I'd like to solicit inputs on troubleshooting a problem I'm seeing with my engine in my 185. I have an IO-520D, and whenever I'm WOT, the #4 cylinder appears to run much leaner than the other 5. This issue shows up anytime WOT, regardless of the fuel flow, so #4's EGT runs much higher than the others when WOT at sea level with 28 gph, or WOT at 10000 ft with only 12 gph. Conversely, if I use partial throttle, all EGT's immediately even out, again regardless of the overall fuel flow (partial at sea level, 24 gph, etc).

I've focused my troubleshooting to the standard three areas (fuel, air, spark)

Fuel:
-swapped fuel flow divider
-cleaned injectors
-swapped #3 and #4 injectors (GAMI equipped)
-removed all injectors and run the pump while letting the fuel collect in glass jars

None of the cleaning/swapping parts made any change, and I didn't see anything noteworthy when letting the fuel pump into the jars (no debris, fairly even levels, etc).

Based on my fuel troubleshooting and that in flight it does not correlate with fuel flow, but throttle position, I'm pretty sure it is not a fuel delivery issue.

Spark:
inflight LOP MAG checks are even and shows expected results: EGT's all rise when on one MAG and the rise is even when comparing the two MAGs.

Air:
I've done the induction leak check suggested by Mike Busch. WOT= #4 acts lean. Any partial setting, all EGTs even. My confusion is that from the readings, I though an induction leak would be indicated by a cylinder running lean at partial throttle since it would be getting extra air through the leak. In my case, the cylinder runs lean when WOT even though the manifold pressure is very nearly ambient.

Some clarifying info:
-the throttle reduction necessary to get the EGT's to run even is about 1.5-2" of throttle, which equates to about 3" of MAP.
-I've thought about an indication issue, but the correlation with throttle position is too strong, plus if left at WOT, the CHT will start to climb on #4, as expected from a cylinder running too lean (on the ROP) at high power settings.
-an example of the issue, at 23" MAP, partial throttle, my GAMI spread is about .3 to .5 gph, with all the peaks around 13 gph. At WOT, 23" MAP, the other 5 cylinders all peak ~13 gph, but #4 peaks at 18 gph. Needless to say, the engine does not run smooth LOP or anywhere near peak when WOT.
-the plane was down all summer, but when I first started flying again in September, the problem temporarily went away (it had this problem prior to the down time). It started doing it again in Nov and has been consistently doing it ever since.

Grasping at straws, I'm wondering if for some reason there is an air vortex or something going on in the intake when WOT. I'm at a loss, and think I'll try taping over the alternate air door this weekend to try something. My long shot thought being maybe the spring on the door is too weak and it is coming partially open when WOT, which is disturbing the airflow in the induction.

Anyone have any ideas or other tests to help narrow this down?

I might have missed it, sorry if so.. but did you change EGT probes around/new probe?

I did not change the probe. I don't think there is a probe problem for two reasons: 1, the CHT changes appropriately for how the cylinder is running (it increase in temp when WOT, full rich, due to that cylinder not being rich enough, and it dramatically decreases in temp when LOP and WOT, because that cylinder becomes too lean for combustion and goes cold.
2. my experience with bad EGT probe has been the time before complete failure, the indication becomes very erratic. My indication is not erratic at all. It is 100% repeatable and is always steady at whatever temp it is showing.

You said the #4 CHT will climb but how high does it get to above the others before you change the throttle position?

a3holerman wrote:You said the #4 CHT will climb but how high does it get to above the others before you change the throttle position?

It will climb about 200 degs higher than where it stabilizes at partial throttle. Two examples below

initial takeoff:



powering up to climb after an initial level off:

Have you done a physical induction leak check (pressurize the intake and use soapy water to look for bubbles)?

The times I had obvious rough or abnormal running, seems like the rubber intake hose material was the culprit. A small intake leak can make a dramatic impact on a 185.

https://m.youtube.com/watch?v=CfG4QGv-r64

I have seen two low time TCM overhauled engines with induction leaks right from the factory. One was a newly overhauled O-470 that had high CHT and egt on one jug and the mech looked at it twice before he discovered a distorted intake gasket on the back side. It turned out during assembly the gasket was quite off center and one intake flange bolt actually had too punch an almost a completely new hole in the gasket during assembly. Another, my IO-520, had a similar issue but not as severe.
Quality control at TCM was/is terrible.

Look at the fuel distributor (top of the engine). The same pressure of fuel is sitting right there. If one cylinder is getting less fuel, then its (pretty much) gotta be downstream.
Assuming all easy stuff checked already, (spark/compressions/injectors/intake leaks/etc etc) its about gotta be there. Check and make sure line from distributor to injector isn't kinked/dinged first.

I had a piece of metal (like the size of a hair and 3/8" long in one and I'd get some similar indications. Then one day it moved from one cylinder to the other. (just floated in front of the next port) That'll make you think!
Anyway, I think you need to look there- swap it (not supposed to takem apart in field I'm told.....) or get someone with experience to inspect it. Don't tear the diaphragm, and you likely will if its very old.

Good luck.

Is there any chance that the line going from the distibuter to that cylinder is a different length? On the continental system it's very important that the line be the right length. Seems like you've covered most other bases, so I'm just throwing out the only other thing I can think if right now.

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Fuel Line?
Ignition Line?

Thanks all for the inputs so far. I haven't had a chance for troubleshooting this weekend. The fuel distributor has been swapped with no change. The fuel line from the distributor to the injectors are all the same length; I've thought about swapping the 3 and 4 lines since those two are the closest to having the same bends, but I'm concerned about bending them too much, so I haven't gone down that road yet.

The SaavyAnalysis folks asked me to run a few more inflight tests but I haven't had a chance yet with the weather. I also got a suggestion to try swapping to a smaller intake manifold crossover tube, since that can cause weird distribution issues (so I've heard) if not sized properly. I like the idea, but I need to see if I can source one to borrow for troubleshooting before I buy one.

Thanks for the inputs; I'll keep the forum posted as/if I make any progress.

I wanted to post an update since it may help someone else in the future. I got my problem fixed. I didn't touch the fuel system, for all the reasons I posted above. I replaced the spring for the alternate air door in the induction airbox. The new spring has just a bit more pressure keeping the door closed. No other changes (outside of a routine oil change) were made and during today's flight I was able to use WOT without cooking my #4 cylinder for the first time in longer than I'd like to admit I had been living with this issue. Thanks again for the inputs along the way.

This is a big relief to finally get this solved.

Glad you found the problem and thanks for reporting back on it.