Over the past few days my car has developed a knocking noise. This comes on mainly in first and second gear when accelerating away but not until the engine is up to full normal working temperature. It disappears as the engine accelerates and performance and fuel consumption do not seem to be affected.
The other symptom is that the car has recently been a bit hesitant and spluttering when starting from cold. I took it to my local Mercedes independent outlet who said that they thought that it could be a problem with one of the fuel injectors but they warned me that this could be fairly expensive to cure.
They suggested that I use an additive in my fuel for a couple of fill ups to see if this might clean things up and overcome the problem (e.g. Wynns).
Does anyone have any theories about this problem, and if i go down the additives route what specific product is the most recommendable?
Thanks
Bob
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You don`t mention how old the vehicle is and the mileage covered, but injectors becoming clogged would mainly occur if the car is used a lot for short journeys. I have personally used Millers Diesel Power Plus in my 320d and found it to be beneficial along with an "Italian Tune up" ( blasting the engine up to 4500 rpm)
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Sounds like a fault with the post fuel injection i.e a small amount of fuel is injected prior to the main fuel injection to reduce ignition delay and diesel knock. Either the injector(s) aren't recieving the pulse that switches them on for the post injection or they are clogged/faulty ie dribbling or spray too streaky and not atomising I would guess.
Could be a way of disabling each cylinder in turn to find the faulty injector and use a scope to check the electronic pulses either way needs someone that has correct diagnostic equipment and knowledge.
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Sorry made mistake earlier - should read pilot fuel injection not post injection, thanks
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Thanks for replies on this.
The car is a 1999 model (manual) and has done 79000 miles.
I put a bottle of injector cleaner into the fuel and it seemed to work brilliantly - the noise seemed to almost disappear for a couple of days. However it seems to be back again. Is it worth trying a second bottle or should I just book it in for attention? Also can anyone speculate on whether running it is likely to cause any longer term damage? - I have to drive about 400 miles in the next two days
Thanks again.
Bob
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I think dieselhead is correct with his diagnosis of pilot injection problem. You need to take it somewhere with correct diagnostic gear.
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Thanks - I feel a bit happier now that the garage's diagnosis appears to have been supported by this site's experts.
Update - car refused to start at all this morning. Just spluttered and engine would not pick up.
Brambob
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Find a Bosch Diesel service centre and you'll get a v-f-m repair and you'll not be contributing towards a dealership service maanager's monthly bonus!
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groups.msn.com/honestjohn - Pictures say a thousand words.....
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For anyone who may be interested - an update
I took the car into the local independent Merc outlet who have always serviced the car and given me excellent service.
They had the car for over 2 days, did all sorts of diagnostics test, and rang me to say that they could not find a fault. They suspect, as has been commented on here, that it is one of the injectors, but their electronic diagnostics do not show which one. They were talking of swapping them out one by one to see if they could cure the fault but they must have decided, at £250 plus VAT per injector, that this would be too time-consuming, and they told me that the existing injectors were completely bedded in and would be almost impossible to remove.
As a result they handed the car back to me in the same state.
To summarise:
1. The car is very difficult to start and needs about 10 seconds of key turning before the engine splutters into life. It misfires badly until it gets moving properly. This seems to be getting gradually worse.
2. It knocks badly at low revs but sounds normal when on the move.
3. Fuel consumption is fine and there does not seem to be excessive smpke from the exhaust
Any suggestions on what are the best options from here please (or alternative thoughts on what may be wrong)?
Thanks
Bob
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