Troubleshooting High CHTs

[FlyingMonkey]

Apparently I can't get from maintenance mode to flying mode this Spring... 

I have noticed over the last few months that my CHTs have been running hotter than usual, 210-225° in cruise.  Today I noticed temps getting hot in taxi...where my oil temp was 140-150°F, my CHTs were 205-210°F.  As I took off, my CHTs started climbing fast.  By the time I turned crosswind, they were at 240°F and pushing toward the 250°F redline.  I aborted and landed.  I checked everything over and found nothing obvious.  No coolant seeps or leaks, there was plenty of coolant in both the plastic reservoir and the metal "spider" tank.

I thought (hoped) it might be a sensor issue, and my hangar neighbor offered to switch in one of his CHT sensors to test.  when he took his out coolant came out, and when we pulled mine it was dry.  We started thinking about a water pump failure...but another Rotax owner since told me there were two different head designs and the older like mine have CHT and the newer (I think with scallop cut valve covers) have Coolant Temp.  That would explain why mine might be dry.

I have another unused/not connected sensor on the other side of the engine.  Is it worth swapping the sensors to troubleshoot, or is this definitely a water pump failure?  Or something else and probably worse?

[Anticept]

CHTs that go in the bottom are true CHTs.

CHT probes that go in the top are coolant sensors. There's two subvariants of these but basically they're installed in a way to pick up the state of the coolant.

Your rotax operator manual will tell you temperature tolerances for each type. Flight Designs were set to coolant limits from the factory, even the SWs.

Failed water pumps heat up to boiling point extremely fast.

Your radiator might be having issues too. Make sure baffling is in place and blow out the radiator fins with either a pressure washer stream at a lower setting or use an engine bay cleaner with mineral spirits followed by alcohol.

[FlyingMonkey]

22 minutes ago, Anticept said:

CHTs that go in the bottom are true CHTs.

CHT probes that go in the top are coolant sensors. There's two subvariants of these but basically they're installed in a way to pick up the state of the coolant.

Your rotax operator manual will tell you temperature tolerances for each type. Flight Designs were set to coolant limits from the factory, even the SWs.

Failed water pumps heat up to boiling point extremely fast.

Your radiator might be having issues too. Make sure baffling is in place and blow out the radiator fins with either a pressure washer stream at a lower setting or use an engine bay cleaner with mineral spirits followed by alcohol.

Thanks Corey! 

Yeah, mine are in the bottom (CHT) and my buddy's are in the top (Coolant).  Until recently my oil temps were always the limiting factor, my CHTs would heat to to about 200-210°F in the summer and then just stay there.  Oil temp gets to be a problem here in Georgia in the Summer, requiring shallow or step climbs sometimes.

During my current unending maintenance cycle I replaced worn baffles both under the radiator and around the muffler shroud inlet.  Coincidentally before today's flight I washed the airplane and carefully washed out the radiator fins with water and Dawn to break up any oil.  I don't think the radiator is the problem unless there's something wrong internally. 

So my first attempt to fly today CHT hit 230°F just taxiing to the runway (89°F OAT).  I went back to the hangars and checked everything over and replaced a suspicious carb vent/balance tube.  When I went back out later the temps were better (205°F in taxi), but it was also a little cooler then (85°F).  Still too hot, IIRC I usually see 175-190° in taxi this time of year.

It feels like it has to be a sensor or the pump, but I'm guessing if a sensor fails it reads zero or just reads a fixed number.  The "heat up to boiling point extremely fast" seems to be what I experienced.  Once I broke ground the CHT went up about 10°F every 30 seconds or so, and when I pulled back and set up for an immediate landing it looked like it was going to keep going and blast right through the 250°F redline on my gauge.  I've never seen CHT do anything like that.  Interestingly, temps were back down to about 205-210°F when I rolled cleared the runway and got on the taxiway...not sure if temps would come down that fast with no circulation. 

As I said, I have noticed higher CHTs for a couple of months now, I wonder if the pump was struggling and finally gave up the ghost.  I looked at the pump diagrams...it looks like pulling off the housing and replacing the following part numbers might be warranted:

Impeller                           #922224    #8 below

Pump Shaft                     #837112     #5 below

Seal Rotary Assbly          #850946   #4 below

Oil Seal                             #850978   #3 below

Gasket                              #850981   #9 below

Can anyone confirm if that is correct or if there's more to it?  Is there one part like the impeller that is normally the culprit and the rest can be left alone?

