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Only 3 years after its last cleaning my diesel particulate filter showed a service warning again (after running fine for 10 years before the previous cleaning). I set up an appointment with my mechanic and between then and the time I had the appointment also the check engine light had popped up. This has happened a few times in the past few years only for it to disappear again within a couple of hours. I attributed those to a fluke. However this time it coincided with my appointment and I had the mechanic check it out.

He told me the particulate filter is more than 50% filled up with soot/ash which is why the service warning showed up. The check engine light mentioned issues with the glow plugs and the throttle plate no longer properly responding.

I was thinking if there could be a link between the issue of the throttle plate and the surprisingly fast built up particulate filter. My train of thought was that the throttle plate (which according to my research in diesel engines still regulates airflow to the engine) malfunctioning would mean that it can no longer properly regulate the airflow to match the injected diesel creating less than ideal fuel/air ratios during combustion.

I usually work with small diesel engines with manual control of the fuel pump and no throttle plate and there sudden changes in the amount of fuel causes the engine to smoke excessively until the air/fuel mixture has stabilized again a couple of seconds later. I was wondering if this malfunctioning plate could have caused similar conditions with excessive soot in the exhaust clogging up the filter beyond its capacity for self cleaning.

My mechanic - whom I know quite well and trust completely - said he would not categorally say no but it seems very unlikely, only potentially in combination with the broken glow plugs.

Is there any possibility my train of thought could still be correct or is this indeed extremely unlikely? Id love to have someone else think about it.

I have a 2010 Mitsubishi Outlander 2.2 DI-D.

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I don't think there's a plausible link between a malfunctioning throttle plate and the clogged DPF because no matter what the throttle plate is doing, the MAF is still continuously measuring the air mass so the ECU can calculate the correct amount of fuel to inject.

Glow plugs? Also not a strong likelihood. At worst, a cylinder with a bad glow plug will smoke for a few seconds after startup, but probably not enough to clog a DPF.

If you take short trips, especially in winter, the DPF will not get hot enough to burn off the soot. Even longer trips at less than highway speed can prevent the exhaust system from fully warming up and regenerating the DPF. If you are not driving a DPF-equipped diesel at 100 km/hr for a continuous 20 minutes at least once per week, I think you will eventually get a DPF warning light.

In my own experience with a different diesel vehicle, if I spend more than about 90 minutes plowing snow at low speed (lots of forward and reverse maneuvering, all at low speed) my DPF loads up to 80% and gives me a warning. In that case I have to stop what I'm doing and go for a drive at highway speed for 20-30 minutes to allow the DPF to regenerate.

If you are definitely not taking short or slow trips that prevent DPF regeneration, you should ask your mechanic to use a smoke machine to search for leaks in your air induction system. A loose turbo hose, a crack in an air box, a hole in the intercooler, all can cause air to leak. (So what? Read on!)

As mentioned, the MAF sensor is constantly measuring the mass of air that flows into the engine so the ECU can calculate how much fuel to inject. But if there is an air leak, the engine gets less air than the ECU thinks, which means the engine gets too much fuel for the amount of air actually delivered to the engine. That will create excessive soot, and that will eventually clog your DPF.

It's very hard to find an air leak without a smoke machine, so if your mechanic does not have one, you might consider finding another mechanic with a smoke machine just to very the integrity of your air induction system.

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  • Yea, I thought I might be grasping at straws here, since either the MAF or the exhaust sensor could detect bad fuel/air mixtures, but you never know :D. I usually drive around 4-5 hours week at 130+km/h, so the self cleaner should be able to take care of it. Guess I might have to bite the bullet and eventually buy a new filter... Mar 8, 2023 at 18:56
  • @YanickSalzmann If there are no detectable air leaks and your MAF gives a plausible reading at a given rpm, then yes, perhaps a new DPF is in your future.
    – MTA
    Mar 8, 2023 at 19:21
  • @MTA Presumably in your example of an air leak where the engine gets less air than expected, you are talking about during periods of positive air pressure provided by the turbo charger.
    – HandyHowie
    Mar 9, 2023 at 7:40
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    @MTA Just a thought, you may know the answer to this - Since the main reason for a diesel throttle is to increase EGR efficiency, if the throttle is permanently not opening correctly, could there be too much EGR happening? Would this cause an increase in particulates?
    – HandyHowie
    Mar 9, 2023 at 7:58
  • @HandyHowie Yes, air leak under turbo pressure was my thought. Regarding too much EGR, the ECM would have to be commanding too much EGR (or the EGR valve would have to be stuck open) for a stuck throttle body to increase EGR efficiency by limiting fresh air in favor or recirculated exhaust. That would cause more soot, but I think performance under load would also suffer, there might be an engine knock and idle would be rough. OP mentioned none of these. Yet it's certainly worth investigating all low-cost options before replacing the DPF!
    – MTA
    Mar 9, 2023 at 12:46

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