Improving poor fuel economy - W124 multivalve diesel

[AndyD]

Hi all,

Fuel is getting mighty expensive these days so it seems worthwhile to spend some time and money on improving my cars fuel economy.

I have a W124 1995 E300 diesel estate (auto) (185,000 miles). I struggle to get 30 mpg for a tank (400 miles) - typically only getting around 370 miles per tank. Ok, sometimes I'm a bit heavy on the right foot - but I was a good boy for the last tank and still only achieved 31 mpg - with almost all motorway miles, no faster than 70 and no heavy acceleration at all.

Everyone else reports better mpg for these cars (even the petrol ones!). Important things: it starts and runs faultlessly, pulls well, has good oil pressure and never overheats. It blows some smoke if pushed hard up hill. There is some soot inside around the exit of the exhaust pipe. I have put a couple of diesel cleaner bottles through it but with no improvement in economy. It has been serviced regularly throughout its life and there are no obvious fuel leaks. The car is used frequently for long journeys. The breaks and suspension are in excellent condition and there is no abnormal tyre wear.

Thoughts:

+ The fan clutch seems to grip a lot when the engine is cold and the fan never really runs free - is this normal or should I replace it?

+ Diesel injectors - worth testing and servicing?

+ Injector pump - check and adjust (an expensive job) - worthwhile?

+ Caked on grime in inlet manifold pipes - worth cleaning?

Any other suggestions?

Anyone tried to improve the fuel economy of other high mileage mercedes?

Thanks

Andy

 

[bolide]

I think the relatively poor mpg of these cars is caused by three main things:

* soft state of tune means poor burn
* autobox doesn't seem to lock up the torque convertor
* gearing wrong for UK conditions

The manual version pulls a higher top gear (4th in a manual is about the same as 5th in the auto) and is quite a bit quicker through the gears

You could decoke the inlet system & disable the EGR. You could also whip the head off to make sure all the valves are seating nicely and bring back some lost compression. You could also refurb the IP and the injectors

Unfortunately, having done all those things, I think you'd need 10 years driving at a very slightly improved mpg to recoup the costs involved

BTW, I'd expect to get 30-31 mpg as an average when driving hard and cruising between 80 and 90

Nick Froome
www.w124.co.uk

 

[Sp!ke]

You think thats bad? I get 350 miles tops to a tank on me 230 manual and am doing better than my wife in her 1.4 injection Bravo.

 

[grober]

Still got your engine undertray? tyre pressures up to spec? car attitude correct- i.e. self leveling working correctly to avoid a "nose up " car ?

Mercs brochure figures for the E300 diesel saloon 4 speed auto were

urban 30.4 mpg
56mph 47.1mpg
75mph 37.1mpg
bearing in mind the estates may have slightly lower gearing and slightly heavier body then you could only expect a 2-3 mpg improvement at best. If the car is running well otherwise I would be inclined to leave well alone.

 

[AndyD]

thanks all

Hi,

From the figures you've all mentioned, my fuel consumption is still a bit high - I usually only get 350 miles (26mpg) to 370 miles (28mpg) to a tank - that's with cruising at 80-90 on the motorway. On some occasions when I've done a lot of mileage around town plus lots of fast roads over hills with roundabouts at the bottoms of the hill, I've got even less.

However, the consensus seems to be that if it's running well, it ain't broke so don't fix it.

On your other suggestions, the tyres are correctly inflated but this nose-up business is interesting. The rear does seem a little low to me - both with and without the engine running - in fact the tow bar has a habit of hitting the ground on dips. The car remains level when loaded so I presume the self leveling is working but could the springs have collapsed somewhat?

Having been asked about the undertray I can't help the inclination to have a moan. I sent the car to a tyre shop recently to correct the tracking after getting a front front ball joint replaced. They did a good job(using a laser system) but it now appears that they failed to properly replace the retaining screws for the rear undertray - so it flapped around in the wind with the result that it has now broken. I'm very unhappy. The Merc main dealer is going to charge £170 + VAT for a new one. Given what it has cost me to get the tracking corrected (three different garages and £130 pounds) and that the undertray now needs replacing, I would actually have been better off paying the Merc main dealer £230 to do the job right in the first place. What chance of an intact one from a breaker? From my enquiries it is specific to this model.

