I had a similar issue with my Mk4 VR6 Golf, coolant level would drop a tiny bit over time with no obvious sign of leakage..turned out to be a very small leak at the bottom corner seam of the radiator.
Reviews of OEM Volkswagen engine radiator
66 votes
Replaced radiator. Flushed out nasty brown water from cooling lines, heater core and flushed with distilled water 3 times. Pulled vacuum on cooling System and Refilled with G13 premix.
EDIT: SOLVED - noticed no leakage around thermostat, coolant ball, all belts were dry, and upper radiator dry. In the end it was the radiator.
run a mk4 secondary rad.
The radiator on my 2016 is leaking, although leak isn't really the right word. Maybe porous. The hole is so small, it takes months to show a change in the coolant reservoir. On some days, given proper conditions, a small wisp of steam is visible coming from the radiator area. I added a UV dye to the cooling system, and it is visible then.
I will will just add that I have a 2L 16V running no oil cooler and an early Mk1 radiator with built-in overflow tank, two pin thermostat, short 5th gear, and the car does fine in a traffic jam and with spirited driving.
90 16v Passat rad fit just fine in my mk1 Jetta with a single fan mk1 (which mounted to the rad just fine) you can cut the wires and set them up for how ever you like.
Regarding our 1997 Jetta GLS that we've had since it was brand-new, I was topping off the original G11 with G12+ since about 2000, without any problems. A few years ago we had the radiator replaced, and the coolant was all changed out to G12 -- that is, the current G12.
If your going to use a fox radiator becareful cause it will hang pretty low. I ought one and thought it will work but it sits really low if you just drive on streets it should be ok, but mine is a track car so I was worried about going off and possibly hitting it on somehting and causing some problems.
I replaced the coolant tank, blue resevoir cap, thermostat, water pump and coolant but didn't help. Problem was my radiator was partially blocked. I replaced my radiator this spring and it's run like a champ.
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