more mpg out of a 2.5?

Ive got a 2.5 P38 and its currently running just under 20mpg. what simple things can i do to increase this?

ive heard i can clean out the intercooler? but ive got no idea where to start with that one lol.

any other suggestions?

Not a lot to make any difference......mine does around 18-20 in town and 26 ish on a run at 65mph.....not that bothered as i run on biodiesel which i make for around 20p per litre.
I've heard taking off 1 of the props gives more mpg but cant see how...still wiegh's 2+ tons.

you make it? and how is it that you do that? sounds like it could be saving you a hell of a lot of money if im honest!

coming from a 200bhp deisel astra that can manage 60mpg on a run, 20mpg is really affecting me but i guess thats what i should have expected with a big two tonne truck

20 mpg around town and 26/30 mpg on a run sounds about right.

You can check for fuel leakage from the leak off pipes under the black plastic cover on the top of the engine, make sure your turbo pipes are in good condition, and learn to have a light right foot...

Other than that welcome to the poor club....60 mpg in my dreams....

I've got a '97 2.5dse and I fitted a PSI Powerbox months ago. It's not everyone's favourite BUT my mpg went from 21-22 around town to 25 and from 27-28 on the motorway to 32mpg at a steady 70mph. The performance improvement was instant as well. It pulls through the gears quicker and smoother. At 50 mph it used to kick down when you pressed the throttle, now it just pulls again. I do a lot of miles (20,000 a year) and spend about £110 a week filling up. The powerbox cost about £350 so it paid for itself after about 6 months.

300AJC said:

I've got a '97 2.5dse and I fitted a PSI Powerbox months ago. It's not everyone's favourite BUT my mpg went from 21-22 around town to 25 and from 27-28 on the motorway to 32mpg at a steady 70mph. The performance improvement was instant as well. It pulls through the gears quicker and smoother. At 50 mph it used to kick down when you pressed the throttle, now it just pulls again. I do a lot of miles (20,000 a year) and spend about £110 a week filling up. The powerbox cost about £350 so it paid for itself after about 6 months.

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That is truly remarkable when you consider that the only way to increase torque and power output on a unmodified diesel engine, is to inject more fuel. 20,000 a year is 384.61 miles per week. £110.00 is at todays prices, 81.90 litres of fuel or 17.92gallons. 384.61 divide 17.92 is in round figures 21.45 miles per gallon. Which is over 3 mph worst than, according to your figures, could possibly be your lowest figure. Think your fuel computor is broken.

300AJC said:

I've got a '97 2.5dse and I fitted a PSI Powerbox months ago. It's not everyone's favourite BUT my mpg went from 21-22 around town to 25 and from 27-28 on the motorway to 32mpg at a steady 70mph. The performance improvement was instant as well. It pulls through the gears quicker and smoother. At 50 mph it used to kick down when you pressed the throttle, now it just pulls again. I do a lot of miles (20,000 a year) and spend about £110 a week filling up. The powerbox cost about £350 so it paid for itself after about 6 months.

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Ha Ha ha, that's the reading on the dash display which doesn't know the PSI box is injecting more fuel. Do a brim to brim check and you will find that there is very little difference with or without the PSI box.

BenRK said:

Ive got a 2.5 P38 and its currently running just under 20mpg. what simple things can i do to increase this?

ive heard i can clean out the intercooler? but ive got no idea where to start with that one lol.

any other suggestions?

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Check the leak off pipes on the injectors. Otherwise remove the lead from your right boot. Mine gives a long term average brim to brim of 24mpg around and about and up to 30mpg on the auto route

I'll have a look at what you lot have mentioned, I know my fuel filter housing is leaking a little and needs replacing but that won't cause much damage to mpg as there's only a tiny amount leakin out. Cheers fr the help

BenRK said:

I'll have a look at what you lot have mentioned, I know my fuel filter housing is leaking a little and needs replacing but that won't cause much damage to mpg as there's only a tiny amount leakin out. Cheers fr the help

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What you think is a small leak can cause a bigger loss of MPG than you think

Leaks from the FIP are also common.

Other sources of poor economy are:
a stuck (open) thermostat,
broken or disconnected temp sender on head (or some of the hotstart fix systems too I think)

best way to improve mpg is to be VERY light footed on the gas or of course dont drive it.

Only ever drive it down hills. I reset the fuel computor at the lights on top of a very long hill coming out of Blackburn a few months ago and got 66.6 mpg indicated.

wammers said:

That is truly remarkable when you consider that the only way to increase torque and power output on a unmodified diesel engine, is to inject more fuel. 20,000 a year is 384.61 miles per week. £110.00 is at todays prices, 81.90 litres of fuel or 17.92gallons. 384.61 divide 17.92 is in round figures 21.45 miles per gallon. Which is over 3 mph worst than, according to your figures, could possibly be your lowest figure. Think your fuel computor is broken.

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However, 12 months ago fuel prices were not where they are today, so the maths doesn't work.

I have run 3 diesel cars with fuel mappings and you are correct, what they do is wind up the turbo and remap the injection.

Around town this will mean a lower MPG is acheived, and I think you are being kind. On an X5 previously doing about 26MPG for town type driving I can see 20.5 without trying. However, long and more importantly smooth journeys the turbo works less time as it gets you to and back up to speed more quickly, and therefore has longer periods of inactivity or low activity to maintain a speed. I have seen 38.5 mpg on the dial, and calculated it to be a real work 34.2 mpg based on Brim to Brim filling of 92.5 litres for 703 miles achieved.

I used to commute 160 miles a day and saw a regular change form 590 to a tank to 650, or two gallons worth basically.

