CHTs on an M20F

[danb35]

I spent a little time this weekend massaging the downloaded data from the EI UBG-16 in my M20F, and I'm seeing something that strikes me as odd.  I'd noticed during flight that my coldest cylinder was #1 (front right) and the hottest was #2, but I hadn't really paid attention to the difference.  Turns out that, in cruise, #2 is running around 340 deg., while #1 is running around 240 deg., about 100 deg. cooler.  That just doesn't sound right to me.  I can see a way that the front cylinders would be the hottest ones, and I can see a way that they would be the coolest ones, but I can't really see a way that one would be the hottest and the other the coolest--especially by 100 degrees.

The CHT probe on #1 is a 3/8" washer type under the factory CHT probe, and I'm entirely willing to believe that this has something to do with the probe, but I'm looking for a sanity check on the CHT readings before I go trying to track down a probe problem that may not exist.

[GeorgePerry]

The CHT's on my M20F usually differ by less than 30 Degrees during climb and are all very similar in cruise (See attached Pic).  In your case, 100 degrees seems excessive.  It's possible that you've got a bad probe...at least I'd start there.

[danb35]

Like I said, it didn't seem right to me either.  At ambient temp (cold engine), Cyl #1 agrees with the other three exactly (+/- a degree or so), but as the engine warms up, the discrepancy grows.  It tracks the changes in CHTs very closely (when the other cylinders warm up, so does #1), but it's just way lower on a running, flying engine than the others.  Probably time to check with EI tech support.

As I mentioned, this is a washer probe that sits under the factory CHT probe.  It then has a short section of firesleeve zip-tied around the two probes, which is supposed to contact the cylinder head--I presume the purpose of this is to provide some insulation, to keep the two probes near the same temperature.  Doesn't seem like much to go wrong, but it seems like something is.

[Magnum]

The best setup for the probes is to use a spark plug washer probe for the factory gauge and a "normal" probe screwed into the cylinder (like the other three) for the engine monitor. Then the measurement for all cylinders is the same.

[DaV8or]

Quote: danb35

I can see a way that the front cylinders would be the hottest ones, and I can see a way that they would be the coolest ones, but I can't really see a way that one would be the hottest and the other the coolest--especially by 100 degrees.

[N201MKTurbo]

First, the factory CHT is usually on cylinder 3.

I chased a cylinder misfire problem for way to long one day until I discovered that the probes were on the wrong cylinder. You can verify that they are correct by removing them one at a time and heating them up with a cigarette lighter and verifying that the right one is increasing on the display.

[danb35]

The factory probe on '48Q is definitely on #1 (right side, closest to the prop).  I can't guarantee that it came that way from the factory, but it was that way when I bought her.  I'd just as soon put in the same style probe as I have on the other three cylinders and disable the factory CHT, but I don't think that would be legal.  It'd probably be a worthwhile experiment, though.

Dave, the engine runs quite smoothly LOP, so there can't be much difference in fuel flow.  Spark plugs were clean in all cylinders at annual a few months ago (since I run almost exclusively LOP).

I suspect at this point that the problem is either in the probe itself or in the installation.

[Ron McBride]

On my JPI 700, they installed a split type probe, so that all cyls and the factory gauge are reading from the same point on each cylinder.  I have a stock 69F with the guppy mouth and rarely get over 350 degrees. In fact during the winter I close the cowl flaps even on takeoff, it is hard to get the temps into the green.

 

Ron