My cold starts have been quite bad (since the injection pump replacement, or just a coincidence that weather got colder), and I started suspecting the 4 year old glow plugs… did a continuity check on them without removing, seemed OK, I checked pump timing, was a bit late so advanced a bit to close to top of the spec, better but still not as it should be, considering it in only around -2 in the mornings, I dread to think how embarrassing cold starts might be when I take it somewhere where it is -15...

So the other day I switched to Shell fuel, the normal stuff, not the V-power.  Previously I was filling up at Asda.  And since then cold starts have been on all 6 cylinders, and better by miles.  Anyone have any experience of the V6 really being sensitive to fuel quality/ cetane number perhaps?  I honestly cannot believe the difference to cold start behaviour, no more smoke and misfiring engine for 5 seconds, never experienced such a sensitivity on any car I have had before, simply unbelievable.  I think I will switch back to Asda on the next tank just to prove the theory and that it is indeed the fuel.

Increasing Cetane always improves cold starts. You could try adding 2EHN to the tank to improve the cetane rating but it shouldn't really be necessary.

What is the ambient temperature?

How long does the glowplug light stay on?

All pumpe düse engines are extremely sensitive to the fuel you put in. Higher cetane will increase power output immensely. My old 1.9 could get up to 10-15bhp more from V-Power diesel.

 

Also, depending on where you live, most of Europe is running on winter diesel at the moment. You can read more about the % of additives added in your region here: http://en.wikipedia.org/wiki/Winter_diesel_fuel

 

Additives will of course decrease performance and again, it has a significantly bigger impact on your pumpe düse than newer common rail systems. New CR engines run the pressure of the diesel above 3000 bar (44000 psi) hence making it less tolerant to additives or "bad" cetane diesel fuels. 

The 2.5 V6 is an old rotary pump engine - not a PD.

 

Assuming you have good compressions and that the timings of both the camshafts and fuel pump is correctly set, the difference you are seeing in starting performance between the fuels is more likely to be due to viscosity variation than cetane value.

 

Make sure also that your battery is able to give the highest possible cranking speed. A less than perfect VP44 (ie most of them) will not deliver adequate low viscosity fuel pressure and volume at low cranking speeds. Sorry to sound negative - the VP44 is not my chosen fuel delivery device.

 

rotodiesel.

 

Always test glowplugs with an ammeter. Resistance measurements are useless as the plug resistance is so low (unless it has failed) but if a glowplug is passing, say 10A, you know it's hot. The current draw of a glowplug will reduce as the internal ballast heats up. Testing this way saves taking good plugs out - I only ever replace them singly.

 

rotodiesel.

Edited 9 March, 201412 yr by rotodiesel

I stand corrected by the master himself. 

The only time I see any difference in cetane, is at full load above 3500rpm, ie on autobahn. Below that, not really noticeable, at least on my 2.5V6. Also, cold starts on this engine seem  to be most affected by timing, my Superb's engine had cold start problems when new, and advancing injection timing to ~0.8ATDC (from factory ~2.4ATDC, both within tolerance)  helped a lot. There was a SUK notice about this ~7 years ago.

 

Re the usual VP44's "extreme problems", my VP44 BDG 2.5V6 manual 6spd Superb is 8 years old, and at 126k miles it has yet to see a workshop other than my own garage, just filters, fluids and CV boot replacement, with an occasional mod and of course the big TB job (coming up again next year).  

Compare this to my wife's 1.6CR Roomster, which at 20 months and 16k miles old, has now been at the local dealership on and off for the last 3 weeks, with serious fuel in engine oil problem (ca 1litre of fuel into the sump every 600-700 miles or so, even on motorway). To give you a clear picture, 1 weekend motorway trip to Ikea and back is enough to notice change in oil level, and until the problem is sorted oil needs to be changed every 2-4 weeks to prevent excessive engine wear due to loss of viscosity. To make things worse, SUK keeps ordering tests rather than actual injector replacement (and how exactly do you test injector spray pattern without removing them from the car?), SUK's latest order was to "test drive" our car by the dealership for ~300miles - I am only guessing to check if I am telling the truth and not pouring fuel to the oil myself :( or alternatively to claim there was no increase in level and that we "must be driving the car wrong". Pathetic.  We have a use of courtesy car in the meantime and the local dealership has been helpful enough, so not all is bad

But as you see from my direct experience, the VP44 proves to be far more robust than the 1.6CR and many other engines of similar design.

