Installing a Blow Off Valve (BOV)

Michael O'Sullivan and his buddy Gary Gill did an excellent job of documenting the process of installing a Blow Off Valve (BOV) on Gary's Esprit S4s. They were even kind enough to provide all the images and files in a format very close to what the Esprit Fact File already uses, thus making it easier to include it here. Great job guys!

On a Difficulty Scale of 1 to 10, this procedure rates a 9.


The following step-by-step procedure is for installing the BOV on a 4-cylinder  Esprit (including SE, S4, S4s, GT3, and Sport300).

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Parts The following parts are needed for this installation procedure:
  • 1 x 1.5" x 16" Radiator hose
  • 1 x HKS Racing Red Bypass Valve
  • 1 x Length of 3/8" Heater hose/Fuel line ~ 6'
  • 1 x Length of 5/8" Heater hose/Fuel line ~ 2'
  • 2 x vacuum T kits
  • 2 x hose clamps (~1.5" diameter )
  • 8 x hose clamps (~0.5" diameter )
  • 1 x JB Weld
  • 1 x 1.5' diameter x 2" length PVC tubing
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  1. Remove airbox and filter.
  2. Drill 1.5" hole for pcv fitting. Location is very important here. Our fitting was under the airbox to look less conspicuous, but was a real pain to snake the radiator hose from the BOV outlet back to our new airbox intake fitting. An ideal spot would be on the top side of the airbox, part of the way down the neck that connects to the turbo intake hose (i.e. where the left hand is in the picture opposite). In this position the BOV outlet would be only inches away from the BOV outlet and also require less bending of the hose between the two.
  3. Attach PVC fitting and seal with JB Weld
  4. Allow it to dry (overnight). Paint if desired
  5. Replace airbox with filter and attach/clamp new radiator hose.

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Coolant Overflow Tank
  1. There simply wasn’t enough room to keep the coolant tank in the same place so it had to be moved. We remounted it to the plenum clover with a simple metal bracket.
  2. There are 4 hoses feeding into the coolant tank of differing sizes. The 5/8" line from the bottom needs to be lengthened, as does the 3/8" line from the turbo. The other two will actually be shortened when the tank is remounted as they come from the front of the car (routed via the right hand side trim panel, where ECU is located).
  3. Remove the tank and associated hoses (be careful draining the tank full of coolant), from the turbo (2/3’ long) and the base of the tank (about 2-3" long), and the other two top lines that run through the right hand side trim panel. At this point I recommend skipping to the BOV mounting as there is plenty of room with the tank out of the way (see section below).
  4. The coolant level sensor wires also need to be lengthened. A 2’ 3 core cable was used to lengthen the line and it was routed through the right hand side where the original coolant tank hoses were fed.
  5. Replace the turbo coolant line with the 6’ replacement and route it through the right hand trim panel for neatness.
  6. Connect up the 2’ line feeding the base of the tank.
  7. Shorten the remaining two lines appropriately and remount to the top of the tank.
  8. Reconnect your extended sensor wires to the coolant level sender unit atop the tank.
  9. Fabricate a simple metal bracket, 4 holes drilled in a flat piece of metal and then bent into a |__| shape. Ours was found at the local Home Depot and can be seen in the picture of the relocated tank.

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Mounting the  BOV
  1. Remove the plenum cover (8 bolts 13mm socket). Be very careful not to tear the paper gasket when removing the plenum cover, otherwise you’ll need a new one.
  2. Remove boost line from the end of plenum.
  3. Attach T the boost line. New T line will attach to BOV valve, while the other line from the T boost line will be remounted beneath the plenum.
  4. Drill 1" hole at the end of the plenum to act as the intake source of the BOV. Using a 1" hole saw. Drilling a small pilot hole is recommended.
  5. Drill 2 mounting holes and thread for 3/8" x 1 " bolts (or which ever size bolt/tap set you have available).
  6. Drill a hole for a 3/8" NPT tap hole beneath the plenum and thread for your NPT boost line fitting to remount boost line. We used brass 3/8" NPT tap fittings (from local auto parts store) rather than re-using the ones that were on the car. The reason for this is that we could not get an appropriate metric tap with the correct thread count for the original units. We use 3/8" – 16 thread (16 threads per inch) pattern as the only size we could find in common. In fact the brass NPT fittings were originally 24 threads per inch but we re-threaded it using a die to 16 to match the hole already threaded in the plenum cover.
  7. Drill another hole for a 3/8" NPT tap in the intake manifold. Have someone press down on the accelerator pedal while you insert a clean moist towel into the intake manifold to catch fillings. Then, drill a pilot hole first and go slowly using plenty of coolant on the drill bit. Once tapped you can remove the rag and also clean out other fillings in the area. Screw in one of the brass NPT tap fittings into the intake manifold. This is now your vacuum source for the BOV.
  8. Bolt BOV to plenum.
  9. Replace plenum.
  10. Attach the vacuum source line (the line that is fitted to the intake manifold to the left of the plenum) to the vacuum source of the BOV and clamp either end.
  11. Clamp the radiator hose from the airbox to the BOV outlet.

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Credits Thanks to Lewis Gaskell who recommended the parts and explained the procedure to us. Gary Gill who provided a beautiful S4s to drill holes into, in addition to doing the work. Alan Elliot for tools and figuring out the best way to remove/replace/re-route things.

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