I just replaced that hose on my 82 300D. Yes you have to remove the t-stat housing. Sometimes it's enough to tighten the clamps, but I was doing a coolant change so it was a good time to replace that hose. I went to Kragen with my t-stat housing and bought a radiator hose that fit that opening. Then I cut it at home. The other hoses, other than the radiator hoses, are relatively easy to replace without much coolant loss, so I would just replace them one at a time as they start to leak, unless they are obviously cracked or swollen.

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2004 VW Jetta TDI (manual)

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