They do work......however

I am potentially facing a bill for a new gearbox, as the increased torque has knackered something in the gearbox and I get a tremendous thump on kick down.

On the 320d I had before, I ran it for 5 years and 212k miles. It ate a clutch every 75000 miles wheras BMW recon they are good for 100k. I can't have been treating the clutch badly as you can only do that amount of miles annually in 6th running on the motorway, so it must have been working harder.

The other one got nicked 4 months after I had it chipped so cant comment on the reliability, but the fuel figures were similar for the time I had it. I hope it blew up......

So, in conclusion. If you do lots of motorway miles and don't thrash it, they do increase the MPG. This comes potentially at a cost to the mechanical components.

I am facing a new gearbox on a 54 plate X5 motorwar car with 85k miles on it and I have 2 friends with Disco's which have knackered cylinder heads due to overheating while towing, and the garage has said they do if they are chipped.

I will not be chipping my new (2001) P38, as I am sure it has probably done enough already and I want the bits to last.

If I have to get another reasonably new mile muncher I will do it without hesitation, but for economy and will try not to thrash it. Honestly, I will.

BenRK said:

Ive got a 2.5 P38 and its currently running just under 20mpg. what simple things can i do to increase this?

ive heard i can clean out the intercooler? but ive got no idea where to start with that one lol.

any other suggestions?

Click to expand...

take off the front propshaft,makes a difference.
or lighten the vehicle take out everything you don't need.
give it a service,make sure you have no leaks.

Takson said:

However, 12 months ago fuel prices were not where they are today, so the maths doesn't work.

I have run 3 diesel cars with fuel mappings and you are correct, what they do is wind up the turbo and remap the injection.

Around town this will mean a lower MPG is acheived, and I think you are being kind. On an X5 previously doing about 26MPG for town type driving I can see 20.5 without trying. However, long and more importantly smooth journeys the turbo works less time as it gets you to and back up to speed more quickly, and therefore has longer periods of inactivity or low activity to maintain a speed. I have seen 38.5 mpg on the dial, and calculated it to be a real work 34.2 mpg based on Brim to Brim filling of 92.5 litres for 703 miles achieved.

I used to commute 160 miles a day and saw a regular change form 590 to a tank to 650, or two gallons worth basically.

They do work......however

I am potentially facing a bill for a new gearbox, as the increased torque has knackered something in the gearbox and I get a tremendous thump on kick down.

On the 320d I had before, I ran it for 5 years and 212k miles. It ate a clutch every 75000 miles wheras BMW recon they are good for 100k. I can't have been treating the clutch badly as you can only do that amount of miles annually in 6th running on the motorway, so it must have been working harder.

The other one got nicked 4 months after I had it chipped so cant comment on the reliability, but the fuel figures were similar for the time I had it. I hope it blew up......

So, in conclusion. If you do lots of motorway miles and don't thrash it, they do increase the MPG. This comes potentially at a cost to the mechanical components.

I am facing a new gearbox on a 54 plate X5 motorwar car with 85k miles on it and I have 2 friends with Disco's which have knackered cylinder heads due to overheating while towing, and the garage has said they do if they are chipped.

I will not be chipping my new (2001) P38, as I am sure it has probably done enough already and I want the bits to last.

If I have to get another reasonably new mile muncher I will do it without hesitation, but for economy and will try not to thrash it. Honestly, I will.

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Think OP i answered was talking now, not twelve months ago so maths are near enough. Chips inject more fuel, simple as that, they do nothing to the turbo. The only thing that controls that is exhaust gas flow. The higher the engine revs the faster the turbo spins, that's the way it is. They do work, but if you use the power you use more fuel, it is that easy. Perpetual motion machines are not a possibility, some read the bumf and think they are. The P38 is a gas guzzler at 30 mph, drinks fuel in traffic stop and start, only comes into it's own on the motorway or on long trips. If i drove mine up and down motorways all the time i would be seeing the high twenties low thirties, but around town if i use the power the chip gives i can see 14 mpg without any trouble.

I recently blanked off my EGR valve,on my usual 30 mile mostly A road run with cruise set at 70 mph the computer was saying my MPG was 26,with my EGR blanked it now says 29 MPG on the same run at the same speeds,I know as MPG reading the computer doesn't read true but as a increment of measure I think their accurate enough to tell you what's going on.
I think the fact my temperature generally stays at the cold end of the scale means my EGR may have been overactive giving me the lower MPG though I'm not sure,I'm quite sure my thermostat needs changing.

On the autoroutes there is a very small improvement in MPG with a power box fitted, I put this down the the fact that even modest inclines cause a downshift to maintain speed on cruise control without the power box whereas with the power box speed is maintained without the downshift and more importantly without the torque convertor unlocking. Torque convertor unlocking is the main cause of of the difference in my opinion.

you are right when t/c locked its the same as manual direct drive ,before lock up energy is allways lost as the fluid is the only route of energy transfer and even in the best of conditions this can never be 100%,why a cooler is essential on auto

That would seem to make sense.
The difficulty in lots of these posts is lack of reliable info: mpg off the trip-computer is obviously useless as you`ve already said; and mpg calculated full to full should be several times and be controlled for inconsistencies (do it few times making no changes and see how it varies without any changes); but the hardest thing may be ourselves!
Knowing we`ve just fitted a bit of kit (costing hundreds of quid) aren`t we going to "look for" every little improvement and listen harder to the more aggressive exhaust? And when cruising being aware of possible fuel usage maybe modify our driving to be a little less heavy footed??
I`m not saying there is no effect, of course there is, but we are not the best judge of our own actions.... after all, I think ths is a good post,... you all may not...