Hi new to this so please be help-

I'm looking at buying a 55 plate ST 155 Mondeo what should I be looking out for???

Never heard of it - do you mean the ST-TDCI?

Yes the TDCI, looking at buying one but really worried I buy a banger

The good news about the ST-TDCI (or any mk3 Mondeo diesel) is it does not have a DPF (diesel particulate filter)

So the DPF cannot go wrong, cannot get blocked, you will not ever need to replace it, or waste fuel due to forced regens, or having to drive up and down the M-way or take the long way home to burn off the soot on the DPF, or worry about doing short journeys etc you dont have to waste money on eloys fluid, or getting a DPF delete ) (because it does not have a DPF)

Ok thats sorted

EGR

Exhaust Gas Recirculation - the EGR valve can cause flat spots, hesitation, turbo lag (even when it is working perfectly) over time it contaminates the inlet with carbon/ soot/ gunge that can eventually choke the inlet manifold, it can cause increased smoke output (out of the exhaust) exessive smoke output can fail an MOT . If the EGR valve fails it can put the car into limp home mode, or stop running at all - replacing the EGR valve can be very expensive with getting a genuine Ford one could cost £200 (an identical (same engine) Jag x-type one costs about £120)

You can blank the EGR valve, (a solid plate is about £5) solving all these problems, a Euro3 engine can have a solid plate fitted without any difficulty, a Euro4 engine will tend to put the EML light on this can be ignored or reset every once and a while with an OBD code reader, many FOC members/ Ford owners feel these are better alternatives to the above - a plate with a hole is no use - a solid plate can "fix" a leaking, jammed/ faulty EGR valve by effectively disabling it

DMF -

Dual Mass Flywheel - another source of problems on "modern" diesel cars they work great(ish) when they are new, over time they can wear, break up, gradually destroying itself untill the noise and vibration is exessive, and the bcar undrivable, can also cause the starter motor to sieze and stop working.

Changing the DMF is a big job, typical labor charges are £400, usually the clutch and CSC are changed at the same time, these parts used to come to about £400 (though prices have fallen) making £800 in total, DMFs could last 50k miles, though ive heard of them lasting as little as 25k or as much as 100/110k (depends on the way the car is driven/ what it is used for) - its not universally accepted as the best way to go but i have fitted a solid flywheel/ SMF conversion along with an uprated clutch (heavy duty/ stronger clamping force/ carbon fiber/ kevlar , sprung plate (to put back the "give" the SMF has taken out)) this setup did not cost a lot more than a standard one, and is much stronger, and the solid plate flywheel will never break up - a solid flyweel is a lot smoother than a worn-out DMF, ive run my SMF for years with no problem, many of my mates run one too

Most modern diesels of all makes are fitted with DMFs, not just Fords

Aux belt - the aux belt can rattle (it can be especially bad at tickover) on mk3 mondeo diesels, if it snaps the AC will stop working and the alternator stop turning/ and the battery stop charging - it will probably not damage the engine as a cambelt snapping might, and you could probably still drive home (in daylight/ as long as you did not use too much electrical power) - the belt kit and fitting could come to £400 - you can get a gates kit, i recommend fitting with a genuine crank pulley, that has a damper. The Jag x-type gets a special freewheelling alternator pulley that reduces wear and vibration (some people have done conversions, especially in Europe) i have fitted a special shortened belt along with an AC delete (the stock belt is very long and has a tortuous, over-complicated path)

Injectors - the mk 3 was made between 2000-2007,(ST-TDCI 2004-2007) the (Delphi) TDDI injectors were designed for the old, high-sulphor diesel, that has lubrication properties, when running on the pump fuel you get today, the injectors may not last as long - the injectors fitted to Mondeo mk3 TDCIs are considered "soft" they can be very expensive to replace/ refurbish again most modern diesels of all makes are the same

Its a nice car to drive, has a lot of torque and a decent turn of speed, especially for the MPG, you can pick them up very cheap, especially for the amount of car you get for the money - even with the above problems, i would still recommend one, just keep a bit of money aside for potential repairs