CHT Spread

[lfgbrd]

What's a normal CHT spread for a 1990 201? I'm seeing:

1. 291
2. 329
3. 354
4. 340

Or to put it another way, the spread is pretty consistently 50-75 degrees with #1 seeming pretty cold. They never really get hot unless you really screw up the mixture. I've replaced all 4 sensors, and their EGTs are always within about 75 degrees of each other.

I'm trying to track down the source of an occasional misfire or backfire in flight. Some other issues as well, but I'll have to ask about them later.

[ArtVandelay]

I have a 40 degree spread 1-3

My 3 is the lowest, my 1 is the highest, kinda makes me think they are reversed, since the wires aren't labeled I can see why the would be mixed up

[Houman]

Do you guys have a big difference between the CHT temperatures when engine has been off for a day or 2, and that the probe is giving the ambient temperature. I have a 20 degrees of difference on #6 CHT when looking at the ambient temperature and more than 60 or 70 when running.

I know it is most probably not a cylinder issue since ambient temperature for #1 to #5 CHT would be the same, and #6 with 20 degrees hotter... 

TIM at JPI tech support has been great helping to figure out what is going on.

Till now we have switched #4 CHT probe with #6 and also have bench tested the EDM, so only remaining problem is the K type wires going from the Probe to the EDM.

Hope this is not a hijack of the thread...

[cnoe]

Do you guys have a big difference between the CHT temperatures when engine has been off for a day or 2, and that the probe is giving the ambient temperature. I have a 20 degrees of difference on #6 CHT when looking at the ambient temperature

My 8 probes (CHT & EGT) all read within 1-2 degrees of each other when cold. But they are less than a year old.

Sent from my iPad using Tapatalk

[carusoam]

Like CNOE said...

Expect that they are all the same temperature.

From the modern version of 0^th law of thermodynamics... Everything in the hangar at equilibrium will be at the same temperature.  Even though they may feel differently.

A couple of other things we have learned here...

- JPI sensors can be adjusted through settings for some devices. Check to see that nobody has made adjustments on yours.

- Mixing J and K type thermocouple wires or splicing in ordinary copper or aluminum will generate unexpected results.

In the hangar over time, all the CHTs, EGTs, oilT and OAT should match.  A few degrees one way or the other is not critical.  Five or ten degrees is starting to get out of line for a TC....

Best regards,

-a-

Edited October 9, 2015 by carusoam

[rgpilot]

I would check for a partially clogged injector on #1.

[wishboneash]

At WOT at 70% power, my CHTs are within around 30 deg. #2 was the coldest, the other three within 6 deg of each other.

[lfgbrd]

So part of the "Other Issues" I mentioned were occasional sudden spikes/changes in EGT/CHT. I could not pinpoint what was causing it and it tended to happen so sudden that it didn't register in the EZTrends in any meaningful way. On a flight two days ago we noticed a loss of about 10kts TAS compared to what we normally see, along with an inability to lean the plane to what I normally lean it to. The TAS drop I knew was suspicious (because I could calculate it without the instruments) but we just changed every sensor and instrument in the panel so I couldn't say with absolute certainty that anything else was wrong.

Today I got on the ramp and when I switched to check the left mag, the engine sputtered and died. I restarted it, checked the left mag, and it ran a bit rough. I did the plug cleaning procedure, tried again, and it ran perfectly smooth. I ran the engine up to full power but it topped out at only around 2600RPM. I checked the left mag, and the engine died. Started it again, checked the left mag, and it was fine. Ran it up to 2000 for a minute, checked the left mag, and it died.

The mag is almost 1000 hours old so someone got good use out of it but I guess I'm due for a new one.

I just got this plane out of the shop and now I'm dragging it back in...

Edited October 9, 2015 by lfgbrd

[kortopates]

May not be a mag issue but an ignition switch issue; especially if it immediately dies - i.e. not just rough running to a slow death but immediately cuts out. Corrosion builds up in the switch. But independently of that, especially if its a slick mag, one should not skip the 500 hr Mag IRAN - there is no way to see the internals of the slick mag, including points without opening it up. No need to get a new one, just send it out to shop specializing in magnetos for the 500 inspection such as Aero Accessories in Van Nuys - they turn them around in 24 hrs of shop time and very reasonable. Lots of other choices too.

