I have a 2003 Renault Vel Satis 3.0 dCi – i.e. the V6 Isuzu engine. The car has 90,000 odd miles on the clock, and has been regularly serviced. The water temperature stays steady, it uses no water, and there is no bubbling when the coolant reservoir cap is removed. (Sharp man, no cylinder head problems then, it would seem. Doc) The problem started in the autumn with an intermittent hunting (fox or stag? He he Doc) at about 1,800rpm when under load. The engine behaves as if it is starved of fuel, and as if it is being strangled. At light throttle, applied on the flat, it happily gets it up to motorway speed, but an incline, or a need for acceleration to overtake, causes it to be strangled. It eventually clears with some white smoke and is then right as rain, i.e. at any speed or any throttle setting for the rest of the day. Now it is happening more and more. I have been using BP Ultimate and not supermarket fuel. Could you advise of any possible causes? In hope,
Andrew Smith, Kendal

Long time since I heard anything from a Vel Satis owner. Congratulations on sticking to the faith Andrew. If there was anything stopping combustion whilst fuel was being delivered, then it would usually clear with the emission of black smoke. So it seems to me that you most likely have an interruption to fuel supply. This could be:

Electronic – an intermittent ECU fault that caused fuel starvation, or maybe an interruption of electronic signals from the accelerator. The Vel Satis was one of the early cars to use a drive-by-wire accelerator linkage and there is also a motorised throttle body that I believe is known to sometimes get sticky. I’m afraid I don’t have any information regarding repair or replacement costs.

Fuel supply – there is undoubtedly a fuel supply lift pump in the fuel tank that supplies low pressure fuel to the high pressure injection pump. The lift pump could at times be failing to supply sufficient fuel. It might be worth checking if the problem changes with fuel tank level. There could even be a fine mesh pump intake filter that blocks intermittently. Sometimes you can get bacterial growth in diesel fuel tanks though, particularly if the car is left unused for long periods.

An airlock in the fuel line – more likely the lower pressure fuel line, before or after the fuel filter, which also needs checking. When was it last replaced, and are there any signs of fuel leakage around the filter?

The fact that the car is alright for the rest of the day, after the problem clears, very much suggests the fuel starvation answer, as you suspect. These above are my best bets then, but the good old exhaust gas recirculation (EGR) system can give somewhat similar problems, particularly if you have done a lot of short runs in the winter months. It usually amounts to the system getting blocked with grunge, such that the EGR valves do not work properly. I would also have a good look at the air intake system, before and after the turbocharger, to check that no hoses are kinked, or jubilee clips loose, and no dead birds or mice are in residence. I hope these suggestions are of some help, although I don’t know how DIY capable you are. It’s not easy on complex diesel engines, but a decent diesel specialist garage should be better able to diagnose things than I can at a distance. I would much appreciate you keeping me in the picture though, as I’d love to know if I’ve been of any help, and what the answer is, when you or the garage have found it.
Regards,
Doc Diesel