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My car hasn't done vast amounts of miles (short of 60000), but already at 30-40000 I would say I noticed it's ability to accelerate has dropped. It is still OK for my use and the fuel efficiency is good (60+ US mpg, 70+ UK mpg). I am guessing reasons would overlap with some of these causes, but wanted to ask somebody more knowledgeable than me.

It is a standard turbodiesel (no common rail or variable vanes) with particle filter (DPF) and catalytic converter, manual transmission (and having read other posts here I think the clutch is fine). It is a 1422cc 3 cylinder engine with a balancing shaft, although I don't think that matters for the arguments.

Things I can think of are DPF (although it does not appear to give any new/unusal problems), air filters, worn bearings giving more resistance, perhaps the turbo is worn/less efficient (why?), but apart from that I am not sure what causes this.

I don't think losing the "new car effect" is uncommon or a problem as such, I am curious about the causes. Thank you!

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    What is the condition of your fuel filters? Diesels require constant fuel filter changes.
    – vini_i
    Commented Feb 19, 2017 at 15:21
  • Have you measured the car's acceleration, and compared it with specs? "It feels slower to accelerate" may not be accurate, it is difficult to objectively compare today's acceleration with that of a year ago just from memory.
    – Hobbes
    Commented Feb 20, 2017 at 13:46
  • When was your car last serviced? Do you know ( and are you sure) they replaced all of the filters at the required intervals?
    – Sir Swears-a-lot
    Commented Feb 27, 2017 at 7:41
  • Unless you can objectively measure the power of your car, you (and everyone else ) can only speculate. A dyno run would give you an idea against published claims but even that may not be accurate or reliable.
    – Sir Swears-a-lot
    Commented Feb 27, 2017 at 7:47

2 Answers 2

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I've had this problem on every car I've ever owned over the past 35 years. With only 60k miles, it is highly unlikely to be any of the causes in the link you post.

What is most likely is deposit buildup throughout the fuel system, combustion chambers, exhaust system. Here is the method I use to restore acceleration to 100% capacity:

Start by changing the motor oil and filter with a top synthetic oil like Mobil 1. Substitute one quart of oil with automatic transmission fluid (ATF).

If the car has a PCV valve and EGR valve, make sure both have been replaced and the tubing cleaned with carburetor cleaner. Replace the air filter (and spark plugs, if applicable). Check the intake manifold and vacuum hoses for air leaks.

Add ATF to your gas tank at fill-up at a treat rate of 1 oz. per gallon. Drive the car immediately in stop-and-go traffic for 10 minutes, shut off engine for 10 minutes, then drive on the highway at 70 MPH for 10 minutes. Repeat this cycle multiple times, for at least an hour, and allow the mixture to penetrate at least two days.

When your tank drops to 10 gallons of gas, add two cans (30 oz.) of Cataclean. Immediately drive the car on the highway for 30-60 minutes, then "floor" the car at wide-open-throttle (WOT) from 50-80 MPH. Repeat the full throttle blasts at least six times, but up to 30 times if the cat. & sensors are very dirty. Be sure the cooling system and transmission are flushed and working properly beforehand. Turn the heater and fan on maximum, with windows open, when performing the WOT blasts. Repeat this cycle by driving the car in stop-and-go traffic, shutting engine off for 10 minutes, then driving on the highway (you can substitute highway speed with driving the car in first gear, 25-35 MPH, at high RPM to develop heat).

Cataclean evaporates out of the gas tank after about 24 hours, so be sure to run the car hard right away. The ATF never fully evaporates. The two products – ATF & Cataclean -- work together and will never damage any components of the car.

When the problems are resolved and you refill the tank, continue adding ATF at a treat rate of 4 oz. for every 10 gallons of gas. It will continue to clean deposits. Every fourth fill-up, add one can of Cataclean and drive at highway speeds.

