Idle problems

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When my e300TD i sitting still in P or N the car sits at 900RPM and idles fine and smooth but as soon as i touch the brake pedal the revs drop to 500 RPM and the car starts to judder, also happens when driving below 20mph and touch the brakes (the idle drops to 500rpm and car shudders) Anyone got any ideas what the prob could be? dont know if it is related but the car is very slow to accelerate unti the turbo kicks in. put the foot down and the car crawls for about 3 secs then the turbo kicks in and off it goes. Is this normal?

 

[BlackC55]

Brake light sw?

 

[Dieselman]

It should idle at 600rpm all the time and so far I don't see the connection to applying the brakes causing a problem.

These cars are slow to move off due to underfuelling for emissions and the autobox winding up.

 

[Bazzle]

If it was a petrol car I would blame the vac to brake booster.

Bazzle

 

[Dieselman]

It sounds like the accelerator cable is too tight so the engine idles at 900rpm. When the brakes are applied this causes the ECU to ignore the accelerator potentiometer so the revs drop back to the true idle speed, which should be 600rpm.
If the engine shudders at low revs there is a problem with it. Suspect faulty fuel delivery valve on the IP, bad injector or low compression.

You can ascertain which cylinder is faulty by cracking free each injector line in turn. The one that makes no difference is the faulty one.

 

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So looks like the last owner knew there was a prob and just turned the revs up so it idled fine when sitting. Thanks Dieselman will need to have a look at it and see what i can find. Another question are these engines self bleeding if you run out of diesel or do you need to crack an injector to get the air out?

 

[Dieselman]

Self bleeding, but don't be tempted to fit a new fuel filter without filling it with something, even veg oil, as the battery might go flat before the engine fires.