I've had this symptom lately and I'm baffled. If interested see my previous thread about this for a more extensive discussion:
http://www.peachparts.com/shopforum/showthread.php?t=134756

I don't have any fluid mixing, virtually no coolant loss, and both I and a local shop tested the cooling system with a pressure tester and it held pressure perfectly and with no pressure fluctuation with the engine running. I still suspect a tiny head gasket leak, but with no solid evidence I'm not about to tear the engine apart. I'm wondering if these engines do in fact intentionally accumulate some pressure for cooling purposes or if it's just the nature of the beast. So I'd like some volunteers to check their cars after they completely cool down and feel the upper radiator hose and then listen for pressure release while loosening the cap and post your results here.

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2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL

Come on guys, I know I'm not the only one here with a W210 diesel.

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2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL

I believe you should have some pressure. Not rock hard radiator hose presssure, but slight whistle when cap removed pressure.

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Adam Lumsden
(83) 300D
Vice-President of the MBCA International Stars Section

My wife is out of town with our E300 but she will be return tonight - I'll check for residual pressure tomorrow morning and give you a report.

'96 OM606 18xxxx miles

No pressure present after cool-off. No release from system when cap is loosened.

Hose is soft and no pressure in the tank. This is a 99 E300 so I am not totally sure it is relavent. It is however a 210

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01 Ford Excursion Powerstroke
99 E300 Turbodiesel
91 Vette with 383 motor
05 Polaris Sportsman 800 EFI
06 Polaris Sportsman 500 EFI
03 SeaDoo GTX SC Red
03 SeaDoo GTX SC Yellow
04 Tailgator 21 ft Toy Hauler
11 Harley Davidson 883 SuperLow

Thanks for the responses so far. Tim, how was your pressure? So it appears I'm not supposed to have any pressure, that's what I thought. I wonder if this could have something to do with my pressure cap. Could it be that it's sucking in air when the cold coolant starts to circulate as the car is getting warmed up? I don't have a lot of residual pressure in the system, but enough to feel the difference in the hose and definitely enough to hear the pressure releasing while loosening the cap.

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2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL

Just did a coolant change (and used prestone, so there!) so have been keeping a keen eye. 67k miles. let it fully cool overninght in unheated garage, no pressure. Hope you can figure things out. Regards,

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BobT
2004.5 Dodge Ram 3500 CTD QC SB 4x4
1999 E300D
Deere 770 w/FEL

No, as the coolant warms up, pressure will build in the system, forcing air out. There would be no impetus for air to be pulled into the system. Something has to be putting pressure into the system, and that something is often not a good thing. The good news is, as long as you aren't getting mixed fluids or using any coolant, it isn't something you have to worry about fixing immediately. If it is a cracked head or head gasket, you will eventually start noticing coolant loss as the problem gets worse.

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Keep everything as simple as possible-but no simpler--Albert Einstein