Engine Undershield

johnway · Unread post by **johnway** » 25 Apr 2010, 12:25

Hello,
First the Xantia then the C5 - both cars have suffered from rusting fasteners for the engine undershield, causing instant panic when the shield drops to the road and scrapes when at speed. Are these shields really necessary and, if so, what for exactly? As usual all advice gratefully received.

Regards,
John.

spider · Unread post by **spider** » 25 Apr 2010, 13:45

They serve several purposes really.

To reduce engine and to some degree road noise

To keep the engine bay a bit cleaner

To keep any oil leaks off the road (maybe, although the shield does seem absorbent

To protect the (in most cases alloy) sump area from stones etc

I'm only basing these observations on 406's which had one too (although you can use bolts and large washers instead of the original clips) , the later push in clips are not as good (although easier to remove) than the earlier bolts.

The ones I saw it was about 50/50 if they still had a shield.

Hope that's a bit of help.

Clogzz · Unread post by **Clogzz** » 25 Apr 2010, 13:51

The under shield holds the car together.
That’s why the grease monkeys haven’t thrown it away yet.
The bumper and inner wheel shields are attached to it.
Otherwise, they would flap in the breeze.

I have it off for now, to change the auto box oil.
It even has to come off to change the engine oil.
I don’t drive faster than 50 mph, because I don’t want the whole front to blow away.

Unread post by **myglaren** » 25 Apr 2010, 14:04

Mine is in my garden too since I changed the oil/filters last week, was attached with tie-wraps.

I noticed while under the car that the 'hose' from the wheel arch to the airbox seemed to be stuffed with something. Has anyone else noticed this? Is it some form of pre-filter or have I just collected a ton of leaves and muck

Clogzz · Unread post by **Clogzz** » 25 Apr 2010, 14:30

Hi Steve,

Mine is hidden behind the rubbish bins, for in case the sun melted it.
Four mountings are bolts, so as to locate the bumper properly.
I’ll look at the air intake by daylight tomorrow, because I’m suspecting something there too.
I cleaned out the air box a few months ago, and by now there’s small leaf matter in it again.

JohnD · Unread post by **JohnD** » 25 Apr 2010, 14:43

When I had my 2.1 Xantia, I drove it down to Spain without the undertray. By the time I got around to refitting it, the ends of the wing liners were broken because of flapping in the wind.

Incidentally the 2.1's undertray had a useful trapflap under the sump plug, so there was no need to remove the tray to do an oil change.

Unread post by **Old-Guy** » 25 Apr 2010, 14:56

Some mechanics are in the habit of throwing away all undershields for at least two reasons:
- it saves the time and trouble not putting it back (especially if some of the fasteners are rusted)
- it saves more time and trouble at the next service not having to struggle to get it off ("there wasn't one, sir").
Vehicle manufacturers don't waste money fitting something whose existence yer average owner will be blissfully unaware, without having sound reasons (including those listed in previous posts).
They also have important and complex effects on the airflow under the vehicle and in the engine bay. These affect both fuel economy and engine compartment temperatures.

Personally, I've repaired the couple of fasteners that had corroded as I take Citroens reasons seriously and I don't like the idea of driving around at speed with the front part of the car like a box with no bottom.

Synergietom · Unread post by **Synergietom** » 25 Apr 2010, 15:37

Speaking as a service mechanic myself ( allbeit VAG ) the originall sole purpose of these undertrays was to stop stones and debri from causing minor damage when flying up from the road but over time manufacturers found they made a small amount of difference on fuel economy when travelling at motorway speeds which coincidently is the only real time you should be should be going fast enough for flying objects to cause any damage my advice would be to get rid unless your doing a lot of motorway driving then keep it on and replace any substandard clips with nut and bolt.

Unread post by **Peter.N.** » 25 Apr 2010, 16:41

Out of seven XMs that I have owned only one had an undertray and that was a pain, I think the main purpose of them on diesels was to make the car sound quieter when new.

Peter

Citroenmad · Unread post by **Citroenmad** » 25 Apr 2010, 16:48

I really dont like under trays, they are usually very difficult to remove and refit.

However i do always keep them on the car. Ive driven the C5 with it off just to test the car after doing something and there was no difference to noise.

C5s trays are held on with funny nail looking things with a belly in the middle. These are a little rubbish and ive lost a few of mine. Usually i hear them clang down the underside of the car just after ive had the undertray off.

Ive replaced most of my pins with small screws, though if its not removed often i can imagine they will rust in, so i always put some copper grease on them.

Its the worst part about servicing the C5s, the dreaded under tray. I much prefer the facelifted C5s oil Filter, cartridge type instead of the screw on units. Which means since I have an oil sucker i dont have to remove the undertray on that to change the oil, same on my Seat too.

jonathan_dyane · Unread post by **jonathan_dyane** » 25 Apr 2010, 19:39

Old-Guy wrote: Vehicle manufacturers don't waste money fitting something whose existence yer average owner will be blissfully unaware, without having sound reasons (including those listed in previous posts)

I don't *entirely* agree that an undertray is always desirable. The S1 XM did not have an undertray, however many of the S2 models did. On cars that suffered having the undertray fitted, if it was not discarded the result was invariably serious rust in the front subframe as it caused a trap for moisture.

Unread post by **CitroJim** » 25 Apr 2010, 19:42

I believe, and correct me if I'm wrong here, that not all Xantias came with an undertray. The V6 and Activa for example...

As said, on the DERV models it was for noise purposes in the same way as the HDi has an engine top cover...

h2ocooler · Unread post by **h2ocooler** » 25 Apr 2010, 20:39

The undertray for my C5 is in bordeax, it came loose (broken mounts) on the way to a friends last year, got to their place with bungee's holding it up and took it off there, its in their barn for "safekeeping"

On of my many trips through france (every 6 weeks, 450 miles each way) the offside wing liner came loose and all hell broke loose too, banging like mad, vibration through the wheel, people pointing and looking, when i stopped it had been hitting the wheel, rubbed along the ground and worn some of it away and pulled the bumper out, Again bungee's helped till I could get to a service area and re-fix it.

Rob

davetherave · Unread post by **davetherave** » 25 Apr 2010, 21:16

I attached the undertray on the xantia with wing nuts, makes it very easy to get on and off.
Ive been trying to find one for the XM with no success so far!

HDI · Unread post by **HDI** » 26 Apr 2010, 15:42

The main purposes of undertrays are to control the airflow through the engine bay both for fuel economy and engine warm up control and also overall aerodynamics.
I know when I replaced the one on my S2 it warmed up noticably quicker this winter.