W124: Do warmer plugs drop fuel efficiency?

[Screwdriver]

Completed a full tune up on my 1996 W124 E220 w/ 46K Kms on the clock. My definition of Tune Up includes:

1) Engine Oil/ Filter/ Fuel Filter Changed
2) Transmission/ Differential Fluid Changed
3) Bosch Spark Plugs replaced with slightly warmer plugs
4) New Engine Shock strut + Engine Mounts + Fan Belt
5) "Seafoam" intake cleaner sprayed through intake manifold/ Throttle Body. "Auto-RX "engine treatment cleaning phase also completed
6) Brakes bled and fluid replaced
7) Rewired Coil wires and grounding (both had degraded to unacceptable levels)
8) K&N Air Filter

Results:

Engine idle much smoother
Car feels stronger, yet quieter
Fuel efficiency has dropped

Used to get 450 kms a tank. Now barely touching 400. Could the car be running more rich? Would the slightly warmer plugs be the culprit (Tropical climate zone)?

 

[SilverSaloon]

many people on here consider K&N filters to be a bad modification. Original filter doing a better job

 

[Screwdriver]

I've noticed. I'm not too worried about it - used K&Ns for the last decade and never seen a downside if treated correctly.

I should've mentioned that the K&N has been on the car for a month prior to the tune up.

 

[Number_Cruncher]

Aside from the comedy air filter, why have you changed the plugs for hottor ones?

 

[Screwdriver]

They were what were available. It's only one grade warmer though

 

[Number_Cruncher]

It sounds like your problem is more likely to be a problem with the wiring to the coils, the coils themselves, or the coil driver in the ECU has been damaged by the wiring degradation. Does the engine run cleanly, without misfires?

Of course, the filter oil could have fouled the maf (if fitted).

 

[Sp!ke]

My 230 gets 450 MILES to a tank.

If you're getting 400 KM on what should be a more economical engine it suggests somethings not right.

My advice would be to stop changing stuff that mercedes spent millions of pounds developing and put everything back to the factory settings. Throw away the K&N, put the correct rated plugs in and then see where you are.

Not sure what you mean by rewiring coil wires but that sounds rather suspect too.

 

[Roger Jones]

Even at 450km per tankful, your fuel consumption seems very high. Lots or short trips, city driving, heavy right foot? Your average figure should be not much less than, and perhaps more than, 30 mpg with normal usage. I get 27 out of my E320 Coupé (>500km per tankful).

 

[grober]

Spark plug temperature is usually set to the highest temperature that prevents pre-ignition due to a localised hot-spot[ i.e the plug] under normal running conditions. If the plug is too "cold" sometimes termed "hard" it doesn't self clean itself from combustion deposits eventually leading a plug misfire. Racing engines are often started on warmer plugs but these are then changed for colder racing plugs. Most spark plugs today have a broad operating temperature range so one grade warmer is probably OK'ish. The only reason for moving to a warmer plug is if you were experiencing plug fouling on the previous plugs. There's a slight chance of pre-ignition if your engine got very hot under load but unlikely under normal road conditions. The most likely thing to effect your mixture control would be the MAS or the Lambda sensor. You may have "poisoned" either of those with your inlet cleaner-the MAF being more likely candidate. The result might be your engine is running a rich mixture which increases your fuel consumption. Best to use the standard air filter and get a new MAF insert for starters as Spike suggested.
If you are in a tropical zone make sure the ignition timing is set correctly for the fuel available. In some countries it was assumed only low octane fuel say 91 ish was available and the engine timing was set accordingly. If better i.e. 95 octane plus fuel is available then you may be able to reset the timing resistor to gain slightly greater fuel efficiency and power. There is a fuel setting wheel not far from the engine ECU mounted on the first bulkhead near the suspension strut top mounting. Its settings are detailed in the car handbook.

 

[Screwdriver]

Yep, its Bombay city driving which usually means very heavy traffic and start/ stop conditions.

The coil wires/ covers were burnt up, which is why they were replaced. The earthing harness had snapped off, which is why it was replaced as well.

We were extremely careful not to spray anything on any sensor (tune up was done by 2 of the best Benz mechanics I know off )

We get 91 Octane fuel here (occassionally adultrated down to 87 Octane), which doesn't make me comfortable advancing ignition timing.

The engine runs smooth, no misfires and much MUCH better throttle response post the tune up.

 

[Roger Jones]

Bombay/Mumbai? It's always a good idea to let us know roughly where you are when posting.

 

[grober]

These pages give good explanation of plug heat range. http://www.globaldenso.com/PLUG/basic_knowledge/heatrange/index.html One thing to note if you are experiencing hidden plug pre-ignition is that the ECU has a knock sensor on the block. If the engine starts to knock the ECU will retard the ignition to prevent this. I suppose its possible that a combination of a warm plug and poorer petrol might mean the car is running with essentially retarded ignition under hard driving conditions which will increase petrol consumption? Many people have experienced problems with K and N filters ( their elements are coated with light oil to improve filtration- like an oil bath filter spread over a large surface area) and this oil can contaminate the MAS over a short period of time after installation. Since the MAS is the major sensor for fine tuning the fuel/air ratio this can lead to poor economy.