Also let me know if I'm off the reservation and need to troubleshoot other possible causes.

 

[cdarza]

I was experiencing high CHT's -  I would be getting close to red line on take off.  I eventually installed a second cht probe and gauge just to get comparison (different cylinder)  and i was seeing normal readings on the newly installed one.    After much troubleshooting,  i had the wires replaced on the old/ original cht.   All good since then.     As to my oil temp - ive had it go from normal to red line when i switch the auto pilot on or even when i press the PTT.   Ive managed to fix that for now by cleaning the connections but i expect the problem to come back as it has in the past.    Hopefully your hot chts are just the 'bad wiring' but obviously best to rule everything else out.

[FlyingMonkey]

55 minutes ago, cdarza said:

I was experiencing high CHT's -  I would be getting close to red line on take off.  I eventually installed a second cht probe and gauge just to get comparison (different cylinder)  and i was seeing normal readings on the newly installed one.    After much troubleshooting,  i had the wires replaced on the old/ original cht.   All good since then.     As to my oil temp - ive had it go from normal to red line when i switch the auto pilot on or even when i press the PTT.   Ive managed to fix that for now by cleaning the connections but i expect the problem to come back as it has in the past.    Hopefully your hot chts are just the 'bad wiring' but obviously best to rule everything else out.

That's good to know, hooefully it is just wiring or a bad probe.

[Madhatter]

I was getting a lot of erroneous indications on my temperatures.  I replaced my connectors with gold plated ones and never had another issue. 

[FlyingMonkey]

45 minutes ago, Madhatter said:

I was getting a lot of erroneous indications on my temperatures.  I replaced my connectors with gold plated ones and never had another issue. 

I guess maybe the first thing to do is switch out the sender and re-test.  Then pull the sender wire out and replace it end-to-end including the connectors.

[Anticept]

Does it read ambient temperature before you start?

If BOTH sensors are reading together, chances are you do have a problem.

As for replacing the water pump impeller: it's not difficult but it does require a crapload of work on a flight design. Engine has to come off to get enough access.

Drain the system, pull the hose off the back of the pump, and stick a borescope in there just around the bend. Have someone rotate the prop. If the impeller turns and you don't see damage or no clearance from impeller to housing, that's not your issue.

PS: installing a new impeller requires you to be very very careful. you can break some serious things, real the manual carefully and check your clearances as you tighten the housing.

[FlyingMonkey]

7 hours ago, Anticept said:

Does it read ambient temperature before you start?

If BOTH sensors are reading together, chances are you do have a problem.

As for replacing the water pump impeller: it's not difficult but it does require a crapload of work on a flight design. Engine has to come off to get enough access.

Drain the system, pull the hose off the back of the pump, and stick a borescope in there just around the bend. Have someone rotate the prop. If the impeller turns and you don't see damage or no clearance from impeller to housing, that's not your issue.

PS: installing a new impeller requires you to be very very careful. you can break some serious things, real the manual carefully and check your clearances as you tighten the housing.

Yes, it reads temp when the master comes on.  I have fixed that wire near the sensor with a butt splice, it got some heat damage and I had to replace the last 3-4 inches near the spade connector.  It's possible the wire just needs to be entirely replaced. 

My setup only has one active CHT sensor, the one on right rear cylinder #3.  I have the small basic UMA analog CHT gauge.  There is a sensor installed in the other side on cylinder #2, but it's not used.  So I can swap those easily and check for a bad sensor.  If that doesn't do anything I can trace, re-run, and re-terminate the sensor wire to make sure there's nothing wrong there...though that might be a PITA to terminate on the gauge end, IIRC it goes through a hard sheath and into a D-Sub connector.  Failing all that I can swing the engine off.  Now that I have a hoist in my hangar it's not a huge deal to get the engine away from the firewall a few inches.

Instead of using a borescope, can I just pop the large hose elbow fitting off the back of the pump housing (the one with the o-ring)?  The impeller should be right there if I do that, and I can inspect the whole thing easily for movement and any damage.