Edited October 9, 2015 by kortopates

[lfgbrd]

We tested the ignition pretty thoroughly and couldn't find any sign of of a short or loose connection. Not ruling it out, though, and the mechanic will check it more thoroughly when the engine cools down.

I'm trying very hard to have the plane flying by next Friday...I think we're just going to express ship a refurb/reman and maybe see if we can repair and sell the old one. Probably not the cheapest way but I'm more worried about time, as much as I hate to be that way.

Edited October 9, 2015 by lfgbrd

[Shadrach]

My F model has a pretty large spread with 1 and 4 being coolest. I have trouble keeping them above 290 in the winter. For whatever reason, I can find a sweet spot around 18-20 LOP where the even out to within 20 degrees of one another. Anything richer or leaner from there and the spread widens.

[Bob_Belville]

My F model has a pretty large spread with 1 and 4 being coolest. I have trouble keeping them above 290 in the winter. For whatever reason, I can find a sweet spot around 18-20 LOP where the even out to within 20 degrees of one another. Anything richer or leaner from there and the spread widens.

Ross, what does your GAMI spread usually look like? Can your EDM show you the LOP delta for all 4 cyls? In other words, is your "18-20 LOP" the leanest, richest, or an in between Cyl?

[Shadrach]

Ross, what does your GAMI spread usually look like? Can your EDM show you the LOP delta for all 4 cyls? In other words, is your "18-20 LOP" the leanest, richest, or an in between Cyl?

Our JPI is pretty archaic (15 yrs old). I installed a transducer with plans to upgrade the box with FF, but have not gotten around to it. I don't know my exact Gami spread. I just know its pretty tight. 18-20 LOP would be cyl#2 which is my richest. Cyl#3 is my leanest and is typically also the hottest in climb. However, since my baffle tweaks it usually runs a few degrees cooler than #2 in level flight. All 4 cylinders are within ~ 15df of one another from peak. That is to say if I lean #2 to peak, all the rest will be between peak and ~15LOP (#3). I don't have GAMIs.  On the deck in winter anything over 30LOP can cause #1 and #4 to dip below 290. 

[Bob_Belville]

Our JPI is pretty archaic (15 yrs old). I installed a transducer with plans to upgrade the box with FF, but have not gotten around to it. I don't know my exact Gami spread. I just know its pretty tight. 18-20 LOP would be cyl#2 which is my richest. Cyl#3 is my leanest and is typically also the hottest in climb. However, since my baffle tweaks it usually runs a few degrees cooler than #2 in level flight. All 4 cylinders are within ~ 15df of one another from peak. That is to say if I lean #2 to peak, all the rest will be between peak and ~15LOP (#3). I don't have GAMIs.  On the deck in winter anything over 30LOP can cause #1 and #4 to dip below 290. 

So, if I get the picture, with # 3 @ 20LOP the others will be closer to peak which will raise their relative CHT compared to #3 and makes the CHT difference tighter.

[Shadrach]

So, if I get the picture, with # 3 @ 20LOP the others will be closer to peak which will raise their relative CHT compared to #3 and makes the CHT difference tighter.

Yes sir.  Although I attribute some of the balancing to baffle work as there was a time when it was always significantly hotter regardless of power setting.

[Oldguy]

I had a similar problem with my mag, i.e. killing the engine when switching to a single-side during run-up test. Fought it for a while until I realized I was within 3 hours of the 500 hour rebuild. Sent it off and have had zero problems since then.

If you say someone got 1000 hours out of the mag, and since you have a J, I am making the assumption you have the -A3B6D engine with the dual headed mag. If yours has not been rebuilt within that 1000 hours, it is way beyond its time to have that done. Misfiring due to arcing within the mag could also explain some of the backfires you mention in your original post.

About 0.5 AMU for a rebuild on my mag earlier this year, but worth every penny! Good luck.

[lfgbrd]

 the -A3B6D engine with the dual headed mag.

Actually it has two mags. I forget the engine suffix. They both had a 500hr inspection, the right one got fixed but it looks like the left didn't have any issues. That was about 500 hours ago, though...

[Shadrach]

Mine are staggered. My rt mag has ~ 300 hrs since IRAN and the left has about 750. I will IRAn the left when it hits 1000 and then the right.  I think this is a good approach based on my experience. Opening them both every 500 is overkill in my view and staggering them means that at least one known quantity is always in service.