Citroën BX – Engine Bay Rattles

4 min read

For almost as long as the BX has had its Mi16 engine back in, I’ve been aware of an engine bay rattles. With the car seemingly being more reliable and pushed into semi-regular service it seemed like a good time to start investigating. Can I solve the mystery of the engine bay rattles?

Whats the rattle with you?

In order to try down any structure born noise, without expensive measuring equipment, you have to define the unwanted sound. For me, this comprised of two key annoyances.
First was a dull thud that sounded like someone was kicking the firewall. It was generally only notable when first starting the engine. I guess an exhaust wrapped in something soft bouncing into ‘transmission’ tunnel would be the best description.
The second is a little harder to describe. Something akin to hot metal expanding or a steel bolt being dragged at an angle through an aluminium hole. I really hope the cause doesn’t match the sound on that one. This one tended to come with the engine running but cold, and gentle acceleration.

Thud thud thu . . . oh, that would do it!

The cure for the first rattle, the thud, sort of came to me without trying. While under the car checking on the route cause of a sticking height correcter I noticed some thread. Actually I noticed a LOT of thread, nearly 20mm of it. For those less used to being under a car, this means either the wrong bolt has been put in, or the nut fell off.

New and old lower engine mount
New and old lower engine mount

And the nut most definitely had fallen off, from the lower engine mount on the sub-frame side. Or maybe I just never put it on? Either way even with a new nut in place, there was more play in the lower mount than all the primary schools in the UK. At just £35 for a ‘Fortune Line‘ replacement, the thud was banished. No improvement on the other rattle though.

Top Mount Rubbers

The second rattle has taken much longer to find. A few months of periodically pushing on things with the engine at idle. On one such occasion, I had my hand on the intake manifold with the suspension on low. I’d been listening to something near the floor and put most of my excess lockdown weight onto the manifold to get back up, the rattle stopped.

A gap between mount and buffer can't be ideal
A gap between mount and buffer can’t be ideal

Some pushing and pulling of the engine ensued until I noticed a pretty big gap between the forward-most longitudinal upper engine mount. The picture doesn’t do the gap justice, it must have been between 1 and 2mm. Sure enough, pushing the engine back so that it couldn’t rock, or pulling the head forward so the rubber and mount contacted, the noise was gone.

As I’ve mentioned in a previous article, this car doesn’t do a lot of mileage. The rubber longitudinal buffers haven’t been on the car for very many years, and less than a few thousand miles. These replacements are for a Peugeot 205 and came from BakerBM, perhaps they are too small.

Shimming the Top Mount

Shim shim-eney I see you!
Shim shim-eney I see you!

But in the back of my mind, I remember that there was a spacer between the old buffer and the body of the car. I spacer I can’t see there now. With the buffer in place, I could just about see the spacer. No amount of wiggling was getting it out, but with the engine supported and the top mount off, there it was.

This immediately starts creating questions. Why does it have a shim? Is it a factory part? I’ve never seen one on any BX other than this. As far as I know, all of these longitudinal buffers are the same size on all BXs, so is there something wrong with the body? Why isn’t it installed? Did it simply drop down when I changed the buffer and I missed putting it back on? And on and on the question flew through my mind.

Peugeot 25 mount has a nipple to keep it in check
Peugeot 25 mount has a nipple to keep it in check

On the bench, I could quickly answer some of these questions. It is most definitely not a factory part! The square is closer to a rhombus than a square. The slot is rough, not parallel and touch oversize.

I’ve also never had it installed with the 205 mounts. A BX doesn’t have the nipple to keep it aligned. I had to drill a hole in the body to fit it originally, and there was no matching hole in the shim. I’ve added one in the picture above.

Putting it back together

Engine mount shimmed to perfection
Engine mount shimmed to perfection

With the shim back in place, both the air gap and the engine bay rattles have gone. The buffer is just touching the mount as it should be and the strange noises on idle have been banished. I also took the opportunity to hit the upper engine mount with the grit blaster at the same time.

Another few problems cured on the BX now, and as it’s almost become a daily commuting car (on the odd days I have to drive), the refinement will be welcome. I still wonder why the shim is there, but it is clearly needed for whatever reason. It will be interesting to find out if anyone else has one!


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