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TL;DR

  • Mercedes 250 diesel 12/1995
  • Rode 350.000km always in low revs
  • Feels clogged, low power
  • Disabled EGR, empty catalyst

Questions:

  • How to restore power to the engine
  • Should an EGR be active without a working catalyst?

My car is a mercedes 250 diesel from 12/1995. It was from my father, who bought it with around 250.000km. At that point it had immense power and would easily accelerate even while climbing steep hills. Shortly after buying it he had the EGR disbaled (it is there but it was clogged/covered from the inside so it does nothing). I am not sure if that caused a power drop or not. More recently (about a year ago? maybe more) he had the inside of the catalyst emptied. It ran until 600.000km, mostly doing city circuits and my dad very very rarely hits the throttle, so the car always felt clogged, with low acceleration and reduced strength.

The car became mine about two months ago. I rode hard for some time and I still push the revs from time to time so that the accumulated soot and thrash gets cleaned. Right now the car feels a lot better (before it didn't even get to the redline in third, now it gets there in any gear up to fourth - did not try fifth) and smother, but I still feel that it lacks power.

I am no mechanical engineer, but I guess the engineers who balanced the engine with the EGR and catalyst knew what they were doing and the engine should have more power with those two components working correctly. Maybe I am wrong, but I did not find a consensus in the large series of questions on this site and others discussing whether the EGR is positive or negative for the engine.

Getting to my question. First, I would be grateful for tips on how to restore some power to the engine. Most likely it would need some deep cleaning since it ran some 350.000km constantly in low revs. Can something along these lines be done cheaply?

Secondly, I am considering to see if the EGR can be reactivated (following the reasoning above) and also due to environmental issues/regulations. Although I would like a definite answer on whether it provides more performance or not, my concern here if the EGR can "work" with an empty catalyst, or if it would cause a catastrophic amount of "trash" being sent into the engine. My reasoning is that if the EGR will bring unburned particles and "trash" into the engine, it would be even worse if these particles are not processed/cleaned by the catalyst.

  • I don't know the whole answer for you, but can tell you the EGR and catalyst are two completely different systems (neither should affect each other and/or runnability). The non-EGR may have a slight detrimental effect on the catalyst (increased NOx), but with the catalyst removed, there shouldn't be an issue with how it runs. – Pᴀᴜʟsᴛᴇʀ2 Jan 26 '17 at 19:18
  • @Paulster2 The two-way oxidation catalytic converter in a diesel vehicle eliminates what little carbon monoxide and unburned hydrocarbons there are in the exhaust. It does not and cannot eliminate NOx. Also, the catalytic converter is in no way harmed by increased NOx. To the catalytic converter, NOx is just a source of oxygen, and there is already plenty of oxygen in the diesel exhaust due to non-stoichiometric combustion. – juhist Jan 27 '17 at 20:25
  • Is the exhaust smoky at all? If so, what color? – Tedwin Jan 28 '17 at 2:29
  • The car makes no white or black smoke. The area around the exhaust exit gets rather sooted black. I can get a pic if needed. – Camandros Jan 28 '17 at 11:33
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Diesels make power one way - injecting more fuel. Turbo diesels have the additional advantage of boost.

Verify the fuel pressure first. New filters etc. If its turbo diesel, make sure the turbo is working.

The only other part on mechanical diesels is the timing - not sure if Mercedes used timing chains or gears on that one. If chains, replace the chain. That will smooth out the motor a bit and put a lot of new life in it.

  • 2nd that on timing chains. Cars with that many miles and any timing chain will have lots of slop in the chain. For diesels, both valve timing and injector timing needs to be spot on. – Dirk Broer Sep 19 '18 at 23:44

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