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TST Air-Fuel Control (AFC) Spring Kit

December 16th, 2010 · No Comments

In the 1994-98 Dodge Rams with Cummins Diesel, the Bosch P7100 injection pump uses an AFC housing assembly to control maximum fuel delivery under low boost conditions. This housing has several adjustments, one of which is its position frontward-rearward on the governor part of the pump. When installing the TST Power Kit, this AFC housing is moved all the way forward. This adjustment minimizes the fuel holdback from the AFC link at low boost, and also allows the governor rocker to move farther forward along the TST torque plate. This forward movement of the rocker corresponds to greater rack travel and more fuel delivery.
The forward AFC link movement, which allows more fuel at low to medium boost is accomplished by boost pressure acting on a diaphragm in the housing. The AFC housings, and especially those on 215 hp engines, use a very stiff spring that opposes the movement of the AFC link forward. Full link travel in some cases requires 34+ pounds of boost.
You can adjust the pre-load on this spring by removing the steel plug from the top of the cast aluminum AFC housing and turning the “star wheel” as described in the installation instructions for the TST Power Kit. This plug takes an 8 mm (5/16″) Allen wrench. It can be difficult to minimize the smoke level without cutting full boost power due to excessive spring tension and mechanical interference to link travel by the spring seat (part of the “star wheel”). This situation reduces engine power below the level possible with the TST Power Kit, sometimes seriously with the “bigger” kits.
Changing this spring to one included in the TST AFC Spring Kit ensures that full AFC link travel occurs with a reasonable amount of boost, and allows smoke control by adjusting the star wheel. The spring tension is sufficient to allow smoke adjustment without reducing power at usually attained boost levels, and the spring is a bit longer to give a bit of “pre-load” without turning the star wheel so far that full link travel may be impeded. Engine responsiveness and power under initial acceleration at medium rpm will be greater, as verified by dynamometer testing. Two springs are included in the kit. The medium tension one usually is best with 215 hp engines, and the lighter tension one with the others (160, 175, and 180 hp). The lighter spring can be used with the 215 hp engine for an even greater increase in low-boost responsiveness, at the expense of more smoke if heavy throttle is used before sufficient boost is achieved. The medium tension spring can be used in the non-215 engines for better smoke control than is practical with the stock AFC spring, especially at high altitude, while retaining good responsiveness and full-boost power. Procedures for installing the TST AFC Spring Kit are described in the following paragraphs. The lighter of the two springs is made from thinner diameter wire, and is noticeably easier to compress by hand.
Remove the rubber boost line (7 mm screw-head clamp) that goes to the turbocharger wastegate, if attached to the AFC housing. Remove the 13 mm nut holding the boost line going from the rear of the AFC housing (towards the firewall) to the intake manifold. Remove the AFC housing in the same manner as is done to install a TST Power Kit. Using a clean cloth to prevent marring the housing, clamp it in a vise.

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