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Perkins, no thermostat?

My engine is a Perkins 100 series with a Perama Marine conversion, indirect cooling. On a sunny day going against the current on a river the engine temp gets to 70°C. On a canal going slower to about 40°C and on a cold day it barely rises off the bottom of the dial. I assumed either a thermostat stuck open, or missing. I have a downloaded copy of the manual and there appear to be two variants, one with the thermostat behind the water pump, one with it in a housing. I can't see a housing so I took the pump off and there's nowhere behind it to fit a thermostat. Am I being stupid? Or is it possible a previous owner has built a Franken-engine using parts from both types so there is no place for a thermostat?

Asked by: Phil Thane  | 4.24pm, Sunday 4 March

WW says:

It certainly sounds as though your engine cooling system doesn't have a thermostat. The engine temperature will simply end up at a level determined by the heat produced versus the temperature of the raw water.
It might be that the boat was previously only used on rivers at high throttle settings, so would be fine, however for canal work it will run cold. This will bring problems with smoke and poor running.
It should be possible to fit an external thermostat or tap, which will send the raw water round a bypass and back overboard until the engine is warm.

Rupert Smedley  | 5.24PM, Sunday 4 March

Another alternative might be to route some of the hot water destined for the wet exhaust back to the pump inlet. This would warm the inlet water, but would need a tap so it can be turned off for fast running when maximum cooling is needed. A small restriction might be needed to make the water go back to the pump inlet if the exhaust spray nozzle does not produce enough back pressure.

Rupert Smedley  | 11.07AM, Monday 5 March

Readers say:

I've been thinking about adding a tap. Manual would be simplest, but then one would have to keep an eye on the temp gauge and constantly adjust it. It would be possible to make an electronic version using the temp sensor as input, but a nuisance to make and to test. A big problem is that there's no 'top hose' on the cooling system, flow from the pump is straight into the cyl head and the output from the head is straight into the heat exchanger via channels cast in the head. Even re-routing the raw water feed from pump to exhaust will be tricky because the hose from heat exchanger to exhaust is very short, there's not much room to tap into it. Ph well...

Phil Thane  | 7.42PM, Sunday 4 March

Please do not start trying to rely on human controlled taps and bypass systems to correct an incorrect or malfunctioning correctly installed system. A thermostat is designed to maintain a set system temperature regardless of usage and coolant temperatures (within limits) and I find it incomprehensible anybody would install an expensive diesel engine without providing the cooling system with thermostatic control. Thermostats do not hide away behind water pumps, they will be installed in the pressure side of the system from the pump, to restrict or divert the flow to open discharge or through secondary heat ex changers. If the temperature detected is below the optimum engine running temperature the full flow will be returned to the engine to continue to carry out its' job, which in the early stages of running is to help warm the engine block, head and lubricant.
For engine temperatures to vary in the way you have mentioned your first thought should point to a failed thermostat which has failed in the open position (normal). Trace the cooling circuit, even a joint in the pipework could conceal a thermostat. Porsche do it so why not on marine installations. In the event you can not find one , do yourself a favor and save money, ask an expert to look at it. Funny thing is even experts are happy to guide people who want to learn.

John Averill  | 4.05PM, Sunday 18 March

I'm not daft, though maybe I didn't explain it well enough. The engine Perkins 100 series was conceived as an industrial unit. Some versions did indeed have the thermostat behind the water pump, mounted inside the cylinder head. Other versions, according to the manual, had the thermostat inside a cover. But the manual doesn't say where the cover was. I guess if you have that engine it's obvious. But not on mine. It turns out after much research and talking to two Perkins agents that my version as originally designed had the thermostat mounted on the side of the cylinder head next to the exhaust manifold under a cover with a top hose connection to the radiator. The Perama marinised version has a heat exchanger integral with the exhaust manifold so the thermostat housing has been discarded and incorporated into the heat exchanger, you have to remove the heat exchanger or at least slide it along its studs, which in turn requires you to drop the starter motor, just to find the thermostat.
Now I've found it and bought a new one from a Perkins dealer, but there is more weirdness. The (industrial engine) manual says the thermostat should begin to open at 82°C and be fully open at 89°C. My thermostat and the new one are both stamped 75°C. I have heard others say that boat engines usually run cooler, possibly to avoid damage to the wet exhaust hose. Even 75°C would be an improvement.
It seems though that a previous owner had fitted the thermostat incorrectly, it is held in place by a spring and *I think* looking at a not-very-clear diagram the spring should be mounted in the head to press the thermostat into the heat exchanger. Previously it was the other side, trying to press the thermostat into the head where there isn't room for it to fit properly so water would flow past it all the time.
Once the weather improves I'll get back to it, perishing cold in the boat yard today!

Phil Thane  | 7.34PM, Sunday 18 March

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