Thanks for all the suggestions

Cheers

Andy

 

[Number_Cruncher]

For various reasons, I've been through most of the suggestions in Nick's list withour E300D estate, and I'm getting similar mileage, say, 370 miles to the tank or so.

One thing I'm going to look at is if the resonance flaps are switching correctly - I'm a bit suspicious of them. I think that over the years, it's possible that upon removing and re-installing the cross over pipe or manifold that either electrical or vacuum connections may not be correct any more. I've rigged up some LEDs on long lengths of wire, which, all being well, I'll wire up to the solenoid valves and see what happens when I'm driving.

Does anyone know what these valves should do? What engine speeds should they switch over at? What should the state of the valves be at low, mid and high engine speed? Ideally, I would like to construct something like a truth table to make sure of what's actually going on.

 

[230K]

Hi

28 mpg cruising at 80-90 mph in a 124 Estate is just about right. Slow her down to 56 mph and she will do 35. This is normal in my opinion. Big speed in these cars kills economy, thrashing them up to that type of speed kills it even more.
If your worried about the cost of it run her on the veg.

230K

 

[grober]

Here's a truth table for the electrical switching of the vacuum valves. Electrical solenoids are operating vacuum controlled valves to open/shut the 2 resonance flaps at different rpms. Without the hardware in front of me I can't test/work out which flap is open and which is closed and in which combination.

Suffice to say that airflow principles dictate the following

low engine speed=low airflow speed----->longer narrower x section inlet manifold=improved low speed torque.

high engine speed=higher airflow speed ----->shorter broader x section inlet manifold=greater power

 

[Number_Cruncher]

Graeme, that's extremely useful information - many thanks!

I'll report back, and hopefully help fill in the remaining gaps.

 

[bolide]

The rear springs on a W124 estate set the static (parked) ride height but, AFAIK, the height corrector sets the ride height when the engine's running

The rear attitude is mostly set by the front springs. The fronts soften with age and the front end droops, producing a nose-down attitude. I normally associate a high front ride height with new springs / lowish mileage

I think a set of tyres with low rolling resistance might be the best bet for reducing fuel consumption. Most E300 Diesel owners do lots of miles so, in the absence of an easy way to improve engine efficiency, I think that might be the best approach. Michelins are famous for being light & having low losses so they may be a good starting point. Other than that I think I'd degunge the inlet system - cheap to do and any gain would be very cost-effective. Final things would be to replace the fuel pre-filter and check the fuel system for air leaks

The suggestion to check tyre pressures & the undertray was a good one

Nick Froome
www.w124.co.uk

 

[AndyD]

Cheers

Thanks Nick, Graeme and others for the knowledgable discussion. The truth table for the vacuum valves is very interesting too - I'll give them a test.

I already have Michelins at the back so I'll replace the Goodyears at the front with Michelins when the are worn. Otherwise, from your suggestions, I might as well de-gunge the inlets, get a bit lighter with my right foot and see how it goes.

Cheers all

Andy

 

[DITTRICH]

other factors which will affect the mpg are:-

1 What are you carrying around? Figs in manuals assume he driver and little else. Hauling heavy loads will increase fuel required for changing speed.
2 What electriocal systems are you regularly running. having the ac on would affect mpg quite alot too - if you have one of course.

your mpg does seem rather low. in town driving i never get less than 36mpg and long motorway jaunts around 40mpg at around 70mph. best ever on motorway was 45mpg iirc. i have an e300d estate m reg diesel 3litre done over 220,000 by now.

Les

 

[Number_Cruncher]

>>I might as well de-gunge the inlets

With the inlet manifold off, it's a good opportunity to;

a) replace all the low pressure fuel line O ring seals
b) replace any broken high pressure pipe clamps
c) replace the rubber crankcase breather push in connectors - these typically become brittle, and break as you remove them to take the manifold off
d) check and note down the glow plug resistances for later reference (assuming they are all working OK now!)
e) lubricate the accelerator linkage pivot on the pump, with a drop or two of ATF.
f) check all of the vacuum connections
g) check the resonance flaps are free to move, and that their vacuum actuators work - i.e., make sure their diaphragms are not split.
h) plug the vacuum pipes, check to make sure nothing's loose near the inlet ports, and briefly run the engine. This will allow you to see if the inlet valves are seating well, as per Nick's point about valves and compressions above.