Which is why I'm looking after my PD (oil lubricated pumping system).

 

Remember we are all at the cross-over point between Winter and Summer fuel and it's possible that you have been using some of each from different suppliers. Winter fuel contains a higher proportion of short-chain hydrocarbons to offset the waxing tendencies of Summer fuel which has longer chains (but a higher calorific value - not to be confused with Cetane value). Correspondingly, Winter fuel is generally of lower viscosity.

 

Low viscosity fuel, thick lubricating oil and sub-optimal battery performance (all a result of basic physical chemistry) will conspire to make starting more difficult, with the added problem of heat release. A diesel engine produces heat for ignition by the compression of air. If the rate of rise of compression is too low, the heat will escape faster into the cold metal than it can be produced - and ignition temperature will not be reached. This is why there is a critical cranking speed below which a diesel engine will not start under given conditions. Low viscosity fuel will result in more leakage past the pump plungers which again is crucial at low cranking speeds.

 

Sometimes, Mother Nature is a bitch - but with a diesel, cranking speed and compression are golden. If you can't reach ignition temperature, it won't start. CR pumps with aluminium bodies are unlikely to meet my standards of long term durability lubricated, as they are, by fuel. Producing PD values of fuel pressure in a fuel lubricated pump is a tall order....

 

rotodiesel.

Compare this to my wife's 1.6CR Roomster, which at 20 months and 16k miles old, has now been at the local dealership on and off for the last 3 weeks, with serious fuel in engine oil problem (ca 1litre of fuel into the sump every 600-700 miles or so, even on motorway). To give you a clear picture, 1 weekend motorway trip to Ikea and back is enough to notice change in oil level, and until the problem is sorted oil needs to be changed every 2-4 weeks to prevent excessive engine wear due to loss of viscosity. To make things worse, SUK keeps ordering tests rather than actual injector replacement (and how exactly do you test injector spray pattern without removing them from the car?), SUK's latest order was to "test drive" our car by the dealership for ~300miles - I am only guessing to check if I am telling the truth and not pouring fuel to the oil myself :( or alternatively to claim there was no increase in level and that we "must be driving the car wrong". 

Bummer, be interesting to see what this is when they find out.  I read somewhere that one of the "features" of some DPF systems is that they inject some extra diesel at the "wrong" time to aid getting the exhaust hot in order to regen... I wonder if yours is stuck trying to regenerate?  This diesel often finds it's way into the sump.

http://avontuning.co.uk/2013/07/mazda-6-oil-level-rising/

My 307 was similarly afflicted...

I'll be gutted when I eventually have to get rid of the Superb, I hated the DPF in that thing with a passion and it seems most new cars "benefit" from one now.

I have had no cold start problems this winter - but I have had a new battery, so the engine gets a good spin as per Roto's comments. This winter hasn't been really cold - I believe that my last winter's VP44 failure might have been due to standing out at -6C and worse for a week which may have led to the fuel waxing in the filter and starving the pump of lube.

During the winter months, I swear by Millers Eco-Max  (cetane enhancer) which makes a big difference to the cold starts and the general smoothness of the V6 in my opinion. Haven't really noticed any difference in mpg, but the car starts cleaner, all six cylinders chime in very quickly, initial smoke is reduced and that V6 burble is attained more quickly.

 

Theory is that if the cetane number is higher, then it ignites more easily .... which makes sense regarding sub zero starts and sub zero cylinder heads.

 

I tend not to use it in the summer as I don't notice any difference (probably as the summer diesel is different), but in the winter I think it makes a definite difference. 

Edited 9 March, 201412 yr by CRC