Is my MPG off?

[vince]

With a 95 1.9TD VSX with 110K on the clock and a new air filter, im getting 36mpg out of it.

My journey is driven sensibly ie no hard foot down action, and is 85% motorway at 70mph and then non stop start A roads at approx 30 mph.

Does it sound like its too low? Also my fuel needle seems a little sticky sometimes....the light doesnt come on but its in the reserve and then sometimes it just comes on....its like its intermittant? Reckon its the float sticking a little and the needle reading lower than it should be.

Anyone come across this ?

[rory_perrett]

Sounds low.

Same car (95 1.9TD SX) - mostly stop start city driving 33 - 36 mpg
Mostly Motorway 44 - 46 mpg.

Rory

[deian]

it does sound low, out of interest, how are u calculating ur mpg?

classic things that could be wrong would be tyre pressures, but even more so, binding brakes!

[CitroJim]

Vince,

I'd agree it's on the low side

You've checked the air cleaner is not dirty and clogged?

Have the strainer out of the tank, especially given the fuel gauge is playing up a bit. At the age and mileage of the car, it's a fair bet the strainer will be a bit clogged up now. This can affect MPG in a strange way as the engine will run slightly inefficiently.

The strainer and gauge sender is accessed by lifting up the back seat and removing the big bung on the drivers side. Slip the pipes and electrical connector off the unit, undo the big threaded collar securing it into the tank and lift out.

You may just be amazed at the rubbish and gunk that has collected over the years.

Also make sure your fuel return line is clear all the way back from the pump to the tank.

Failing that, you may need to have your injectors overhauled and your pump retimed. Over the years, on mechanical pumps as fitted to yours, slight wear in the injection pump causes the injection timing to retard slightly. Timing is ultra critical on a diesel for best economy.

Does it smoke at all? Black smoke particularly?

Is your boost sense pipe (the little one from the turbo duct to the UFO on top of the injection pump) intact and good? Small boost leaks can also knock economy about due again to loss of engine running efficiency. It's worth checking all the jubilee clips on the turbo trinking for security and persished hoses, especially the short rubber one between the inlet manifold and the last plastic turbo pipe.

[vince]

i tried to get the fuel sender out from under the back seat but the big threaded securing ring is stuck fast! I cant budge it even with a screwdriver and hammer.

It is down on power id say but it doesnt smoke much at all for a diesel. Sometimes on start up but its not that often.

Any chance of an engine shot with the pipes in question pointed out that need checking?

I worked out the mpg on an excel spreadsheet...

£20 divided by 1.21 a litre then divided by 4.55 to give gallons per £20. Then divide the mileage im getting out of £20, circa 130, by the gallons figure gives me the mpg.

[Gregg1100]

Hi Vince,,
I have always worked out my mpg another way;
Number of litres Multiplied by 0.22 gives the gallons. Divide the mileage by galls=MPG. End up with nigh on same figure as you. . Mine is getting 175 miles to £20--driven like a granny, admittantly.
Since I have replaced main exhaust pipe for the non cat one, I have noticed a good difference to the good, on the mpg figures I was getting before.
Greg

[Old-Guy]

Vince, at the risk of insulting your intelligence: the only way to measure the fuel used with any accuracy is to start by filling the tank until the nozzle repeatedly cuts out on slow fill (expensive!), zero the trip, do a load of miles, getting receipts for any intermediate fills, then fill up again using the same pump (so it doesn't matter if the forecourt isn't quite level). You now have reasonably accurate fuel and distance figures. I use the same calculation as Jim.

I have done a couple of extended mileage checks on our 95 Xantia Estate TD SX, when we first had the car at 10 years-old it had done 67k shopping miles - couldn't ever have been driven hard as there was an inch of slack in the throttle cable! First check gave a bit over 39mpg. After another 12-15k with some hard motorway miles to bed in the rings, a couple more checks gave 41-42mpg. This is m'way cruising at a 3,000rpm (82 indicated - about 78 true), rural driving and short local town journeys. Since fuel prices shot up, I haven't wanted to waste money buying more fuel than absolutely necessary - I'm tempted to try an SVO blend.

I reckon you should get 42+.

Guy

[deian]

Hi Vince, at the risk of insulting Old-Guy's (and ur) intelligence here is my way.

