Citroën CX - Fuel Injection Plastic Plenum Repair

If you drive a fuel injected Citroën CX with a plastic air distribution chamber (henceforth called a plenum) you will most likely experience a plenum failure.  A plenum failure is when the plastic plenum weakens to the point where the side of the plenum gets sucked against the intake runners.  The plenum looks like it has imploded and there is a loud low-pitched noise.  Repeated collapsing of the plenum in this way will crack the plastic, making the plenum virtually useless.

There is a solution that can be put in place prior to failure and which may prevent failure for the life of the car.  The fix is not expensive and takes only a couple of hours (less if you are handy).  This page hopes to explain the procedure and convince you of the necessity of the 'fix'.  Since the repair doesn't involve buying anything from me, that should help in the convincing part!

How It Fails

So what happens when the plenum fails?  As stated earlier, the plenum will crack after repeated failures.  Many people believe the plenum cracks first which allows the plenum to collapse.  In our experience that is NOT what happened. This repair was made to a plenum that collapsed after an unrelated repair was completed.  So the question becomes "how does the plenum collapse?"  The answer is it weakens, especially along the seam running around the plenum.  The weak seam flexes and allows the side of the plenum to get sucked up against the runners.  It's that simple.

The Fix


Disclaimer:  The writer and his associates bear no responsibility for the success or failure of this repair.  Proceed at your own risk.  I'll answer questions when possible but the information presented here is provided 'as is'.  Read the entire article prior to starting!  I can be contacted at pgrobin@pgrobinson.com.  



The fix is to strengthen the seam and force the plenum to maintain its shape.  This involves three parts:


  • 1/2" (12.5mm) 'U' channel aluminum of sufficient length to run the length of the plenum top and bottom (about 30")
  • A 1/8" (3mm) threaded rod about 24" long
  • 6 self-locking nuts for the threaded rod with flat washers
And you'll need to following tools at bare minimum:

Overall view of repaired plenum
Overall view of a repaired plenum. Click to enlarge.
  • Hack saw
  • Dremel tool or similar cutting tool
  • High speed drill
  • Drill bits of the proper size
  • Drill punch
  • Wrench for the nuts
  • Long-nose pliers
  • A large bench mounted vise (6" vise required)
  • Measuring tape

As the photos show, the aim is to brace the top and bottom seams with the 'U' channel using three rods evenly spaced over the length of the channels.  If the seam can't expand outward there isn't room for the side to 'implode'.  To see the effect of the repair press on the side with both thumbs and note the amount of deflection (you may have to press very hard).  Try this again after the repair, you shouldn't see any deflection with the same amount of pressure.

The trick is to be able to drill the holes from each side so they line up.  This requires careful measuring and remember, measure twice, drill once!  The drill bit, by the way, should be exactly the same size as the rod, so the plastic plenum will self seal around the rod.

I clamped the plenum, with the 'U' channel installed, in a large vise (I removed the throttle housing for better access and visibility).  The 'U' channel is a tight fit at 1/2", but it does fit.   You must cut two small sections from the 'U' channel to fit it over the 'gussets' on the bottom of the plenum (see photos).
'U' channel detail, throttle spring end
Detail view of plenum at spring clip end.  Click to enlarge
'U' channel detail, throttle plate end
Detail view of plenum at the throttle plate end.  Click to enlarge
End view of repaired plenum
View of the plenum from the end showing the bracing.  Click to enlarge

I then drilled a pair of holes through the 'U' channel and the plenum together, for perfect alignment, from each side.  Then I inserted the threaded rod (not yet cut) into one hole and through the opposite hole.  This requires a bit of luck and some planning of the hole locations.  Study the 'overall' photos for a good idea of where the holes should go. You may also find it helpful to use a small tool, such as an awl or similar sharp instrument inserted through the second hole.  Get it pushed against the end of the rod to help guide the rod through the hole.  This may be the most frustrating part of this task, so be patient and persevere.

Once the rod is through the second hole put a washer and self-locking nut on the rod and tigthen until the rod is just flush with the top of the nut.  Now pull the rod back through and cut it with enough length left to handle it with pliers.  Put the second washer and self-locking nut on that end and tighten.  You should have a bit of rod to grip so you can get the nut on without the rod simply turning in place.  The fit of the rod should be tight enough where it passes through the plenum to assist in holding the rod.

Repeat the process two more times.  As you tighten the nuts on the rods don't distort the plenum, look at the 'U' channel and make sure it remains straight.  When all three rods are inserted use a dremel tool to cut the rod flush with the top of the nut (don't cut the nut).  That's it, you are ready to put the plenum back on the engine!  We have been running with this repaired plenum (it did fail as stated earlier) for more than a year now without incident.


Repaired plenum shown from the bottom
Overall view of the repaired plenum showing the bottom. Click to enlarge.