According to the ATSG manual, the ONLY thing that ONLY is active in fourth is your K2 clutch. Sadly, getting to it is a major affair.

In theory it could also be a valve body problem. Any history, or did it just "pow" start on, say, Saturday?

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James Marriott
2003 Buick Regal
1983 300D (228k, frau Auto)
1996 Suburban K2500 (192k, 6.5 turbo diesel/4WD towmaster 10,000)
www.engineeringworks.biz
1987 300SDL junker 170k
1982 300SD junker, 265k