An impact jar for coupling between opposing uphole and downhole portions (140, 150) of a downhole tool string, wherein the impact jar comprises a shaft extending within at least a portion of a housing, and wherein the housing and the shaft move axially relative to each other such that first and second impact features (490, 495) carried with the shaft and the housing, respectively, impact in response to a tensile force applied to the impact jar exceeding a predetermined threshold, wherein: a port of the housing connects a space external to the housing with an annulus defined between the housing and the shaft, thus permitting automatic equalization of a first pressure of non-wellbore fluid contained within the impact jar with a second pressure of wellbore fluid external to the housing, wherein the port comprises internal threads; and the impact jar further comprises a flow restrictor that reduces a flow area of the port, wherein the flow restrictor comprises a generally cylindrical body having: external threads engaging the internal threads of the port to maintain the flow restrictor within the port; and a passage extending between the annulus and the space external to the housing, wherein the passage has a flow area that is smaller than the flow area of the port and a piston slidably disposed within the annulus defines a first annulus portion and a second annulus portion; the piston fluidly isolates the first annulus portion from the second annulus portion; the flow restrictor fluidly connects the space external to the housing with the first annulus portion such that the first annulus portion contains wellbore fluid at the first pressure; the second annulus portion comprises the non-wellbore fluid at the second pressure; and the flow restrictor and the piston collectively permit the automatic equalization of the first and second pressures; and the shaft comprises a longitudinal bore and a radial bore extending between the longitudinal bore and the second annulus portion. a port of the housing connects a space external to the housing with an annulus defined between the housing and the shaft, thus permitting automatic equalization of a first pressure of non-wellbore fluid contained within the impact jar with a second pressure of wellbore fluid external to the housing, wherein the port comprises internal threads; and the impact jar further comprises a flow restrictor that reduces a flow area of the port, wherein the flow restrictor comprises a generally cylindrical body having: external threads engaging the internal threads of the port to maintain the flow restrictor within the port; and a passage extending between the annulus and the space external to the housing, wherein the passage has a flow area that is smaller than the flow area of the port and external threads engaging the internal threads of the port to maintain the flow restrictor within the port; and a passage extending between the annulus and the space external to the housing, wherein the passage has a flow area that is smaller than the flow area of the port and a piston slidably disposed within the annulus defines a first annulus portion and a second annulus portion; the piston fluidly isolates the first annulus portion from the second annulus portion; the flow restrictor fluidly connects the space external to the housing with the first annulus portion such that the first annulus portion contains wellbore fluid at the first pressure; the second annulus portion comprises the non-wellbore fluid at the second pressure; and the flow restrictor and the piston collectively permit the automatic equalization of the first and second pressures; and the shaft comprises a longitudinal bore and a radial bore extending between the longitudinal bore and the second annulus portion.