When u fill ur car with any amount of fuel (lets say £10) and keep the receipt, reset the mileage counter, before you fill up again, note the mileage you did with that £10 (and reset mileage counter

Next time fill with maybe £12, get the receipt, and drive till you want to fill up, again making a note of the mileage before the next fill.

Do this all the time, making a note of the average driving type for each fill up, e.g. motorway, hill climbs, full car, hard driveing, city etc.

Put all this in a spreadsheet, I can send you one if you want, and you will see exactly your MPG for any amount of fuel and any mileage you do.

The maths is simple, you already know from the receipt you get when you fill up, the cost per litre (say 124p), and the amount of fuel you get. as well as the cost.

You also know the mileage you get against the figures on that receipt.

Now because MPG is gallon, like Old-Guy said, you need to convert the litre to gallon and do the maths from there, here is my spreadsheet cut and paste:

First column is the date, then the cost of a litre of fuel, then the amount of fuel, then the overall cost, then the third bold figure is the amout of fuel in gallons, and then the mileage for that given amount of fuel, then you divide the mileage by the amount of fuel in gallons and there is your mpg for any given amount of fuel.

16/07/08 118.9 33.36 £39.67 7.34 332 45.23
17/07/08 117.9 12.72 £15.00 2.80 147 52.5
19/07/08 125.9 31.77 £40.00 6.99 290 41.49
22/07/08 116.90 8.55 £9.99 1.88 76 40.43
26/07/08 110.90 8.66 £9.60 1.9 74 38.95

Of course with a spreadsheet you can calculate the average mileage too.

Hope that helps.

[myglaren]

Hi Vince, at the risk of insulting Old-Guy's (and ur) intelligence here is my way.

When u fill ur car with any amount of fuel (lets say £10) and keep the receipt, reset the mileage counter, before you fill up again, note the mileage you did with that £10 (and reset mileage counter

Next time fill with maybe £12, get the receipt, and drive till you want to fill up, again making a note of the mileage before the next fill.

Do this all the time, making a note of the average driving type for each fill up, e.g. motorway, hill climbs, full car, hard driveing, city etc.

Put all this in a spreadsheet, I can send you one if you want, and you will see exactly your MPG for any amount of fuel and any mileage you do.

The maths is simple, you already know from the receipt you get when you fill up, the cost per litre (say 124p), and the amount of fuel you get. as well as the cost.

You also know the mileage you get against the figures on that receipt.

Now because MPG is gallon, like Old-Guy said, you need to convert the litre to gallon and do the maths from there, here is my spreadsheet cut and paste:

First column is the date, then the cost of a litre of fuel, then the amount of fuel, then the overall cost, then the third bold figure is the amout of fuel in gallons, and then the mileage for that given amount of fuel, then you divide the mileage by the amount of fuel in gallons and there is your mpg for any given amount of fuel.

16/07/08 118.9 33.36 £39.67 7.34 332 45.23
17/07/08 117.9 12.72 £15.00 2.80 147 52.5
19/07/08 125.9 31.77 £40.00 6.99 290 41.49
22/07/08 116.90 8.55 £9.99 1.88 76 40.43
26/07/08 110.90 8.66 £9.60 1.9 74 38.95

Of course with a spreadsheet you can calculate the average mileage too.

Hope that helps.

Which has been essentially my method for the past few years with the minor exception that I use the main odometer rather than the trip (I do reset the trip to give me an instant reading of that tankful, which varies enormously) and only ever fill up when there are just fumes left in the tank (around 5 litres anyway )
I always use the same fuel station but not the same pump.

Anyone without a spreadsheet, if you have a Gmail account there is one in Google Docs.

