VW Scirocco, small air bubbles in coolant. Negative HG test.
Discussion
VW Scirocco, 1988 MK2 Scala.
On a random day before it started getting cold, I thought it would be a good idea to check the coolant to see if it needed replacing prior to the big UK freeze the media had predicted. Opened up the cap to see that coolant was low, so added a bit more and carried on. A few days later I had another check and saw the coolant level had again dropped, not drastically but enough to take it from just below the MAX to on the MIN line. Topped it up again and began to think about what had caused that leak, chased all pipes and saw nothing. Monitored it over the next few weeks to find it very slowly lost water but I could not find the issue. In a head gasket related panic I checked the oil which was clean, the coolant which was clean and the exhaust which did not blow white smoke. I also had a good look over the engine to see any leaks or coolant crust but found nothing.
My attention then turned to how the car was running. Felt like it was full power, started on the button but it had become a bit lumpy on idle. Did a full service, sparks, filters etc hoping to cure this issue. It improved but the idle was still a bit bumpy - not too concerning. Also checked if the heater matrix was leaking but everything seemed dry. One common fail on these cars was the header tank cap, so I purchased one and for good measure I brought a block testing kit to put my mind to rest.
Cap and test kit arrived today so I went off to garage and did a cold start block test. There was a steady stream of bubbles going into the test kit which seemed to sporadically start and stop. I was thinking the worse and expecting the blue liquid to turn yellow but nothing happened, left the car running with test on for 5 minutes and then stopped. So I stuck the new header tank cap on and went out for a run in the car. When I got back I ran another block test to exactly the same results. Negative... Went and double checked the oil for mayo, the exhaust for steam etc and nothing to indicate anything was amiss, except this stream of bubbles.
Bunch of questions...
1. Is this a head gasket blow which has shown a false negative on the block test?
2. About a month ago/1k miles ago the water pump was changed (not by me). Could they have introduce some sort of air into the system which the old header cap has failed to expel?
3. Could there be a leak somewhere in the coolant circuit which is expelling coolant as steam and then when the systems cools bringing air back in which is now bubbling through the block test kit.
4. Something else.....manifold?
Been scratching my head about it all day hoping for the best and preparing for the worst! Any help will be most appreciated.
Thanks
On a random day before it started getting cold, I thought it would be a good idea to check the coolant to see if it needed replacing prior to the big UK freeze the media had predicted. Opened up the cap to see that coolant was low, so added a bit more and carried on. A few days later I had another check and saw the coolant level had again dropped, not drastically but enough to take it from just below the MAX to on the MIN line. Topped it up again and began to think about what had caused that leak, chased all pipes and saw nothing. Monitored it over the next few weeks to find it very slowly lost water but I could not find the issue. In a head gasket related panic I checked the oil which was clean, the coolant which was clean and the exhaust which did not blow white smoke. I also had a good look over the engine to see any leaks or coolant crust but found nothing.
My attention then turned to how the car was running. Felt like it was full power, started on the button but it had become a bit lumpy on idle. Did a full service, sparks, filters etc hoping to cure this issue. It improved but the idle was still a bit bumpy - not too concerning. Also checked if the heater matrix was leaking but everything seemed dry. One common fail on these cars was the header tank cap, so I purchased one and for good measure I brought a block testing kit to put my mind to rest.
Cap and test kit arrived today so I went off to garage and did a cold start block test. There was a steady stream of bubbles going into the test kit which seemed to sporadically start and stop. I was thinking the worse and expecting the blue liquid to turn yellow but nothing happened, left the car running with test on for 5 minutes and then stopped. So I stuck the new header tank cap on and went out for a run in the car. When I got back I ran another block test to exactly the same results. Negative... Went and double checked the oil for mayo, the exhaust for steam etc and nothing to indicate anything was amiss, except this stream of bubbles.
Bunch of questions...
1. Is this a head gasket blow which has shown a false negative on the block test?
2. About a month ago/1k miles ago the water pump was changed (not by me). Could they have introduce some sort of air into the system which the old header cap has failed to expel?
3. Could there be a leak somewhere in the coolant circuit which is expelling coolant as steam and then when the systems cools bringing air back in which is now bubbling through the block test kit.
4. Something else.....manifold?
Been scratching my head about it all day hoping for the best and preparing for the worst! Any help will be most appreciated.
Thanks
Like mentioned by cptsideways throw an exp cap on it (dont do pattern parts) about £20 from a VAG dealer (TVR V8 cars use em too) these system are totally self bleeding but just make sure the small bore vent pipe from cylinder head to the clear expansion bottle is clear and flowing coolant easy to see with the cap off 
If it's leaking slightly when hot you will usually find the coolant evaporates at the point of the leak and leaves a colored stain behind. This might not be in plain sight if it's leaking from a matrix, joint etc but ought to be obvious for leaks at the pressure cap once you know what you're looking for.
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