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I have a 2010 Ford S-max 1.8 TDCi with 70K miles. The engine starts most of the time, sometimes I have to try 2-3 times. If it happens to not start, the engine barely runs, maybe for a second if at all. But it starts eventually, and then it feels weak. When I drive, there is very little power felt at low revs, and pressing down the accelerator makes no difference at all. Eventually as the revs do come up, around 2.5k or so when the turbo kicks in, the car begins to feel normal, i.e. accelerating is possible beyond just a feather light foot on the gas pedal.

Air filter, oil filter, glow plugs, engine oil is all new. I only use BP and Shell diesel.

I took it to a garage a long time ago after it broke down completely, where the chaps said they found metal dust or shavings in the fuel system (either pump or filter, I cannot remember word for word), and pump + injectors need to be replaced and fuel lines cleaned out. To save me some money there and then, instead they cleaned the injectors, tank, fuel lines and changed the fuel filter - so they said at least. It's been fine from then, until a few months ago. But I really don't want to fork out for 4 injectors and a fuel pump for a 12 year old car.

Before the car started playing up, my better half drove it very close to empty tank but I am unsure if this could have contributed - my understanding is that fuel is sucked from the bottom of the tank.

Just to be sure, I am aware what diesel lags feels like - this is not it. Starting on hill used to be simple, ample power, it's a diesel after all. Now I have to be much more careful, more clutch, to the point people behind must be wondering why I am driving slowly.

Grateful for insight as to what might be going on here.

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I doubt this is an ignition or fuel supply problem, those would have very different symptoms. Turbos are air compressors driven by exhaust pressure, modern ones have electronic sensors that measure air temperature, pressure and air flow and feed the information to a computer which controls boost pressure, ignition timing and fuel delivery. If these sensors fail you will usually get a check engine light and a diagnostic code. As you haven't gotten a code the most likely culprits are:

  • Air leaks: there could be an air leak in the intake or output side of the turbo which is causing lost pressure and giving incorrect sensor readings. Check for loose or degraded hoses and re-attach/replace as necessary
  • Stuck Exhaust Gas Recirculation (EGR) valve: An EGR valve is an emissions control device which allows a small amount of exhaust into the intake manifold to help control emissions. As EGR valves are passing exhaust over time they get fouled by carbon deposits and can get stuck open or closed. If the EGR gets stuck open you get a loss of pressure on the turbo, which consequently needs more pressure to spool up. Removing and replacing or cleaning the valve is a common fix
  • Problems with the turbo itself: a lack of boost can be an indication that your turbo is on its way out

I'd start with a EGR valve, personally. They are usually easy to remove and you can often get them working again using a bottle of cleaner and a brush.

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    I'd start with a visual inspection under the hood and under the car. My son once drove off the road and tore the turbo pipe off the intercooler on my '03 Passat 1.8T (gas). It drove, but had zero power. I stopped, pushed the hose back on by hand and it drove fine again until the air pressure popped it back off. Unfortunately, he'd also broken chunks out of hard plastic, so even with the hose clamped back on, it was low on power. Could be something similar going on here, and looking at the air plumbing is really simple. Fixing it, maybe not so much...
    – FreeMan
    Jul 8, 2022 at 21:12
  • Thanks for this. Just to clarify, the turbo appears to working well. It kicks in as expected and with a lot of force. The throttle issue is experienced is at 900-2.5K revs. Also, let me add, we get no dashboard warnings. Anyway, EGR valve sounds like a good place to start. As always, much grateful for help and insight!
    – Jepper
    Jul 12, 2022 at 11:35
  • FWIW, just saw a youtube video youtu.be/eKdre36aMHo . The symptoms I have appear to be consistent with a stuck open egr valve, low power, and while the engine runs smooth, there's a weird judder in the first minutes from cold - which I initially thought was a bad injector (because I know nothing about diesel engines!!).
    – Jepper
    Jul 12, 2022 at 13:07
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    Rough running when cold is also a classic stuck EGR symptom, like I said in my answer it's the best place to start @Jepper. Let me know how you get on.
    – GdD
    Jul 12, 2022 at 13:37

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