Air In Cooling System ** NEED HELP**

Hi All,

Recently i changed the expansion tank & coolant (step by step as mentioned in one of the how to guides) on my 98 tdi .. Even after replacing the old damaged expansion tank still loosing coolant .. So i took it to me local garage and they pressure checked the the cooling system ** no sign of any leak and no sign of head gasket being faulty ** .. So today i took the car out on motorway (p.s only loose coolant if i rev the car driving fast) .. did few miles at 70 - 75 mph and the coolant check light came on .. stopped the car and checked the upper hose connecting to radiator and it was warm/hot .. checked the bottom hose connecting and it was cold ... then i opened the drain valve and coolant came out like it was a bottle of champagne and after few seconds it went stable .. does that mean theres an air packet in cooling system ... One more thing the car doesnt over heat at all ... when i was driving earlier today at motorway temp was stable at 90 and when the coolant check light came on it dropped .
After draining some coolant from drain valve (as mentioned above) i topped up the expansion tank and when switchd on the car the temp needle was back to 90 mark and stable .. **CONFUSED**

Please Help.

Thanks
Sal

sounds like perhpas just air if it keep re occuring then you have a leaking head gasket pushing compression into the coolant

So if its just the air how can i bleed it from the cooling system .

Thanks

How do you know that it is loosing coolant? Maybe the level went down because you were working air pockets out of the system?

Maybe it's running hot and blowing coolant out because the waterpump has failed?

I think if it is really just air in the system you should be able to work it out by running it gently (maybe just idling) until it is fully warm and the thermostat opens. After it runs for a few more minutes, let it cool and top up the tank.

Does the return line on the top of the tank get warm?

HopefulFred said:

How do you know that it is loosing coolant? Maybe the level went down because you were working air pockets out of the system?

Maybe it's running hot and blowing coolant out because the waterpump has failed?

I think if it is really just air in the system you should be able to work it out by running it gently (maybe just idling) until it is fully warm and the thermostat opens. After it runs for a few more minutes, let it cool and top up the tank.

Does the return line on the top of the tank get warm?

Click to expand...

Hi ,

Theres no visible evidence of coolant leak ... just the coolant check light came on ... secondly the car doesnt overheat so i reckon the water pump is fine ... and yes the return pipe on top of expansion tank gets warm ...

Today the cars been running fine no loss of coolant etc ... the lower hose pipe on radiator was warm when i checked this morning after driving for 10-15 miles .. but the temp do fluctuate alot (temp guage).. and if you dont mind me asking which pipe/hose connects to the heater matrix and where can i find it ??

Thanks

How much total coolant do you think you've put in? That may eliminate the 'working the air out' theory.

I assume you are using the Pink G12 coolant from VW? If so, any leaks will be readily visible via the pink stain.

When was the water pump last replaced? Have you looked around the water pump area? Depending on year, you may beed to remove the timing belt cover. One possibility is the pump shaft seals are failing, but you would see a trail of pink.

If the temp is fluctuating, it could be low on fluid or a sticking thermostat. I would not rule out the thermostat. They have been known to fail.

HTH

Tony

If the temp is fluctuating something is going on.

There are only a few options: air, thermostat, waterpump.

The engine will be cooled enough not to overheat if you keep air moving across it. I know you said the return line gets warm, but it might be worth pulling the hose while the engine is running to see the water actually returning (cool engine).

The thermostat is on the lower hose and is a cheap and easy replacement. If the lower hose does not warm up, try that.

With a bad waterpump, the engine can get very hot without the temp sensor really knowing about it. Keep looking - good luck.

Fred

HopefulFred said:

If the temp is fluctuating something is going on.
There are only a few options: air, thermostat, waterpump.
Fred

Click to expand...

If the temp gauge is fluctuating, add engine ground to the short list. I had a fluctuating temp gauge in a non-TDI that turned out to be the alternator ground output going through the temp sensor, and thus the gauge, when the normal engine ground connection was compromised.
Now, this has nothing to do with this coolant loss issue in this thread, but it is something to consider when there is no loss of fluid, no thermal abnormalities and no flow interruptions.

The TDI coolant system is self bleeding. The coolant sphere is plumbed in the high point of the system. Any entrapped air will eventually be dislodged and bubble up to this high point, the fluid from the sphere filling in the pocket in the process. If there is no external evidence of fluid loss, no drips, no puddles, no steam, then keep filling the sphere to the recommended level until the exchange of fluid in the sphere for air pockets in the system ends, and it will.

My fluctuating temp. gauge was cured with a new coolant temp. sensor,
part no. 059-919-501-A(green). Ditto on the self bleeding feature, because I lost about two quarts of fluid in the process of changing the sensor, and all I did was replenish the lost coolant(G12) until the warning light went out. Then again the engine was cold at the time, and the thermostat had not opened, if it ever does.
My lower radiator hose is always cold, even after a 20 mile drive.