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[HoldenSS]

My 2.5v6 (2000) is loosing coolant at the rate of 1/2 a liter per week. I drive 350 miles per week. The engine oil is clean so is the coolant reservoir. Can anyone tell me where the coolant might be going? and if this is a common fault with these cars? I have recently bought this car and it is a pleasure to drive with 145k miles on the clock. 

[tunnie]

Check the HBV (Heater Bypass Valve) rear of the block 

[humbucker]

Weeping gasket might also be at fault, check the corners of the heads (rear). Not head gasket failure in the traditional sense, but still a compromised gasket. Happened to one of my engines.

[feeutfo]

My 2.5v6 (2000) is loosing coolant at the rate of 1/2 a liter per week. I drive 350 miles per week. The engine oil is clean so is the coolant reservoir. Can anyone tell me where the coolant might be going? and if this is a common fault with these cars? I have recently bought this car and it is a pleasure to drive with 145k miles on the clock. 

Welcome to the Forum.
Yes it's is common sadly. Most common cause of coolant loss is the heater by-pass valve at the back of he engine under the scuttle.
However, yours sounds very un dramatic and consistent, which makes me think it might well be the oil cooler cover plate or some other associated coolant grubbins like the thermostat seal, both in the v of the engine.
A pressure test will confirm.

[HoldenSS]

Thanks to all who have kindly given me a quick reply to my query of coolant loss. I think I already have a lot of meat to chew. An hour ago, as I was checking how much coolant had been lost this week after the car warning system showed up again, I noticed that the top of the expansion tank was wet with coolant around the edges. I am now also suspecting leakage from the pressure cap. Next time the coolant reaches high operational temperature I will stop the car and investigate whether any coolant is sipping out. Keep you posted.

[Entwood]

It is not unheard of for tiny cracks to appear around the neck of the expansion bottle, or for the seal in the cap to fail. Either will prevent the system attaining full pressure and allow "evaporation" of coolant at a higher rate than normal.

Leakage can sometimes be checked by using a soap solution brushed in the area when the system is cold, then running until hot and watching for bubbles, whilst reapplying soap solution....  takes a while but might help ..

[HoldenSS]

Thanks for the further tips on cracking my coolant loss problem. I'll be going out in few minutes, on a 30 minute journey. May be at the end of the trip I will be able to observe something to help me pin-point the source of my coolant loss. Cheers.

[Paddy Flannery]

If you do trace your leak to a cracked header tank, there is a replacement available locally to you.
Genuine part, bit cheaper.
I think I'm right in thinking that the header tanks are the same size across the range.

http://www.carpartsconnexion.com/vauxhall/searchresults.asp?f=Vauxhall&m=OMEGA&y=2003&cat=0&p=12&