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Old 17th June 2016, 09:39   #1
Alanbp
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Default CDT Auto Question

I have been reading up on the merits or otherwise of slipping the car in neutral at lights rather than staying in drive and holding it on the brake.

I have noticed when I start from cold and pull up at the first stop, I hold the car on the brake in drive, the revs start to "hunt" and you can feel it through the car. This does not happen when warm. If I slip into neutral it is less noticable. I assume its a cold start fueling effect? I notice the idle although almost smooth undulates a wee bit. I have a EGR bypass fitted and the PCV is all changed etc along with all fluids and filters.

I have also noticed that the "box" changes into top as soon as possible and the revs are more in the 1500-1700 at normal A-road speeds. I remember in my Xantia TD that the optimum power with turbo boost was 2100-2300 which improved fuel economy. As my mpg is around 36 which I am a little disappointed with I wondered if this might be the cause?

I dont have any noticable black smoke, although I noticed after fitting my exhaust that there was a little soot on a cold start rev.

Any suggestions?

Many thanks,
Alan
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Old 17th June 2016, 10:55   #2
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Hi Alan

I have this exact same "issue" (although I don't think it is a problem) :

http://www.the75andztclub.co.uk/foru...d.php?t=236124

In that thread, Spyder says upping the idle by 30rpm is the solution. Need a T4 diagnostic tool to do that (or maybe TOAF can do it).

The gearbox when in "D" will change up at around 1800rpm on lightish throttle. It is very keen to change up. This was the Rover strategy to get the best economy. If you use the switch and set to "S" (sport) mode it will change up at around 2100 rpm and be more responsive, but may harm economy.

Your MPG sounds about right for mostly town driving. I will get around 38 MPG in my ZT auto mostly around town - some fast A road blasts - maybe 70/30 (bigger wheels on the ZT so probably not so efficient as smaller ones - 16/17's).

On a good motorway run you should see around 45-47 MPG.

Slight black soot is normal - give the car a good run and revs on a motorway to clear the engine from time to time. Driving around town all the time at low revs is not brilliant for diesels.

You can try Millers or Redex additive to give the injectors a clean (or use Shell Optimax, etc.) which should help soot.

Also the manifold probably could do with a clean - does wonders for engine breathing - loads of oily, sludgy carbon deposits in there - do a search at the top - the Google one for details - a very messy job but worth it.

Cheers

Andrew
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Old 17th June 2016, 12:22   #3
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That is exactly the same. That's good to know it appears to be a characteristic of the car. I cant help thinking that sitting in neutral must save fuel, as the engine note smooths out and its not "fighting the car"

I ran a bottle of millers through the tank to clean the injectors but not a filter/injector soak treatment yet. Will try optimax too as a treat for it!

Mean to do the inlet manifold and clean the MAP at some point. My driving is a mix of 40% city rush hour commute and 60% open A roads, sitting at 50-60. Over the last 2 tanks brim-brim it was 35 then 37, but I have been doing more "static running" with work on the electrics (towbar) and exhaust replacment, EGR etc testing for things. Would be happy to have the mpg around 40-42 as this is what I used to get from PSA 1.9 TD lumps. Tales of 50-60 mpg make me very jealous!!
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Old 17th June 2016, 14:28   #4
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Take the 60 MPG claims with a pinch of salt - a nun out on her first long drive may just possibly get that.

Around 50 MPG should be on the cards for manual cars. Around 45 MPG for autos. Both of these in taking it easy at around 60 MPH scenario's.

When going "with the flow" of normal traffic I get around 42 MPG.
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Old 17th June 2016, 21:55   #5
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Quote:
Originally Posted by Alanbp View Post
I have been reading up on the merits or otherwise of slipping the car in neutral at lights rather than staying in drive and holding it on the brake.
Alan,

You might be interested in this.

But that apart, call me old fashioned but I believe that it is good practice manually to engage neutral and use the handbrake for extended stops at traffic lights for these reasons:
  • It prevents the transmission fighting the brakes which is probably detrimental to fuel consumption and stresses mechanical components.
  • It keeps the handbrake linkage in good working order.
  • Use of the handbrake is safer in the event of a rear collision. It was said when I learned to drive that your foot would be dislodged from the brake pedal by the impact allowing your car to lurch forward and hit the car in front; similar to a motorway 'pile-up' but at lower speed.
  • Your high intensity brake lamp doesn't dazzle the occupants of the vehicle behind you.

