees. This is a carburetored non-turbocharged 87 octane O-320 engine.
> >
> > I have always thought from past leaning experiences that 1500 degrees or slightly higher was the theoretical limit on 100% non-turbo gasoline powered engines.
> >
> > I'm thinking and hoping that this is simply a bad probe.
> > Note: I also intend to run a differential compression test on the cylinder in question in the next couple of days.
> >
> > Is there anyone out there that has any knowledge on this subject?
>
> Ken,
> Let's start with the bad side. . .
> In my decades of lab work, T/couples failed reading low or open. (our systems could detect open)
> Let's hope for the good side...
> You were at cruise - probably 20-24" map????
> If you have developed and intake runner leak, that cylinder would go lean. The lean mixture will burn more slowly - hence high EGT. CHT might go up a little, but that is more dependent of BMEP.
>
> Let's hope it is one of those gaskets where the intake goes into the head.
>
> Matt
Matt Here is my point. On an airplane we have control of the mixture and we lean out the engine when flying over 5000 feet. We lean the mixture until we see peak EGT. If we continue lean the engine past peak EGT very much the engine starts to miss due to an over lean condition. We richen (if that is a word) the mixture so the EGT on the highest cylinder goes down by 50 degrees. (some people go 75 degrees) I have never seen a peak EGT higher than 1520. The reading of 1642 really confuses me. The probes are indeed a thermocouple. I thought thermocouples fail by reading low and not high. This is why I am questioning what I think I know about this whole thing.
Some aircraft have only one EGT probe and some like mine have probes on all cylinders. My past experience has been the peak EGT reading never exceeds 1520 degrees and usually it is around 1500 so I can not understand a reading of 1642 degrees with no change in CHT or engine running smoothness. I would buy your idea of an intake leak on one cylinder if the EGT reading peaked around 1500 and or the engine ran roughly.
I'll still check for an intake leak on that cylinder.
Please tell me more.
--
Ken Burton - N9KB
_______________________________________________
GMCnet mailing list
List Information and Subscription Options:
http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
|
|
|
|
| Re: [GMCnet] Non-GMC engine question [message #89130 is a reply to message #89090] |
Fri, 18 June 2010 14:46  |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
|
Senior Member |
|
|
| Gary Casey wrote on Fri, 18 June 2010 07:24 |
My apoligies for continuing this somewhat off-topic thread, but I think Ken is "hanging by a thread", so to speak 
The high EGT being caused by an air leak is highly unlikely - maybe impossible. As Ken said, the highest EGT previously seen was about 1500 regardless of mixture. Second, compared to the air flow through the essentially open throttle, any "normal" air leak caused by a bad gasket or crack in the intake tube would have to be miniscule. It could make a noticeable change in idle characteristic, but in flight? I don't think so.
The only problem with an EGT probe that I can think of is if the probe shorted to ground. It would still read temperature, but with an error. Some aircraft systems use grounded probes and some ungrounded - I don't remember who uses which.
Compression test? I wouldn't bother. A simple way to perform a surprising accurate compression test is to simply crank the engine with the fuel turned off. Are the compression "lumps" uniform. You'll easily detect a difference between cylinders that way. I do that test occasionally, usually inadvertently during a hot start  . Any leakage in a cylinder that could even slightly effect performance at a high engine speed will be easily detectable during cranking.
There is a natural limit, but I wouldn't call it a theoretical limit, on EGT. It depends primarily on compression ratio and ignition timing. Running on one plug effectively slows combustion (one flame front instead of two), raising the EGT above the maximum that could be obtained with both firing.
The temperature of one cylinder going up by 100 degrees pretty much correlates to a spark plug that ceases firing. The cylinder will continue to put out about 90% of normal power (may or may not be noticeable) and all other temperatures will remain close to normal. Only the higher EGT will indicate the problem, which Ken reported. Did Ken do a mag check in the air after he saw the problem? I know I would not have done that at night at 3500 feet (why so low? At night?), but there are those more "metallically endowed" than myself that routinely do mag checks in the air. In this case it would have immediately identified the problem if it were indeed an ignition-related failure.
My bets are on either a spark plug going shorted or an ignition wire doing the same. Problem is, it might not show up on the ground, so another, "diagnostic", flight (my wife hates the phrase "test" flight) might be in order.
Good luck,
Gary Casey
N422SG
|
I just cleaned and gapped all of the plugs about 3 hours ago.
It passed a mag check prior to take off. I did not try a mag check in the air. I would prefer to do that over the airport in daylight.
I did not think of a bad or shorted plug. I'll swap the #3 and #1 plugs and see if the symptoms move. I always swap upper and lower plugs when I clean them but now I'll move them to another cylinder.
I agree that a compression / leak down test probably will not show anything but it only takes 4 or 5 minutes to do it while I have the plugs out.
Why 3500 feet at night? We were just out sight seeing (looking at windmills). It was scattered at 5000 and it just turned dark so I stayed at that 3500 on the return trip. We probably would have had to have gone to 7500 or filed IFR and it was not worth the bother.
Thanks for the ideas.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
|
|
|
|
|
|
| Re: [GMCnet] Non-GMC engine question [message #89279 is a reply to message #89278] |
Sat, 19 June 2010 22:40 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
|
Senior Member |
|
|
| Hal Kading wrote on Sat, 19 June 2010 22:24 |
Ken, Gary,
As one who routinely does in flight mag checks, I wonder why you would not do the check at night at 3500 ft? What could go wrong that hasn't already gone wrong? Bad mag will show up, bad lead or plug will be confirmed - go back to both and engine will run as before. Sometimes the engine will run fine on one mag when it won't on both. Please enlighten me as I don't see any downside.
