QuestionQUESTION: I have a 89' VW Jetta 1.6L non-turbo diesel. The engine temp is fine low speeds but drops at highway speed. It's almost like the thermostat is stuck open but it tests good. Also, the colder the outside temp is the colder the engine runs.
ANSWER: Robert,
Your sending unit or dash gauge are not designed to give you a precise temp reading, but rather a general indication of engine coolant temp. An apparent high or low reading can be qualified by using a thermometer to read the coolant temp in the coolant reservoir...If you get a reading of 150F or higher, you should be OK. As I recall, the thermostat in the 1.6L is designed to be full open at 190F, and will read cooler at the reservoir. Understand that heat is the enemy of these small diesels...if your operating temp is a bit low, you may just have an efficient cooling system. Rejoice! Jerry
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QUESTION: Hi Jerry,
Thanks for the response. I wish it was as simple as an inaccurate gauge or sending unit. Example, I bought the car in Feb and drove it home from PA. The outside temp was about 28 degrees F. The temp gauge was reading about 110 (it's an after market gauge) at 65 mph. The heater would was putting out cold air. I had to drive at 55 or 60 to get the temp gauge to come up to 140 just to get some heat. When the sun came up and the outside temp got up to about 40 I could drive at 60 mph and the engine temp gauge was reading about 170. The engine RPM also has some affect, I believe do to the water pump. The higher the RPM the cooler the engine temp is. I can also travel at 45 and shift to 4th gear and the engine will cool down. This is a very strange issue. I've talked to one other person who had a car that did the same thing but he never figured out the cause. Thanks for your help.
AnswerRobert,
Diesels produce heat, and the cooling system is designed to do several things, remove the heat,allow internal engine parts to expand to their designed running dimensions, and keep the engine at an operating temp. that allows oil to flow and lubricate the system efficiently. In cold climates, diesel trucks will use tarping to restrict the flow of air through their radiators, and thus raise the engine temp. The purpose of this is two fold...to allow their cab heaters to work, and to ensure that the oil and engine will work within its temp. parameters. If your system is unusually effective, and you seem to think it is, just restrict part of your radiator air flow by securing the two small flaps on the radiator shroud, and run your vehicle to see how it responds (they are designed to open through air flow when driving at highway speeds)...also, make sure that your radiator fan is not running at lower temps than it should...it works on a thermostatic switch located on the driver side bottom of the radiator, and I believe they are designed to kick on somewhere around 210F. As long as you can get heat out of the dash heater, you should be OK.
