my first time in this forum and in hope someone had a similar problem or could help me. So i restored a golf mk3, took the engine out and swapped for an used one with 75k km, the old radiator was leaking so basically was a FULL Service. The car warms up to 70ºC and stays there until I'm in traffic or traffic lights, if i stop the car reaches 90ºC and the fan kicks in, the fan doesn't kick in before 90ºC

New spark plugs, new fuel pump, new thermostat (tried 4 thermostats, one was OEM) new radiator (aftermarket), G12 coolant mixed with destilled water, new housing for the thermostat (distribution for the hoses), new temperature sensor and even tried with 3 different old ones, Tried with another temperature gauge and the reading was still the same. My core heater is working fine.

What I'm basically doing now to get "good" MPG and performance is blocking a portion of the radiator with a plastic and now the car stays around 85/90ºC but i shouldn't do that...

I've also made sure all the hoses are connected to their place with a vw parts website.

I think it's the radiator, although it's specific to my car it's not OEM...

What do you guys think? weird isn't it?, Thanks!

  • Welcome to Motor Vehicle Maintenance & Repair! Are you using the dash gauge to read the temp? You state you "tired another temperature gauge", but I'm not sure what that means (a new dash gauge or something else?). Dec 15 '19 at 15:04
  • Hello, yes i'm using the gauge from the dash, i only tried with another gauge from another dash, i haven't used an external reader. Thanks!
    – Bruno NXR
    Dec 15 '19 at 15:55
  • The dash gauge is really just an indicator. Put a "real" temp gauge on it and see where it reads. Depending on the year (whether OBDI or OBDII) you may be able to pull the temp directly from the computer. This will give you a far more accurate reading than what you're getting from the dash. Dec 15 '19 at 15:58
  • I have to try that but i think that it is reading fine because once it passes 90ºC you can hear the radiator fan kicking in...
    – Bruno NXR
    Dec 15 '19 at 16:12

One of two possibilities here; the first one being a jammed or missing thermostat. On these cars it is possible to remove the thermostat from the housing and reassemble everything. This then means that the car never really warms up. I have a 1.6 Mk3 Golf like this and it only gets up to temperature when it's stationary on a hot day after enthusiastic use. I also have a 2.0 GTI Mk3 with no thermostat and the temperature gauge never lifts from zero.

The other possibility, assuming you have the correct temperature sensor installed (consider swapping the one from your old engine to check) is that you have a lower temperature thermostat installed. I'd have to double check but off the top of my head, the standard Mk3 item opens fully at around 78 degrees C. Is it possible to buy thermostats that open at 72 degrees and if one of these is installed, you'd experience the behaviour you describe.

At any rate, the recommended course of action would be first to replace the temperature sensor as they are very inexpensive and then, either at the same time or if the sensor doesn't resolve the issue, switch out the thermostat.

A personal note; well done for restoring a Mk3, they're great cars (we have two at the moment) and very overlooked by the VW community.

  • Hello, i've replaced both, the temperature sensor and the thermostat, I currently have one fully opening at 84ºC and the temperature sensor is brand new, i've also tried with the other engine temp sensor. I think the problem is related to the new radiator, it has less 10mm in thickness to the original one... the water is probably coming back from the top hose... what do you think?
    – Bruno NXR
    Dec 19 '19 at 22:48
  • It could be the radiator, I've also seen air-locks create odd behaviour in Golfs. Try "burping" the radiator (i.e. running it with the cap off and squeezing the bottom hose to see if any air is trapped in the system). Don't do this with the coolant at full temperature / pressure though! Dec 20 '19 at 10:53

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