 

If it's not the sensor, wiring, gauge, or pump, what could be the problem?  I've never heard of a 912 cooking a cylinder without a really obvious reason.

Thanks!

[Tom Baker]

A quick check would be to run it and measure the temperature on the outside of the coolant tank on top of the engine. You can use either an infrared thermometer or one of those strips like they sometimes stick on the ignition modules and regulators.

Also since you just did your hose change, check your grounds. 

[FlyingMonkey]

11 minutes ago, Tom Baker said:

A quick check would be to run it and measure the temperature on the outside of the coolant tank on top of the engine. You can use either an infrared thermometer or one of those strips like they sometimes stick on the ignition modules and regulators.

Also since you just did your hose change, check your grounds. 

Good points.  What temps should I be seeing from the spider tank if the cooling system working normally?  Is there any way to diagnose whether a pump is failed or not without opening it?

[Anticept]

You would be reading colder than your CHT, but not way colder.

I suggest you get a second cht installed even if only temporarily.

With the fact you only have one sensor, I would consider that you have a restriction in your system possibly only to that cylinder. I have seen it be a problem before.

[FlyingMonkey]

14 minutes ago, Anticept said:

You would be reading colder than your CHT, but not way colder.

I suggest you get a second cht installed even if only temporarily.

With the fact you only have one sensor, I would consider that you have a restriction in your system possibly only to that cylinder. I have seen it be a problem before.

I don't have a way to read out two CHT's I'd need a whole new gauge installed.  I could run the existing wire across the engine to the #2 cylinder probe and read that out and compare it, but I could not compare them in real time.

How would a restriction occur to just one cylinder?  I can see the hoses in/out of that cylinder and they are not warped or kinked in any way.

[Anticept]

Rotax CHT sensors are not special. They're just VDO Temperature Sensors, thermistor type.

The rotax installation manual, 76-00-00, has a resistance graph that correlates temperature to resistance.

Using that, since you are E-LSA, you can literally go to a car parts store and buy a gauge for cheap as long as you match it all up. Or even just stick an ohmmeter on wires running to the cockpit and keep the chart handy.

You don't even have to buy a rotax sensor either. As said, they're VDO temperature sensors, which comes in various shapes and sizes, you could find a third party source.

[FlyingMonkey]

2 minutes ago, Anticept said:

Rotax CHT sensors are not special. They're just VDO Temperature Sensors, thermistor type.

The rotax installation manual, 76-00-00, has a resistance graph that correlates temperature to resistance.

Using that, since you are E-LSA, you can literally go to a car parts store and buy a gauge for cheap as long as you match it all up. Or even just stick an ohmmeter on wires running to the cockpit and keep the chart handy.

I see, thanks.

 

[Anticept]

PS I believe the thread well size of the sensors are 10mm x 1.5mm pitch.

You still have to take a depth reading and make sure the thread engagement area isn't too long either, as said there are many variations available, but if you find one it will be all of 25 dollars instead of 200.

[FlyingMonkey]

I switched the sensor for the unused one on the other side of the engine and cleaned up all my grounds (again...recurring CT theme).  This problem seems to have resolved itself -- I made four flights over the long weekend in OATs around 75°F with CHTs all normal.  CHTs come up to about 205°F taxiing to the runway, then once in the air they stayed around 190-205°F depending on phase of flight and engine rpm.  If I pull the power back to 4000rpm and/or descend at reduced power the CHT drops pretty quickly to 150-180°F.  

To my memory this is exactly the behavior I remember previous to this issue coming up.  Still waiting for a typical Georgia early Summer day at 85°F or higher to say for sure (it's been unseasonably mild last the couple of weeks), but I think this is a done deal.

[Anticept]

Don't throw away sensors unless you confirm they're bad, those things are $$$$ from rotax!

[FlyingMonkey]

24 minutes ago, Anticept said:

Don't throw away sensors unless you confirm they're bad, those things are $$$$ from rotax!

Yeah, I just swapped them.  The possibly bad one is on the other side now where there is no monitoring on my airplane.  I never throw anything away, ask my wife!  

I think I still have every part that ever came off my airplane.