Points a to h are all quick, easy, cheap, and IMO well worth doing while you're there.

 

[230K]

Dittrich

That is fantastic economy neither of our diesel estates could manage anywhere near that. I wish the 210 would do 40mpg at 70 on the motorway she will just about do it if i sit at 50mph (think it did it once on a 50 mile trip). 70mph on motorway gives me approx 32-34 mpg. To regularly get above 36 i need to stay below 60 mph. Winter/summer makes a big difference as does putting on my 18" AMG's.

Are you manual??

230K

 

[Dieselman]

Not sure whether the O/p car is manual or auto but the best way to improve economy is to fit a higher ratio differential. All MB diesels are undergeared for rapid accelleration. The Manual runs about 3.5:1 and the auto 3.07:1.

I have a 2.65:1 diff available if interested, thats 15% uplift for the auto.

 

[SilverSaloon]

i got over 500 miles to a tankfull on the run back from Hampshire when i bought the car - speed approx 80mph but this was 500 miles almost non-stop. - i generally get 400 miles to a tank on every day driving (in town + motorway mix).

car is 1994 E300D Estate.

Interesting point though is that my 1995 E300D Saloon does the exact same milage but does accelerate slightly faster than the estate.

 

[DITTRICH]

Dittrich

That is fantastic economy neither of our diesel estates could manage anywhere near that. I wish the 210 would do 40mpg at 70 on the motorway she will just about do it if i sit at 50mph (think it did it once on a 50 mile trip). 70mph on motorway gives me approx 32-34 mpg. To regularly get above 36 i need to stay below 60 mph. Winter/summer makes a big difference as does putting on my 18" AMG's.

Are you manual??

230K

The miraculous figure of around 45mpg was done under very carefully controlled motorway conditions on a trip to wales 3 years ago. I did around 56mph all the way and was very careful on acceleration. i was trying to beat the mpg figures specified in the manual. The car was fully loaded with the family, mother-in-law and my father. Mine was a 4 speed auto. fuel economy does suffer on a 4 speed auto but keeping the revs around 2,500 and certainly below 3,000 helps. To be honest the car has been SORN for some time [wife still trying to pass test] and I can't swear 100% to the figs but they wouldn't be off by much. I once took that car to Russia and the long distance economy was very very good. perhaps I need to check some earlier posts to be sure

Rgds

Les

 

[Number_Cruncher]

Here's how the resonance flaps work on my car;

The valve in the crossover pipe, I'll call X/O, and the valve in the manifold itself, I'll call 3/6.

The solenoid for the X/O valve is connected to the Yellow / Brown wire pair, and is the upper of the two solenoids when in the as installed position.

The solenoid for the 3/6 valve is connected to the Grey / Brown wire pair.

In the relaxed position with no vacuum, the X/O valve is closed - i.e. the crossover pipes remain independent for their full length.

With no vacuum, the 3/6 valve is closed, and the manifold behaves as an independent pair of 3 cylinder manifolds.

<2400 rpm 3/6 valve Closed, X/O valve Closed

>2400 rpm &
<3500 rpm 3/6 valve Closed, X/O valve Open

>3500 rpm 3/6 valve Open, X/O valve Open

I didn't reliably obtain any switching above 3800 rpm as indicated by the table in Graham's post above, but, for me, that's quite academic; it's not an area of the rev range that I use often, or for any duration.

The sequence above seems logical enough, and so, I'll have to look elsewhere for an improvement in fuel economy. Perhaps I should just drive a little slower?

 

[bolide]

I didn't reliably obtain any switching above 3800 rpm as indicated by the table in Graham's post above, but, for me, that's quite academic; it's not an area of the rev range that I use often, or for any duration.

That's the best bit! Above 3200 rpm these engines start to percolate and from 4000 to 4700 they're quite entertaining in top gear

Nick Froome
www.w124.co.uk

 

[skemball]

I assume that you are making the caculation by filling the tank - driving till almost empty then refilling a dividing the mileage covered by the ammount of fuel used to re-fill the tank?