Caterham 485 Coolant Temp Drop

[Lancefitzgiben]

Hey guys, having a little issue with the cat.

 

I was having a little leak from the water rail, as the coating on it was compromised and letting a bit of coolant out. That got air in the system and I had big temperature spikes. The water rail got a cerakote and we filled the system with an airlift. The car now warms up just fine (heater valve open) and temps act normal - until I exceed 4k rpm, at which point the coolant temp begins to fall off. If I'm warmed up (80c) and give it a squirt up to about 7,500 rpm, the coolant temp drops by about 15c. Once cruising again the temp slowly rises back to normal and stays, unless I use the upper half of the tach.

 

General symptom searches indicate the thermostat as a possible culprit. Sounds logical enough, but I'm looking for some more opinions. I'd rather get the issue fixed with one more drain/fill, as it's been done a few times now. I'm guessing the other possibility is a sensor?

 

2014 485S dry sump, with heater, about 4200 miles.

[TurboWood]

What does the gauge read if the sensor wire is grounded or open circuit? Sudden changes like that strike me as electrical. It's hard for me to buy a thermostat issue unless somehow it's letting surges of cold water out (not sure how that would work). Maybe the higher pump speed is driving something? Does it happen if you free rev the engine?

 

One wild thought would be a plugged radiator. Maybe the higher pump speed is able to momentarily force flow through?

 

Daniel

[Lancefitzgiben]

Temp does creep down when revved over 4k while stationary, but doesn't cool as quickly (no air flow?)

 

Your theory about the extra flow forcing through a restriction was how I was making sense of the thermostat theory. I was thinking that maybe with lower rpm flow, coolant wasn't really getting to the radiator as much.. then some extra flow was forcing that cooler fluid into the system.

 

Do I have more than the one coolant temp sensor that's on the back of the head? I'm not even sure how to verify the functionality of a sensor.

 

- Tim

[Lancefitzgiben]

Tony at Caterham said it's normal for the temp to drop some with rpm as it forces coolant through the thermostat. I'll go for a longer drive this weekend and see if things stabilize as the car gets fully warmed through; though I don't anticipate change. Going to order in a stat and sender in the meantime.

[maurice1]

My Duratec Birkin does exactly the same thing. Once I exceed 4000rpm especially in 4th and 5th gears the temperature drops. Its is very reassuring during spirited driving or track days to know I don't have cooling issues. I just put it down to the increased airflow through the radiator and maybe also the water pump/cooling system working more efficiently at higher rpm.

 

Maurice

[Lancefitzgiben]

To update this:

 

I took that longer drive and got the car really and truly fully warmed through. The condition persisted, though not as dramatically. This is certainly new behavior, so nothing 'normal' about the extent of the temp swing. A bit of swing I could see, but this is a drop of like 20C.

 

New thermostat is in the car, currently waiting on the temp sender, which I figured I may as well do while the system is drained again. I'll report back in a couple weeks - I miss driving the stupid little thing!