vlocci Posted December 5, 2009 Share Posted December 5, 2009 I'm trying to diagnose what I believe is a thermostat that is stuck open. My newly acquired 300TDi Defender runs very cool. It has a recent radiator replacement and a new water pump. The thermostat was replaced at the same time, and I fear the thermostat is bad and stuck open. After 30-40 minutes of driving, it will just reach the "C" on the temperature gauge. My newly installed MadMan EMS (incredible product) tells me that the maximum coolant temperature is 55 degrees while driving around town and on the motorway. Needless to say, the heat is about useless. I've tested the viscous fan, and it appears to be free. I can stop it while at idle with a rolled up newspaper. Do 300tdi's simply run very cool, or does this sound like a bad thermostat? Vin Quote Link to comment Share on other sites More sharing options...
gaz489 Posted December 5, 2009 Share Posted December 5, 2009 this is my first post on this site but if im right it should run a little hotter than that but not much and again if im right thermostats have different temp ratings and it will have it stamped on the side somewhere. if you think its the thermostat...... to test it, take the thermostat out of the housing dangle it in a pan of water on the stove along with a thermometer and see if it opens at the temperature stated then when upto temperature if it opens take it out and see if it closes again as it cools down..... if it has been replaced there is a chance it will have been replaced with one of a different temp rating Quote Link to comment Share on other sites More sharing options...
Retroanaconda Posted December 5, 2009 Share Posted December 5, 2009 Definitely sounds like it's over-cooling to me. For the price of one, chuck a new thermostat in and see if it improves things. What temperature are the rad top & bottom and bypass hoses? Quote Link to comment Share on other sites More sharing options...
western Posted December 5, 2009 Share Posted December 5, 2009 fit another 88 degree C thermostat in, make sure it has the small 'jiggle' pin in the central disc to help prevent airlocks Quote Link to comment Share on other sites More sharing options...
mobyone Posted December 5, 2009 Share Posted December 5, 2009 was it running at the correct temp by the old gauge when you got it? did you have to set any perameters in your madman ems? if so, is it correct? what does the original temp senser say, or are you using it to feed the ems? you may not find a thermostat when you check. if all the other options are correct. before you remove it, at 55 degrees that is cold, that will cool your oil too much also. try reconnecting the old sender to the old gauge and see if that reads too cold; if it is easily reconnected. richard Quote Link to comment Share on other sites More sharing options...
suggs Posted December 5, 2009 Share Posted December 5, 2009 mine runs pretty cold too (200tdi), the gauge only just moves into the white under normal running. been like this since ive had the 90 about 3yrs and not had any issues, bar the heater's rubbish. but nothing new there lol Quote Link to comment Share on other sites More sharing options...
vlocci Posted December 5, 2009 Author Share Posted December 5, 2009 gents, The water pump,thermostat and hoses were done before I bought the truck. I can't say what temp it ran at before they were done. I'm using both the original temperature gauge (and sender) as well as the newly added Madman gauge. I verified that the madman gauge is recording the right temperature using a cup of boiling water. 46-50 degrees is as warm as I can get the truck is daily driving. I managed to see 65 degrees on a long steep hill climb, but this was after an hour on the M5 Looks like I'll pop in a new t-stat and see if there is any improvement. Msny thanks, Vin Quote Link to comment Share on other sites More sharing options...
Retroanaconda Posted December 5, 2009 Share Posted December 5, 2009 My 200Tdi runs with the needle at half way, vertical. Always has, and as far as I know that's the normal case Over-cooling won't do a massive amount of harm (certainly better than over-heating), but it doesn't help engine wear and fuel consumption. A nice hot heater is always handy too. Quote Link to comment Share on other sites More sharing options...
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