[bonnyman750]

Jim,
Thanks for the tip about the strainer on the fuel sender unit. I have been having occasional problems lately with the engine revs intermittently dropping on tickover to the point where the engine has actually cut out. I have read all the threads about air leaks, leak off pipes etc. etc. and done all the necessary checks but up till now found nothing. (I replaced all the leak off pipes when I replaced the head gasket. For info the car has 208500 miles on the clock and the head gasket was replaced at 206000). This morning I checked the strainer. First I found that the return pipe clip was loose, and some of the spines on the plastic ring were missing, suggesting that someone had had a go before me. When the strainer emerged from the tank I was amazed to see that the gauze filter was completely clogged up with what could best be described as black jelly . Also there were large pieces of black contaminate in the bottom of the white insert in the tank (which the gauze filter is located in when fitted!) which, due to the tank being nearly full, I removed with some long nosed pliers. Put it all back together and started up almost immediately and ticked over properly and more quietly than before (I always thought that this gold Xantia was noisier than the blue one). Took her for a good run and found her much smoother than before and only had the low rev problem once! So, hopefully that's cured the problem. again thanks for the tip...

Glyn

[deian]

how is the normal fuel filter?

there is whats called an idle control valve that can affect idling, this can also get clogged up and stuck too

[bonnyman750]

Deian

The normal fuel filter is quite new. Did a full service at 206000 miles. I make a point of changing the three filters and the oil every 6000 miles and often change the airfilter half way through this period. As for the idle control valve I don't know (yet!) if one is fitted and, if so, where! If this is fitted and could cause this intermittent idle problem then I will definitely check it out...

Thanks...
Glyn

[vince]

Hi Glyn,

I want to get my pick up filter out and de gunge it but i cant get the black ring loose. its stuck fast. Any tips?

[deian]

Oh yes, the presence of an idle control valve on a diesel may be slim to none, i did write that bit late at night, so excuse my ignorance, still, you never know!

But I know on petrol cars such as a bmw 325i which did have an ICV, it was the cause of bad idle, as the valve got stuck closed.

[bonnyman750]

Hi Vince,

To get the black ring off mine I used the time old method of using a broad flat bladed screwdriver and a hammer. Once the ring starts to move, apply pressure downwards on the sender unit as it is under spring pressure and it will make the ring unscrew easier.
Before unscrewing the ring undo the two clips on the fuel pipes, ease the pipes off the sender unit carefully as the white plastic they are made out of tends to grow brittle as it ages! I find that putting a flat bladed screwdriver in between the white inlet/outlet shoulder and the end of the pipe and pushing the pipe off the easier and better way. Put some paper tissue down to soak up any fuel spillage.
Once the pipes are off, take the clips off the pipes and store them somewhere safe as you can guarantee they will fall off and end up somewhere on top of the tank where you just can't get to them! I put mine on the nearest seat fixing stud welded to the floor. Then tuck the pipes out of the way.
Unclip the electrical connector block and stow out of harms way then undo the ring. To get the ring out of the aperture, press the sender unit down and ease the ring out. Have a bowl ready and ease the sender unit out and drop into the bowl. Tip it upside down to inspect the filter.
I cleaned mine out in a parts washer so that I could get all the small rubbish out from behind the filter and flush it out of the feed pipe at the top of the sender unit.
While it is out check the circuit board that sends the empty/full message to the fuel gauge. Any damage may cause false readings on the gauge in which case a replacement unit may be the way to go.
Before replacing the unit back in the tank, check down the hole in the tank for any debris that may be lurking in there. My filter was completely clogged in jelly like stuff and there were more large pieces of it inside the tank, which I had to remove with long nosed pliers as I had just refilled the tank before reading the original thread.
When looking into the tank take note of the white plastic peg sticking out (on the right hand side of the aperture looking towards the front of the car) as this locates in a space in the side of the sender unit main body. To make the locating of the unit easier, turn the unit so that the pipe outlets are pointing towards the pillar between the right hand doors (B pillar) and the peg/space location will be easily seen. Locate the sender in the hole and push down against the spring pressure. Fit the ring and screw it down as far as you can just by hand first. Then tap the ring round until the web on the ring with the arrow on it lines up with the arrow moulded into the top of the sender unit and no further! Fit the pipes, clips, electrical block connector and large rubber grommet into the floor. Start the car to reprime the system...job done!

Just for further info, I found it easier to take the seat cushion(s) out...depending on what level of trim you have, as they tend to keep falling down and it also makes it easier to keep the carpet up and out of the way. Also make sure that the large floor grommet is put back properly as it will let water in which will sit undetected under the seat carpet.

I checked the sender filter and tank on the blue Xantia (162500 miles) today and it was clear apart from a few bits...

Hope this helps...
Glyn