poor mpg

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fastjet
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poor mpg

Post by fastjet »

Hi, can anyone help solve my c5's low mpg problem please.

My 2004 C5 2.0 HDI 110 bhp mpg has suddenly dropped. I've checked with Lexia, no are faults shown.
The best I can get at the moment is around 36 mpg driving at 55 mph on a virtually empty road.
Any ideas on what could be cause this?
Thanks.
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Re: poor mpg

Post by GiveMeABreak »

Is it running rough (these engines have been known to run on 3 cylinders without a diagnostic code being generated). Has it been regularly serviced? When was the air and oil / oil filter last replaced?

Also, are you using cheaper supermarket fuel? Get a diesel fuel system cleaner and put that in the next time you fill up. Do this for the next couple of tankfulls and allow most of the fuel in the tank to run through before filling up again. That will give the injectors a good clean. Also switch to some premium diesel and use a tank full of premium every 3 refills to keep the fuel system in good order.
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fastjet
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Re: poor mpg

Post by fastjet »

Hi, thank you for reply.

The car is serviced every 9000/10000 miles. At last service the air filter, oil filter and fuel filter were replaced approximately 4500 miles ago.
The car does not run rough, the only rough running occasional happens when you are on very light throttle at low revs (approximately 1100/1200 rpm) pulling out of a corner. Increasing the throttle clears the rough patch.
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Re: poor mpg

Post by GiveMeABreak »

So get the fuel system cleaned - the premium fuels have fuel cleaners included hence the extra cost and regular use of these should avoid the need for stand alone fuel cleaners - but I would certainly invest on a few bottles of some injector cleaner / fuel system conditioner. These are the things that are often overlooked, despite getting the other servicing items done regularly - none of those directly clean the fuel system.
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Re: poor mpg

Post by Michel »

I'd just drive it like you stole it for a few days, should be enough to blow the cobwebs out of it. I've had this on many older diesel cars in the past, most notably a C5 that wouldn't do more than 37mpg until I drove it to Brussels and back, at highly illegal speeds all the way to and from Calais. It regularly did more than 50mpg after that.
fastjet
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Re: poor mpg

Post by fastjet »

Thanks for the replies.

I tend to potter around in it at a steady 50/55 mph in 5th. There seem to be quite a few different injector cleaners out there, which are the better ones to use?
Fuel system conditioner?
Thanks for your help.
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Re: poor mpg

Post by admiral51 »

not very well informed with the early C5 but are the MPG figures from the on board computer or based on miles done and re fill ?
reading between the lines it seems that the OP has suddenly noticed a drop in MPG whilst driving similar routes and driving style.
could it be an injector that is leaking and by default is giving false readings to the onboard computer?
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Re: poor mpg

Post by GiveMeABreak »

fastjet wrote: 03 Oct 2019, 16:27 Thanks for the replies.

I tend to potter around in it at a steady 50/55 mph in 5th. There seem to be quite a few different injector cleaners out there, which are the better ones to use?
Fuel system conditioner?
Thanks for your help.
I just put the other names for it here - some are injector only and some do the whole fuel system - STP do a range
and son do Wynns:

https://www.halfords.com/motoring/engin ... ines-325ml
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Re: poor mpg

Post by myglaren »

The Forte products seem to be a good bet.
Here on Amazon
They can be difficult to get at factors as they are only supposed to be available in workshops, there are exceptions though. eBay often has listings for them.
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Re: poor mpg

Post by Peter.N. »

I used to get around 50 mpg on mine and I don't drive fast but I do quite a few miles - or I did when I had it. Do as Michael says and give it a good thrash, you will probably see a lot of what looks like black smoke out the back, it is actually soot that has accumulated in the system due to low throttle operation.

If there is a lot of soot it could be causing back pressure in the exhaust.

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Re: poor mpg

Post by moizeau »

Don't pay any attention to how mpg Peter can get!!!! I'm sure he could achieve 40mpg in a 1980's Range Rover V8 !
When you say suddenly, is that within a tank full, like a switch, or over a period of a month or two?
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Re: poor mpg

Post by Peter.N. »

Yes all right then :roll: 50 mpg is easily obtainable from a 2.0. 8 valve engine by a normal person on the motorway - driven sensibly, I have had a lot more.

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Re: poor mpg

Post by moizeau »

Just pulling it, no offence meant, quite the opposite. I went from 52 to 45 once the condenser was changed and re gassed.
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Re: poor mpg

Post by Peter.N. »

I know, I don't take offence. 8-)

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Re: poor mpg

Post by Doo »

I used to have to take my wife's C4 hdi for an Italian tune up at least every couple of weeks.

I asked her (well, begged really :rofl2: ) to try BP Ultimate (other premium fuels are available...) for 3 months. That was 7yrs ago now 8-)

The car hasn't needed an Italian tune up since (apart from the other day after sitting for 2 months due toABS issue).

She drives to/from work with the odd trip to Tesco or her friends. But the car does average 5 miles a "day" :shock: It barely touches 30mph any given day...

Every mot the emissions are 0 8-) 8-)

I bought a Mercedes E320 estate with 182,000 miles on it. Straight six diesel was in good tune and the car had a weeks mot left so I stuck it in and remember clearly the emissions were 1.26 which was well within reason. The car was allowed 3.50.

Anyhoo, I owned that car for a year and never put any other diesel near it other than BP Ultimate. It was always driven hard and always performed well. The next mot, the tester did the emissions and scratched his head...He removed the probe and changed the filter and re-did the test, same result. So he did something else and did a 3rd emissions test while I stood there grinning ear to ear.

He asked if I had put a new engine or fiddled with the pump or injectors? Of course not, you can see it's all original! Told him about the fuel and he was surprised that was all I had done. He told me that newer cars aren't even this clean!! :o

The emissions result? 0.24 which pleased me and proved a very valid point that while the petrol additives have a little effect, the diesel additives do what they claim.

Anyway, it is also possible the OP has a sticky brake caliper which would account for the "sudden" onset :wink:

I had a Peugeot 207 come in needing an mot but the brakes were very poor. Turns out the ladies brother had done them all at the roadside...

I stripped each caliper and used the wire brush attachment on my grinder to scrub off any muck & rust, then applied a little grease and new pads all round.

The lady came back a couple of weeks later and handed me an envelope with some cash in it! I asked why? She was delighted that not only was the car restored, but it accelerated with ease and better still her fuel use had quartered. She was using almost a full tank to do 200 miles before, but now she was getting upwards of 650 miles to a tank (she never let it go lower than between empty and 1/4).

Diesels are so torquey you barely notice a sticking brake so it's worth a look :wink:

Good luck :)
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