912 high coolant temps, but CHT's OK?

Hi,
I'm trying to diagnose a persistent issue with high coolant temperatures, with a 912 engine in a 1999 Zenair Zodiac HDS. It has the rads inside the cowl, on either side of the engine. Unlike a properly designed system, there isn't a duct that forces air into the front face of the coolant rad, only some metal that attempts to channel air in and out, within the cowl. It isn't admittedly the best design. (The design is a semi home made system, at a time when Zenair owners were trying to get away from the draggy VW rad installation below the fuselage, but before the factory had a cowling that properly integrated a small rad into it.)

I could summarize the issue by something I wrote years back about the aircraft:

"During summer weather of 25-30 C on the ground, CHTs never rise above 210
at any time on the ground or in the air. Which sounds great if one is using
CHT to indirectly monitor coolant temp. And it is lower than the CHT's some
others have reported for their 601s with in-cowl rads, after a climb out in
hot weather.
But the actual coolant temperature sensor would often show a rise up to 225
or 230 F after a 1000' to 1500' climb out. The temperature would continue
to rise unless I reduced power (or perhaps levelled off). I never let the
temp go over 240. "

The engine does have the newer 1.2 bar rad cap.

I hardly ever hear of CHT's being higher than coolant temps.

Both are measured with a Grand Rapids Technologies Engine Information System, a digital LCD engine monitor. Two CHT's are measured with ring type senders, and one of the original Rotax supplied CHT senders, a probe type from VDO, is moved to the coolant system. In this aircraft, that is threaded into the top of the radiator.

The only time I've read of CHT's higher than coolant temps, is unusual cases where the coolant circulation system was totally inadequate, having very limited flow rate such as from undersized fittings. Can it also occur if the coolant rad isn't getting adequate airflow through it?

Indeed, in a recent Rotax manual (installation manual?), Rotax gives a procedure for setting CHT limits based on the concept that the coolant temp is typically about a fixed number of degrees lower than the CHT. Although on another page it only says that CHT's are "usually" higher.

In the old days of the 912, people didn't even bother to measure coolant temp -- if the CHT's were good, coolant must be good. And I even heard people call their CHT's "coolant temperature" as it was supposed to be measuring the temperatures of the coolant cooled heads. Later on (~2006) Rotax got pickier about measuring both CHT's and coolant.

I'm just in the process of getting a decent thermometer to better confirm that the EIS is reading right. I test with hot water on the ground with an old meat thermometer was inconclusive whether the EIS was reading right or a little high.

Any ideas about whether or why I should be seeing coolant temps well above CHT's?

Peter C.
Toronto, Canada

We are seeing the same issue with our Rotax 912ULS and a Zenith 601HDS we recently bought. On climbs, the Coolant reads up to 257 and even 260, while both the oil and CHTs are in 230s, closely matching each other. On straight and level, I get 220 oil temp, 215 Coolant while the CHT reads 176. (Outside air temp 85).

It seems odd to me that the coolant would read higher than the CHT, given that the cylinders are where the heat is generated. How can the coolant be higher in temp than its source?

Could it be that the wiring for CHTs and coolant are cross-connected? Seeing your posted issue, could it be that there was a mistake in Zenith wiring diagrams?

I would appreciate if anyone could share their ideas. Maybe we can solve this mystery together.

I had the exact same scenario and I hear more and more similar cases like it. I am changing the cap for $80 as we speak. . I think in the hot weather just like any other engine our temperatures run hotter than normal and it is not anything the engines aren't built to run in. Just like in the winter they run cooler. Mine blew out the coolant so it can be a problem.