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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.

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  • 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 ♦
    Commented Jan 26, 2017 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
    Commented Jan 27, 2017 at 20:25
  • Is the exhaust smoky at all? If so, what color?
    – Tedwin
    Commented Jan 28, 2017 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
    Commented Jan 28, 2017 at 11:33
  • EGR in a diesel works the opposite way that it works on a gasoline engine. In a diesel, the EGR is wide open at idle during the leanest mixture. The EGR shuts off progressively as the throttle is increased. This is compared to a gasoline engine where the EGR is closed at idle and progressively increases with throttle up to a point. Past that point, the EGR progressivly shuts again.
    – vini_i
    Commented Aug 20, 2020 at 15:27

2 Answers 2

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You don't state which model you have, so I'm conveying information relevant to the W124, W202 and W210 series. If your engine is a OM602 or OM605, then the EGR is notorious for clogging up the intake runner. Disabling the EGR will improve performance because it leaves the butterfly valve in the intake runner fully open, with the added benefit of never drawing in exhaust soot. On later cars the EGR cannot be disabled or bypassed without affecting the ECU, but on earlier models without ECU the EGR can be safely disabled; some of these engines even were sold without EGR according to the electronic parts catalog. Do an internet search for "OM606 EGR remove".

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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.

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  • 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
    Commented Sep 19, 2018 at 23:44

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