ZZ8S Temperature Fluctuations

[BasaltRMK]

Hello all. 2024 G23 with the ZZ8S. We've had a couple of coolant leaks (hose fitting leaks) that the dealer has twice repaired under warranty, including this past winter. The dealer also had to replace the thermostat shortly after we bought the boat - we got a temp spike issue on one of our first outings.

This season, we're seeing some new temperature fluctuations. The normal operating temps for our boat typically range between 163 - 167 F. This season, every time we first get under throttle (to pull up a surfer, accelerate to cruise at speed, etc.), the engine temp climbs to 174 - 176 for 15 - 20 seconds, before slowly lowering back down to the typical range and staying there. Sometimes spikes as high as 178. In addition, when just cruising at idle speed, the temp will sometimes climb up to 178, lower back down to 163, and continue this fluctuation every few minutes.

Any idea what might be causing this? Thermostat going bad again? But we did not see these fluctuations the first two seasons with the boat, so would like to diagnose and address it. I replaced the impeller at the start of the season, the old one was in good shape and not missing any chunks that could be causing a clog. I pulled / reconnected the 6 winterization drain lines to see if may there was a trapped air bubble. Sea strainer is clean, o-ring is in place and doesn't appear to be pulling air or leaking. No water in the bilge. Coolant level is good, not leaking for a change.

Hoping it's the thermostat, but curious if anyone has ideas. Dealer is a couple of hours (each way) away, so hoping this is a DYI fix.

Thanks,

- bob

[jpwhit]

How long since the engine impeller was last changed? These kind of fluctuations are most often caused by insufficient raw cooling water flow. If the impeller is fine, then I'd suspect an air leak on the source side of the raw water pump.

[BasaltRMK]

JP - impeller was replaced in May of this year. As noted, no damage / issues with the old impeller. Thanks,

- bob

[Scooter G]

I'm guessing that these closed systems also have a water pump to circulate the antifreeze, or is this a function of the intercooler? Am I way off base on this? This is a new to me system, so not in my wheelhouse (yet). It's bizarre to me that the thermostat would be bad from the get go, if there is one out there, there are 2. I have seen a bad thermostat cause temp flucs. I do like jpwhit on the air theory, you wouldn't likely see anything at idle, but under load, doesn't take much to suck some air. I know that Natique had a run of bad intercoolers there for a bit, but I think you good with a '24.

[BasaltRMK]

Thanks gents. Will pull the strainer again, check and lube the o-ring, and inspect to try and rule out an air leak on the intake side. And also maybe pick up that $66 (minus PND discount) thermostat... Anyone else please feel free to weigh in!

- bob

[jpwhit]

Can also be caused by partially blocked heat exchanger. You’re describing temp going up when the engine is generating a lot of heat but when the boat isn’t moving fast initially. That’s when the boat is relying on having a lot of raw cooling water coming primarily from the raw water pump. Once the boat is moving, raw water flow naturally increases due to boat forward motion. The fact that it cools down fairly quickly shows that the thermostat is likely working by opening fairy quickly to take advantage of the available raw water. The exhaust system gets hot much more quickly than the engine. I bet if you had a way to check exhaust system temps, you’d see it’s getting even hotter than your engine.

Not saying it’s impossible the thermostat is the problem, just not the most likely cause in my experience. Also wouldn’t 100% assume that just because the dealer changed the thermostat earlier that it was the cause of the earlier issue. Intermittent cooling issue are very time consuming to diagnose. So dealer will often try a thermostat because it’s quick and easy.