I expect that many will consider this view highly controversial since 99% of drivers sit with their foot on the footbrake when stationary, so I will don my tin hat and head for the trenches; unarmed I might add.

Simon
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Last edited by SD1too; 18th June 2016 at 08:13.. Reason: Adding a further reason
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Old 18th June 2016, 12:27   #6
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Quote:
Originally Posted by first-things-first View Post
Take the 60 MPG claims with a pinch of salt - a nun out on her first long drive may just possibly get that.

Around 50 MPG should be on the cards for manual cars. Around 45 MPG for autos. Both of these in taking it easy at around 60 MPH scenario's.

When going "with the flow" of normal traffic I get around 42 MPG.
Well to contradict what you say, my cdt auto returned 52.4 mpg on a run from Newcastle to Essex last year,. Brimmed the car before I left, did about 320 miles, filled the car to come back home,and divided the milage by the number of litres required to refill,then divided the litres by 4.5 to convert to gallons.
On the same run the previous year ,with a MANUAL, I got 51mpg
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Old 18th June 2016, 13:33   #7
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Just spent a week in Oldham On a VOSA course and to and from (Isle of Sheppey Kent) on the motorway doing normal M/way speeds 70 and above ish (maybe another 10mph maybe not lol) I averaged 44.8mpg, at average speed of 47mph, was rather chuffed with that as i wasn't tickling the throttle and showing BMW's that the 75 auto estate (tourer) isn't an old mans car capable of only 60mph.
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Old 18th June 2016, 14:12   #8
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1 Imperial Gallon (UK) = 4.54609188 litres so calc on 4.5 will be a bit out..

My diesel auto will give me 45-47 mpg on a motor way, the speed doesen't seem to matter very much, as long as top (5th) gear is used.

In town you wont get more than (approx) 30 mpg due to the gearbox limiting your car to gears 1-2-3...

I never shift out of gear and use the hand brake except, if I going to be stationary for a long time...

I prefer to manually shift down on long steep hills.

I have never needed to use sport mode, and use snow mode a lot on slippery surfaces...

I am the cars 2nd owner from new, and have had the car for 7 years. mileage is now 180,000 miles..

My car has been 161 modded by MarinaBrian...
...

Last edited by James.uk; 18th June 2016 at 14:16..
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Old 18th June 2016, 19:14   #9
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Quote:
Originally Posted by SD1too View Post
Alan,

You might be interested in this.

But that apart, call me old fashioned but I believe that it is good practice manually to engage neutral and use the handbrake for extended stops at traffic lights for these reasons:
  • It prevents the transmission fighting the brakes which is probably detrimental to fuel consumption and stresses mechanical components.
  • It keeps the handbrake linkage in good working order.
  • Use of the handbrake is safer in the event of a rear collision. It was said when I learned to drive that your foot would be dislodged from the brake pedal by the impact allowing your car to lurch forward and hit the car in front; similar to a motorway 'pile-up' but at lower speed.
  • Your high intensity brake lamp doesn't dazzle the occupants of the vehicle behind you.

I expect that many will consider this view highly controversial since 99% of drivers sit with their foot on the footbrake when stationary, so I will don my tin hat and head for the trenches; unarmed I might add.

Simon
Adding to above advice:

If first in queue it is best to keep in gear;
on average each car will move off 1.5 seconds behind the car in front, that gives you plenty of time to put into gear and take off handbrake.

This advice applies equally to manual or auto box for reasons given by Simon.
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Old 18th June 2016, 19:54   #10
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Quote:
Originally Posted by RPWC View Post
Well to contradict what you say, my cdt auto returned 52.4 mpg on a run from Newcastle to Essex last year,. Brimmed the car before I left, did about 320 miles, filled the car to come back home,and divided the milage by the number of litres required to refill,then divided the litres by 4.5 to convert to gallons.
On the same run the previous year ,with a MANUAL, I got 51mpg
Well done. I suppose smaller wheels and Dunlop / Goodyear Efficiency tyres make the difference. When did you take up the habit.
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