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can I kindly ask your opinion on my car issue? I'm not deep in field of engines and this problem is causing my serious headache and a total waste of cash without any results.

I've a 2015 VW Polo TDI. All began with cars signaling a low cooler levels, but I was in a hurry and 60 miles away from home so I decided to came back monitoring engine temps on the car's computer anyway. During the way back the engine never had gone back over 90° Celsius. When at home I refilled the expansion tank (was empty) and immediately leaked. So I took it to mechanic, which diagnosed radiator failing because damaged by metal debris on the street floor (what a luck). I saw the broken part. So we proceeded with new one.

When I got back the car I immediately noticed that levels of cooler was 2 inch above the maximum in the tank: he said isn't a problem. While casual city driving in the next days I immediately got a new problem: randomly after a while, during driving (maybe 5 - 15 miles) engine temps ramped to over 100° Celsius and air cooling fan was activated at maximum speed.

I took the car back in store, and mechanic said that was a problem of termostatic valve of cooler system and changed it with a new. Problem again, exactly the same (I monitored that he made quite a lot of miles, so I assume he was testing it).

Took back in, it changed termostatic sensor. Again tests done, again problem.

Took back in and he dropped a car to VW colleagues for a new plunging in the cooler system (everytime this was done) and he said that the replacement sensor was fault and changed it again. Problem after 2 days of normal use was back. More interstingly when I took the car yesterday with the issue "live" (engine over 100°C) coolant wasn't boiling and when we made some engine speed test temp was rising, but after just some seconds, turn off and turn on again temp was 90°C.

Now he is saying that it's surely a problem of blown head gasket, probably caused by my initial trip back home.

I'm a medic and I recently moved so I do not have any mechanic of trust nearby. What it leaves me really bothered is that he seems to proceed by trial and error and from what I googled blown head gasket is a serious problem with really evident symptoms, which are totally absent. I'm really afraid to leave the car stopped for maintenace another month (2 months now) and spend again $2000 more ($1000 spent already) on a car that is $9000 worth now.

What you would do? May the overfilled cooler tank be the cause?

update 1! Done what Steve Matthews suggested and the solution seems to be near. With cold engine I've turned on the car, heater at maximum temperature maximum vent, removed 0,5L of coolant exceeding the maximum line in the ball tank. With cap open, squeezed the bottom hose and 2 bubbles burped out. Test drive#1: 5 miles all ok. Test drive#2: 10 miles all ok. Test drive#3: 10 miles and suddendly again the temp ramped with obvious turning on of the fan. With issue present opened car hood: ball tank of coolant not boiling (neither very hot), level above 1/2 inch above the max and when I turn on at maximum the heaters problem disappears (temp returning suddendly to 90°C).

Final update A late update, just to share the solution with everyone. I brought the car to my old mechanic and he said that was the previous bad work that caused water pump fail that he fixed. He also unmounted the head gasket to make sure that all was right. Shed another 1200$ but problem was gone then. I can't totally trust what he said, but that's all.

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  • The overfilled coolant tank shouldn't be relevant. Possibly he deliberately overfilled it slightly in case there was still a bit of air trapped somewhere in the cooling system - that is quite normal if the system has been completely drained and refilled to fit a new radiator. Blown head gaskets don't necessarily cause any obvious symptoms if there is only a slight leak, but I agree it sounds like your mechanic is guessing rather than doing some tests and then diagnosing the problem. But I'm sure you know that medics also have to use trial and error to fix problems sometimes!
    – alephzero
    Dec 14, 2018 at 11:06
  • Thank you for reply! Plunging and refilling was done multiple times from now, I was hoping on that problem. As for the overfill, you say, shouldn't, but can it happen? Because as a simple user the only thing that I can do is to open the user's manual which is saying at the proper chapter that light overfilling is ok, but great overfilling can damage the system for high pressure building in. Do you consider that removing some liquid is worth trying? As for the mech, I didn't want to offend. Absolutely also in medicine there is some sort of trial and error.
    – Andrea Roncaglia
    Dec 14, 2018 at 11:17
  • If you overfill so much that the pressure relief valve opens, you just get the excess coolant sprayed out of the filler cap, which makes a mess (dried antifreeze all over the engine) but doesn't harm anything. Of course if several things go wrong at once, anything might happen.
    – alephzero
    Dec 14, 2018 at 13:25
  • The filler cap in the Polo is really really strong and air-tight, so I think that before the cap "explodes" air will dissolve in solution. So, for the sake of completeness I have removed 6 cups of fluid and see what happens. I'll keep you updated!
    – Andrea Roncaglia
    Dec 14, 2018 at 14:16

3 Answers 3

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The real problem here is what you already stated: that you drove home for 60 miles with little or no coolant in the engine because of a damaged radiator. The temperature gauge was unreliable in this case because it is designed to read coolant temperature. If there isn't any it will never show the actual overheating that the engine did.

In my opinion you are lucky if the headgasket is indeed the only other part that failed. The entire engine head could be warped because of overheating and there could also be other serious damage. I'd suggest to have the engine examined more closely to see if there is more damage, before any more parts are replaced. This will cost you some money but one cannot decide if and how a patient can be treated without a good diagnosis. Trial and error could indeed be an endless path.

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  • Thank you for replying! I do understand. I made an error moving the car. As written, I'm trying to understand if such a subtle symptom is probable or just compatibile by exclusion. In this case I would try to made at least the try of plunging some liquid away as it tooks really just some minutes.
    – Andrea Roncaglia
    Dec 14, 2018 at 11:30
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I've seen this exact behaviour on Volkswagens with overfilled coolant systems. It's typical of a car with an air-lock and when the coolant comes above the max level, it covers the overflow pipe which vents into the coolant tank (the little ball shaped tank). This prevents the flow of water up from the system when it's trying to vent air from the radiator.

Whenever I add coolant to any of my VW's, I make sure that the heater controls are set fully to hot. I then top the coolant up to the max line. Start the car with the cap off and watch for bubbles. Squeeze the radiator bottom hose (keeping away from the electric radiator fans). If the coolant level drops, add more but only up to the max line. Then refit the cap.

Be cautious not to do this when the car is hot.

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    Thank you a lot for replying. Tomorrow I've got some spare time and I'll surely try out and post the results.
    – Andrea Roncaglia
    Dec 15, 2018 at 16:07
  • Done what you suggested and solution seems near (see update1 above!). Is the representing of the issue related to another bubble stuck in the system or other cause? If so, the problem is that bubbles are still stucked in or are them actually formed by a malfunctioning? Is the head/gasket ruled out or not? Professional burping was done multiple times... Thank you in advance for support!
    – Andrea Roncaglia
    Dec 16, 2018 at 18:37
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I have a 2011 Vento (Petrol), I ran into the same issue and the mechanic at VW service showed a live demo on what exactly is the problem. All the below tests were done through the dealer's diagnostics system/laptop.

  1. Leave your car with Engine running and Air Conditioner turned-off, monitor the temperature of the coolant - the coolant temperature starts at 76C and starts going-up till reaches 90C (in about 10 seconds); at this point, your radiator fan should kick-in and bring down the temperature. In my car, the fan did not kick-in and ended-up boiling the coolant (90C+).
  2. Leave your car with Engine running and Air Conditioner turned-on, monitor the temperature of the coolant - the coolant temperature will be consistently at 76C - 78C. Here your air conditioner is helping the engine to maintain the coolant temperature maintain at 76-78C.

The problem here is faulty wiring harness (as per the service center mechanic).

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