VW Passat overheating - coolant not flowing under pressure



Problem and a little history - is mechanic right?

My 4-cylinder turbo VW passat overheats very quickly when the cap is tight the coolant reservoir; not when the cap is loose enough to vent and prevent pressure from building up. Car has 70K miles, early 2001 model.


1. Small leak in radiator noted by dealer in May 2007, when doing service, including cooling system. Dealer says reapir can wait.

2. Heard Tom and Ray recommend trying leak repair to another caller, so I add Prestone leak repair, per instructions running warm engine more than 30 minutes, driving. etc.

I topped off reservoir about 1/2 pint with Peak coolant (green color) to max line, not the pink VW coolant.

3. Car runs fine for next 6 weeks, including a vacation journey more than 200 miles at 60 MPH, fully loaded with family and luggage, 90 degrees air outside, AC on, climbing 5000+ feet on highway.

4. Three days later, driving around town, outside temp in 90s, with lots of stopping and starting, AC on, car display, says ‘check coolant’, then ‘STOP, check service manual’.

5. Attempted repair: local mechanic replaces thermostat (uses VW part, not brand x) and fills with Peak global coolant, including bleeding air from system. Problem persists.

When cap is off reservoir, system holds 190 F steady. When cap is sealed, temp starts climibly quickly and overheats.

-heater core is cool

-bottom hose to radiator is cool

-top hose is warm near radiator, hot near engine.

-there is no noise suggesting a failed water pump; fans running as expected

Mechanic thinks maybe a head gasket?

No sign of oil in the coolant. No sign of coolant in the oil. Engine runs fine.

What do you think?


Adding leak stop to a radiator is totally foolish in my opinion.

The only thing that comes to mind on this one is the thermostat may be partly plugged with that leak stop gunk now or could it have been placed in there backwards?? I suppose there could be other areas gummed up by the leak stop as well.

I could not rule out the head gasket, but it just does not sound right.  Did your mechanic do any specific test of this or is he guessing?  There is a test he can do.


Top rad hose near engine is hot and near rad is warm and the bottom rad hose is cold?

Is this when the reservoir cap is sealed tight?

If so, you may have a collapsed upper rad hose or a plugged rad.


Still no edit button here!

Add: I’m not sure about VWs’ coolant but with some you can’t mix different kinds as this dilutes the original.



  1. stat was replaced with VW original. I asked about backwards - it won’t fit that way, only fits in correct direction.
  2. Radiator is clear, not clogged.


I think he is guessing.

What is the test you mention?

What about flushing the cooling system under pressure with tap water hose conneted to the system?


upper radiator hose isn’t collapsed or clogged


the temp difference I mentioned appears when the system is sealed.


Using antifreeze other than the G12 that was in the car is bad news . Incompatible mixes can gel and corrode . Your problem may be due to a plastic water pump impeller failing resulting in no circulation .


Here’s what it was:
A noiseless water pump failure that isn’t evident at idle speed:

the water pump’s plastic impeller cracked at the bushing where it connects to the shaft. At idle speed and low pressure, the impeller spins and coolant flows. Under pressure and slightly higher RPM (2000+), the impeller and shaft are uncoupled with the shaft spinning free and the impeller floating; coolant flow stops.

Covered repair under 100K drive train warranty.