W124 diesel fast idle fault

[greenw124]

On my 1987 300D saloon (233k miles, auto with a/c, latter defunct) - occasionally when driving at low speed, the engine will 'switch' to a very fast idle speed, with the tachometer showing zero. It stays likes this even if the engine is revved. Can carry on driving under this condition, and it's only a problem when I come to slow down and stop, when it makes control a bit tricky. ALWAYS cured by stopping and re-starting the engine, SOMETIMES it cures itself when driving. Without the tacho I can't be sure of the engine speed when the fault appears, but I'd guess it is at least 1500 rpm compared to normal idle speed of 600-ish

Seems to occur in batches, ie will do it several times in one journey, then I may get several days trouble-free. The 'fast-idle' doesn't seem to be related to level of fuel in the tank, but this car has always had a problem with loss of power when the tank is getting close to empty, so I wonder if the two are related. I had the fuel filter replaced when I bought the car (at 81k) to try to fix the 'empty-tank' problem, but it made no difference.

Someone once said the fast-idle problem was likely to be a sign of a worn fuel pump, and suggested lubricating it as a stop gap - putting some gearbox oil in the tank.

Any other ideas welcome.

many thanks

 

[Parrot of Doom]

I wonder if the tank has a breather thats blocked?

 

[Bolide]

I've not come across a fast idle problem on the early diesels before. I don't know what sets the idle speed but I'd look at that first, followed by the cold-start enrichment device

I don't think adding engine oil will do anything other than produce smoke. The pump is lubricated by the diesel fuel

Have you changed the fuel pre-filter?

Loss of power can be caused by the shutoff valve not opening fully

I think you need to take the car to a diesel specialist

Nick Froome
www.w124.co.uk