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    Dont you think it would be best to find out the cause of the problem instead of spending heaps of money on changing/cleaning parts which may be in perfect working condition?
    – Evren Yurtesen
    Commented Feb 20, 2017 at 8:00
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    also that catalytic converter cleaner stuff do they actually work? I have seen some videos saying otherwise
    – method
    Commented Feb 20, 2017 at 8:24
  • A PCV valve and EGR are not much money. Since the poster has no codes, low mileage, and diesel engine, the problem is likely carbon deposits.
    – Carguy
    Commented Feb 20, 2017 at 9:13
  • The catalytic converter stuff has definitely been working for me for 35 years on all kinds of cars, even two late model Mercedes-Benz with poor idle / poor acceleration / poor gas mileage. It is mostly acetone & xylene mix, which have chemical properties that have been proven to clean deposits since the 1950s. Some scientists even argue that acetone should be used as a fuel additive by major oil companies.
    – Carguy
    Commented Feb 20, 2017 at 9:15
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    @method I think that the additives actually do not do anything. Because they would burn in the engine to begin with. For example, see combustion formula for acetone C3H6O + 4 O2 --> 3 CO2 + 3 H2O Maybe acetone can be a good solvent, but it won't be acetone what reaches your catalyst. I searched online for actual test results. Some people tried to make comparison of exhaust emissions etc. using empirical data from exhaust gas test machines after using cataclean. It does not appear to improve things. Of course this won't stop people from feeling improvements :)
    – Evren Yurtesen
    Commented Feb 20, 2017 at 18:41
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There can be many reasons to your problem. It can be as simple as your turbo charger does not perform or ecu detecting a problem and limiting performance. You should take your car to a shop and get it checked. It won't help to mindlessly use cleaning additives or changing parts which may be in perfect condition.

The cars computer (ecu) records all sort of information about the performance of the engine and it's parts. Detail of the information changes from model to model. They can communicate with it at repair shop to determine exactly what is wrong. In some places they can even measure the performance of the car by running/tesing it on special measuring rigs.

The performance of your car should not get reduced by time as you explained unless something is wrong with it.

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  • Sure, there are many possible reasons for the problem. Not all reasons are as major as a failed turbo charger. Many minor problems accumulate -- dirty fuel injectors, valves, rings, cat. converter, etc. -- which are very common during slow, city driving. These small issues can be resolved by mindlessly using cleaning additives. The ECU will indeed record all sorts of information, but mechanics cannot easily determine exactly what is wrong and will charge money heavily by the hour to troubleshoot!
    – Carguy
    Commented Feb 20, 2017 at 17:44
  • With that logic, you can just replace the engine and exhaust system to fix the problem. It is a sure fix. But the logical approach would be finding out what is the cause of the problem first or if there is even a problem to begin with. Additives are just money traps. If they really worked, car manufacturers would recommend using them once in a while to clean the engine for optimal performance. Yet I have never seen any car manual which mention such stuff. Don't you think they want happy customers? Unfortunately most people feel the placebo effect of such additives.
    – Evren Yurtesen
    Commented Feb 20, 2017 at 18:07
  • My Mercedes Benz manual recommends Techron in the fuel. When I used it on my '08 E350, it solved a rough idle problem. Small aircraft manufactures officially recommended Marvel Mystery Oil. Of course it would be ideal to determine exactly what is wrong with an engine, but that approach can be very time consuming, costly, and really unnecessary for minor issues. Most fuel and oil additives are either variations of mineral oil or volatile organic compounds, which are both effective and cost very little.
    – Carguy
    Commented Feb 20, 2017 at 18:29
  • You are right, but I wonder from what year that manual is because today fuels include necessary additives in correct amounts. Even Mercedes says you shouldn't use secondary additives unless your fuel does not have additives (at least I don't know where I can get fuel without additives...hmm...): bevo.mercedes-benz.com/bevolisten/119.0_en.html "The use of secondary additives however, is mostly an additional cost burden that is not necessary and in the worst case it can lead to permanent damage."
    – Evren Yurtesen
    Commented Feb 20, 2017 at 19:30

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