Hal Kading 77 Kingsley 455 Las Cruces NM
|
I did not want to experience a possible second failure like a bad mag switch or a failing / grounded P lead. I also had a passenger that I did not want to scare.
I swapped the EGT probes between 3 and 1 today. I will swap the plugs between 3 and 4 tomorrow.
I did not fly it today because a friend showed up and wanted me to go get a motorcycle that he just bought. We spent the rest of the day fixing little things on the motorcycle.
Maybe I'll fly it tomorrow and see if the problem moved.
I'll do the in flight mag check when I am alone in the plane.
Ken B.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
|
|
|
|
| Re: [GMCnet] Non-GMC engine question [message #89280 is a reply to message #89278] |
Sat, 19 June 2010 22:44 |
James Hupy
Messages: 6806
Registered: May 2010
Karma: -62
|
Senior Member |
|
|
My uncle Al Andersen who was a wild and crazy guy and ex WWII fighter pilot
who came back after the war and flew aerial dust and spray planes told me
when I was about 15 "There are old pilots and there are bold pilots, but
there are Da## few Old Bold pilots". I don't fly full size aircraft but I
have built and flown a number of Radio Controlled models. My instructor told
me when I was learning, "Always fly 9 dumb thumbs high." Does that apply
here?
Jim Hupy
Salem, Or
1978 Royale 403
On Sat, Jun 19, 2010 at 8:24 PM, Hal Kading <halkading@fastwave.biz> wrote:
>
>
> Ken, Gary,
>
> As one who routinely does in flight mag checks, I wonder why you would not
> do the check at night at 3500 ft? What could go wrong that hasn't already
> gone wrong? Bad mag will show up, bad lead or plug will be confirmed - go
> back to both and engine will run as before. Sometimes the engine will run
> fine on one mag when it won't on both. Please enlighten me as I don't see
> any downside.
>
> Hal Kading 77 Kingsley 455 Las Cruces NM
> _______________________________________________
> GMCnet mailing list
> List Information and Subscription Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
>
_______________________________________________
GMCnet mailing list
List Information and Subscription Options:
http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
|
|
|
|
| Re: [GMCnet] Non-GMC engine question [message #90385 is a reply to message #89279] |
Tue, 29 June 2010 00:24 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
|
Senior Member |
|
|
Well I finally got around to flying it again tonight with the probe positions swapped. It is a bad probe.
Now for the replacement cost.
$94.00 -
That is just for the thermocouple with a one foot lead on it.
I was going to get two but at that price I'll have to rethink my purchase.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
|
|
|
|
|
|
| Re: [GMCnet] Non-GMC engine question [message #90455 is a reply to message #90401] |
Tue, 29 June 2010 16:43 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
|
Senior Member |
|
|
I do not understand it either. I assumed the TC's failed by going open.
I'm going to try one thing. I'm going to pull the TC probably tonight at Two Wheel Tuesday and bead blast the surface to the point it is shiny again. I do not expect this to do any thing but It will only take a few minutes to find out.
Ken
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
|
|
|
|
| Re: [GMCnet] Non-GMC engine question [message #90493 is a reply to message #90455] |
Wed, 30 June 2010 02:38 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
|
Senior Member |
|
|
| Ken Burton wrote on Tue, 29 June 2010 16:43 |
I do not understand it either. I assumed the TC's failed by going open.
I'm going to try one thing. I'm going to pull the TC probably tonight at Two Wheel Tuesday and bead blast the surface to the point it is shiny again. I do not expect this to do any thing but It will only take a few minutes to find out.
Ken
|
Well I did it and the temperature dropped about 210 degrees. Now it is only reading about 50 degrees high. I'm really confused at this point. I guess it is time to clean up all of them in the bead blaster and see what happens.
I would just replace all of them but at $94.00+ per cylinder that is a little steep.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
|
|
|
|
|
|
|
|
| Re: [GMCnet] Non-GMC engine question [message #90609 is a reply to message #90511] |
Thu, 01 July 2010 00:16 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
|
Senior Member |
|
|
| mcolie wrote on Wed, 30 June 2010 08:34 |
| Ken Burton wrote on Wed, 30 June 2010 03:38 |
Well I did it and the temperature dropped about 210 degrees. Now it is only reading about 50 degrees high. I'm really confused at this point. I guess it is time to clean up all of them in the bead blaster and see what happens.
I would just replace all of them but at $94.00+ per cylinder that is a little steep.
|
If you get figured out what is going on here, I would love to know more about it.
The panel instrument is most often just a millivolt meter and a switch - no active components at all.
T/Cs do typically fail open or partly (depending on whether they are isolated or grounded junction), but the other common mode failure is to short and form another junction out of the area of interest. I have never even seen an active readout that reported higher with failure.
While you are in there, find out if the T/C is a grounded or isolated junction for me.
Matt
|
Type K thermocouple. Isolated from ground. Red and yellow wires.
One of the guys at the airport has a tester for these things. He is working 12 and 16 hours shifts so I have not got my hands on it yet. If I do I'll have to figure out how to run it.
I found a place in Troy, MI that also makes these. They are $50.00 each and have a nice rigid 90degree bend in the lead. I have not figured out yet if the ones they make are the correct diameter. They specifically do not mention use in aircraft. That is understandable since they would have to go through the FAA BS, spend years ,and thousands of dollars to get approved. If they are they are available in the correct diameter I might order one and try it.
Years ago on another airplane I needed a flasher for the light on top of the tail. It was priced as a $35.00 item. I bought the thing locally at the auto parts store for $1.08 . It was even the same Tungsol part number. It just did not have the FAA stamp of approval on it.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
|
|
|
|
Goto Forum:
Current Time: Mon Oct 21 18:41:37 CDT 2024
Total time taken to generate the page: 0.00858 seconds
|
GMCforum
Search
Help
Register
Login
Home
