Disclaimer: if your car explodes it's not my fault.

This procedure applies to the C25XE and some installations of the X25XE engines (see Vauxhall/Opel engine identification conventions).

The C25XE is the engine used in the Vauxhall Cavalier V6 (in Britain - it's called the Opel Vectra (A) in the rest of Europe). It is also fitted in V6 Calibras up to about 1995, after which the X25XE was used, although it retains the induction system of the C25XE so this procedure is still needed to change the spark plugs. The Vectra-B V6 also uses the X25XE but has a different induction system, with which changing the spark plugs is easier.

It is unlikely you will find instructions for doing this in any commercially available car maintenance manual; even such engineering tomes as the Haynes manual do not include this, as the V6-engined cars were not produced in sufficient numbers to warrant it. Any specialist procedures for the Cavalier Tub are omitted for the same reason.

The kicker about replacing the rear spark plugs in the C25XE (and 'Calibra-version' X25XE) is the arrangement of the induction system. Changing the front bank of spark plugs is fine - remove the decorative engine cover and they're accessible. Unfortunately the rear bank are underneath the inlet manifold, which must be removed before they can be reached. This is why garages charge so much for this job. The air trunking system tracks around behind the rear cylinder bank and reaches foward, over the top of the rear cylinder bank and down the centre of the 'V'. The inlet manifold is in the way.

Later iterations of the V6 engines (such as the X25XE as installed in the Vectra-B and Omega (not the Calibra)) improved things, keeping the induction system over the centre of the 'V'. It was probably not helped that the Cavalier/Vectra and Calibra were front-wheel drive, which in a massmarket car necessitated a transverse engine. With longitudinal engines there is more room to place the induction system at one end of the engine (usually the end at the front of the engine bay) as lengthening the engine bay is probably more feasible than widening it.

The design of the Vectra-B (which was/is also front-wheel drive) resolved this issue by lowering the mounting height of the engine; this gave more room to mount the air inlet and throttle system on top of the engine instead of to one side (try to put a Vectra induction system onto a Cavalier engine and you won't be able to close the bonnet/hood afterwards). I assume this to be the case because as far as I know the X25XE is otherwise virtually identical in design to the C25XE.

To remove the inlet manifold and access the rear sparkplugs you'll need:
- A standard socket set
- A star socket set (although you may find the standard socket set will suffice, so check before you buy any new tools)
- A set of torx bits
- A replacement inlet manifold gasket (GM part number 90412126); as with all gaskets, any that are disturbed should be replaced.
- A flat-headed screwdriver.

1. Remove the cosmetic engine cover, by undoing the four screws that hold it on - you will need a T30 torx bit and possibly a star socket for this.

2. Remove the throttle cable from its retaining bracket, which is screwed onto to the rear of the inlet manifold (be careful not to disturb the throttle cable adjuster clip while doing this). If you want, you can remove the U-shaped retaining clip on the throttle cable ball joint andprise the ball joint off the throttle lever, so you can get the cable out of the way. This is useful because otherwise as it tends to flop about and annoy you while you're working. Much like certain other things.

3. After that, remove the black rubber hose that is attached to the throttle body intake pipe, about halfway along. It is about 15-10mm wide, on the front side of the piping. It goes underneath the inlet manifold, to the idle air control valve. After that, undo the jubilee clips at either end of the pipe, using the flat head screwdriver. After you unclip the wiring connector that may be clipped to the side of the pipe, the pipe can then be pulled away the throttle body at one end, and the air box at the other - this can be rather tight, so might take a bit of force.

4. Remove the brake servo pipe from the back of the inlet manifold, using a 19mm spanner to undo the bolt that screws it in place. You should be able to hook this pipe behind the weather strip underneath the windscreen to hold it out the way.

5. Remove the four torx screws that hold the wiring harness onto the inlet manifold, using an E10 star socket and/or a 13mm socket. Looking from the front of the car, there are two screws on the right edge of the manifold, and two on the back. One of the two screws on the back is attached to a bracket, which itself is secured by one or two torx screws. Depending on the tools you are using, you might find it easier to loosen them before removing the screw on the manifold, as it is rather a tight space.

6. Undo the seven bolts that hold the inlet manifold in place. These should be obvious: seven bolts about 15-20mm at the front of the manifold in two rows - two on the top and five on the bottom. Note that with these bolts, sometimes just the head will undo (as in a nut), with others the threaded rod will come out with the nut, more like a bolt.

7.Remove the small hose that attaches to the underneath of the inlet manifold on the right-hand side (looking from the front of the car), and fits onto a nipple on the rear cylinder head. Do this by undoing one of the jubilee clips that hold it in place, with the screwdriver.

8. Lift the manifold up slowly, checking the piping as you go. There is a thin white hose that goes into the underneath of the throttle body from the fuel pressure regulator which may come out when you do this, so make sure you check it's in place after you've finished. Once you've lifted the manifold off of any threaded rods that remain, move it forwards as much as the attached piping will allow. You will now be able to get at the rear spark plugs. Note: After removing the inlet manifold I suggest you cover the now-exposed inlet ducts at the centre of the 'V' because if you drop anything down there, spark plugs will be the least of your worries.

Refitting is a reversal of removal, remembering to check all piping is attached, and to make sure the inlet manifold gasket is renewed (you can't miss it, it's bright green). In particular make sure the pipe that connects to the air trunking just before it reaches the throttle body is put back; it goes to the idle air control valve, and will cause the engine to die as soon as it is started if it isn't connected properly. By "die", I mean the engine will cut out, not that it will smash itself to bits leaving you with a car-shaped paperweight.

This procedure is also necessary if you want to:


On a couple of issues I have done a bit of supposition, so please /msg me with any corrections.

Sources:
  • Personal experience (I have done this dozens of times)
  • Burgess, Jason; "Changing V6 Spark Plugs";
    <http://www.concept-autosport.co.uk/v6spark.htm> (tool sizes and part numbers)

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