drop in fuel economy

[cheesebox]

With my previous 318is I was getting between 320 and 380 mile out of a full tank, which I gather is about right for the standard engine.

Now I've got my lower mileage (80k miles) 318is on the road I'm struggling to get more than about 300 miles out of a tank, even with an economical driving style.

The engine set up isn't quite standard, but I don't think any of the mods should contribute to such a drop in economy.

It's a UK car. I've replaced the fuel injectors with Bosch 0280155710-F6VE injectors from five-o motorsport. It's the standard chip as far as I know. There's a K&N cone filter on the intake, and a big bore throttle body. Everything else is standard.

I replaced the timing case profile gasket while I had the other car on the road. Whilst I had it apart I replaced the timing sprockets, chain, tensioner, and had the valve stem seals replaced and a very light head skim. After the rebuild it seemed to lack power, and fuel economy was poor, so I took it to an indy bmw specialist to have the valve timing set properly (I aligned it with a straight edge, so wasn't 100% confident it was set optimally). The power seems better now (I think), but fuel economy is still poor. Of course I can only take his word that he did actually do the job he said he did! It does sound like it's detonating slightly when accelerating from 1800-3000 rpm, but it's so slight I can't be sure it actually is detonaton.

I'm not sure what the problem could be. Are those injectors ok on a uk car? I'm sure the oem injectors are the same on us and uk cars. I've heard big bore throttle bodies can actually impede the flow of air into the engine due to turbulence - maybe that's it.

Could it also be the O2 sensor? I'm not sure if my car even has one - I don't think the cat was fitted to all uk cars,

Any suggestions would be appreciated. I do 300 miles a week so this is a significant problem for me, especially as a full tank costs about $90 over here!

[cheesebox]

An update...

Over the weekend I managed 410 miles on a tank!

This was mostly motorway driving at a steady 65-75 mph, with around 40 miles of steady A road driving.

The warmer weather has also improved matters. In cold weather I noticed it would take an hour or so for the engine to reach maximum temperature, and even then the temperature guage was only reading around 30%.

Conclusion - a combination of driving style and an over-active thermostat were behind the poor fuel economy. A big bore throttle body may have also contributed, with more fuel being allowed into the engine at full throttle. I've got an oem thermostat to fit - it'll be interesting to see if that improves the economy for the shorter journeys.

[mikedz]

I'm having the same issues.  It takes forever to heat up and the gauge gets  just out of the blue.  Keep updates comeing after you put in the new thermo. I think that is my issue as well.

[no1_jazz]

Hi there Peers

For fuel economy do check the o2 sensor as that pack up as for a car this age. It is possible for your car not to be getting the rite air and fuel mixture and hence drinking more fuel then it should.

Also have you change the changed the blue and brown temp sensors this could affect fuel economy so do check that out.

So the car never gets to full operating temp?

Thermostat stuck open in that case, and that will increase fuel usage.

Hope this helps & let us know how you get on.
Jazz