Fuel Temp Sensor Alarm and Fuel Filter Can Hot

[IMK1230]

Hello,
My friend VW eng. just did a 100k mile service on my golf 5 1.9 TDi 2004.
Replaced CAM sensor which was faulty (Hot Engine Start Problem, now fixed) , and all filters including fuel filter.
He was driving car back to my house about 5 miles on really hot day and the engine ecu light came on on.
He got to my house a plugged in the VAG dia PC and alarm was Fuel Temperature, i touched the fuel filter can and it was really hot.
He is returning to tomorrow to look at car and asked me to post for suggestion as to what might be wrong.
-- Only Gal or so of fuel in tank --
Many thanks in advance Ian

[Rob69]

Unless your's is vastly different from the PD engines I've had, there is a thermostatic fuel return valve on top of the filter, or somewhere else along the fuel hoses, this circulates fuel around the engine and filter until it is up to a certain temperature to prevent the filter waxing in very cold weather, it then opens and starts to return fuel to the tank via the fuel return cooler. your thermostatic valve sounds like it is stuck, maybe the recent filter change disturbed something or your fuel cooler is really dirty. Only having a gallon in the tank would mean the tank would be warming up if your valve had opened but the cooler is not working very well. Google fuel system diagrams and you should see the valve I'm on about.