The restrictor plug addresses a design flaw with the cooling system. See my thread on Winter Thermostat from post 26 for further details. The plug fits all Smart ForTwos with OM660 Mercedes engines. Does not fit Smarts with Mitsubishi engine. The plug will also fit the OM160 petrol engine but probably no significant gain as the thermostat fitted on this engine already has a restrictor built in.
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Below photo shows the restrictor plug on the left. Plug shown is an earlier version with smaller bore size. The plug fits in the V shaped branch piece (shown on right) in the radiator bypass line from thermostat housing to coolant pump.
Kindly note that the V shaped branch piece on the right is not included. The restrictor plug fits in the upper branch of above V shaped branch piece. There is one of these V shaped branch pieces already in your Smart so please note that you don't need to buy one of them.
Above photo shows the restrictor plug (current version is near white in colour as normally turned out of white nylon bar) fitted into upper branch of V shaped branch piece.
Kindly be advised that the EGR cooler which is fitted to Canadian specification Smart cars, is not shown in above diagram.
The plug creates a small back pressure, thereby slightly increasing pressure in thermostat housing. The small pressure increase is sufficient to make the check valve* fitted in the thermostat disc seal positively so loosing less valuable heat when engine is cold and warming up. Flow through heater matrix is increased as well so higher heat output and warmer cabin. The check valve in the thermostat will wear over time mainly due to vibration causing increased leakage through check valve. The restrictor plug will increase service life of thermostat since the higher pressure differential will keep check valve hard up against its seat causing less wear due to vibration.
* The check valve is simply a piece of brass fitted loosely in a hole drilled in thermostat disc.
The main reason why check valve does not seal is due to the orientation of the thermostat on the engine - thermostat is sitting on its side and not horizontally which is more common in engines. The brass check valve therefore sits at an angle and does not seal against its seat unless there is a small differential pressure accross the thermostat. Smart could have corrected this design flaw by fitting a check valve made out of a plastic material having neutral bouyancy in the coolant.
The restrictor plug relies on a fully functional thermostat. I recommend you test your thermostat before fitting this device or one you have made yourself.
The branch where water trickles out goes to radiator. The branch that is sealed by my thumb goes to heater matrix. The branch to the right, which is partly hidden, is radiator bypass. This branch goes to cooling pump.
The thermostat shown is brand new and leaks a wee bit, which is normal and main reason why the engine takes a long time to come up to operating temperature. Try sucking on it. It should seal. Suck from the barb that connects to radiator hose. Replace the thermostat if yours does not seal when sucking hard.
You need to slightly drop engine and subframe to gain sufficient access to fit the plug.
There is info about development of the plug on the Canadian Club Smart Car forum. Post no 49 is worth while reading.
There is information about the special "Clic" hose clamps on OnSmart forum.
I recommend you reuse the existing high quality Clic hose clamps but you will then need a special "Clic" compatible fitting tool. Alternatively use good quality stainless steel Jubilee clamps.
Hose clamp diameters:
V-piece at water pump 33 mm.
Thermostat outlet to radiator 33 mm.
Thermostat outlet to heater matrix 22.5 mm.
Thermostat outlet to V-piece 27 mm.
Now is the time to fit the plug if you wish to enjoy a cosy Smart when temperature gets lower.
There are three items of important maintenance you can easily carry out same time once you have partly lowered engine and subframe to get access for installation of restrictor plug:
1) Starter motor. The spade type connector to solenoid is open to the elements so prone to bad contact and failure to start. I recommend you clean the connector and apply some protective non conductive grease such as white grease, battery pole grease or Vaseline.
2) Hand brake equaliser mechanism. The mechanism will rust and seize up resulting in hard hand brake lever. Keeping the mechanism wetted in grease is good preventative maintenance. You can also open out the complete mechanism and overhaul it, see thread on OnSmart forum for details: http://forum.onsmart...;p=26279#p26279
3) Support cradle for intercooler. The version up to late 2006 eats and cuts into lower part of intercooler resulting in leakage and costly repairs. See post on OnSmart for details: http://forum.onsmart...php?f=13&t=3070
Note regarding Gates thermostat: Gates TH40390G1 thermostat is very poorly made and both its disc and check valve do not seal. The restrictor plug will therefore not improve the performance of the Gates thermostat.
Edited by tolsen, 17 December 2015 - 01:05 AM.