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IQ Fuel Efficiency

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I own a 10 year old IQ (mileage = 75,000 miles). Recently the original Battery failed and had to be replaced. This I did.

The consistent MPG I was getting before the Battery change was 54-55 MPG. After the Battery change it is about 50 MPG. For some reason I have lost 5 MPG somewhere. I realise the ECU was reset when the Battery failed but should this account for 5MPG difference? I have done about 500 miles since the Battery change. Any ideas as to the problem.



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I would say approximately 2 to 3 weeks. It was back in May so I can't say for sure. Like you, I was wondering how my MPG could increase significantly. Albeit, only for a  couple of  weeks

It's a bit long winded, but this is my theory.

The engine management ECU has a FUEL TRIM programme. This IS a fact.

What this means is this.

You will probably know that the perfect air to fuel mixture for best combustion is 14.7 parts of air to 1 part of fuel. This is called the stoichmetric ratio. The engine ECU knows the actual fuel to air ratio by measuring the oxygen content in the burnt exhaust gases. It is located in the exhaust manifold. The oxygen sensor continuously monitors the EXHAUST GAS oxygen levels , and a corresponding signal is sent to the ECU proportional to the actual oxygen content. Too much oxygen content in the exhaust gases means the fuel mixture is too lean. The ECU will command the fuel injectors to inject more or less fuel in order to maintain the 14.7 : 1 ratio. Obviously this action is carried out many times each second.

Note that a FUEL rich mixture will soon render the catalytic converter as scrap ! 

If a fault exists which affects the fuel ratio - such as  slight vacuum leak - say due to a small split in a vacuum pipe - then there will be too much air in the mixture and there will be an excess of air (oxygen) in the exhaust gases, because there will be too much oxygen for good combustion.

The oxygen sensor will create a corresponding signal to the ECU.

The ECU will then interprete this signal and allow more fuel to be injected to compensate for the excess oxygen - thus maintaining the 14.7 : 1 combustion ratio. This compensation is known as FUEL TRIM.

The ECU will do what it can to maintain the correct fuel to air ratio.

You need a diagnostic scanner to view fuel trims. The fuel trims will tell you as a  percentage, the deviation of fuel being added (or subtracted) from the manufacturers programmed values in order to maintain the correct fuel to air ratio.

The ECU can only compensate up to a limit though. You could have a minor fault which the ECU  can be compensate for - but without access to a  diagnostic scanner, you may not even know a fault is present. However, most manufactures will allow about  25% TRIM deviation before illuminating the engine management light on your dash board. Then the driver will know there is fault in the engine emissions.

There are 2 fuel trims ; short term and long term .

Short term fuel trim is the IMMEDIATE fuel trim adjustment to maintain 14.7 : 1 air fuel mixture. As the throttle pedal position is varied when driving, the fuel trim values are constantly changing positive and negative many times a second.

If the short term fuel trim is continuously compensating for a fault, then the long term fuel trim will move (positive or negative), from it's datum near zero point to allow for the on going fault. This will allow the short term fuel trim to be be within its new adapted  limits near zero - hopefully !

If the fault is rectified, the fuel trims will gradually re-learn their new values. The short term fuel trim will act quickly, and the long term fuel trim will eventually learn to reset to within a couple of per cent of zero. Ideally fuel trims should be near to zero but certainly less than 10 %. In practice they are never at zero. The short term fuel trim wil be moving all the time whilst the engine is running.

Anyway back to "why has my MPG changed after fitting a new battery". It may mean that a) the ECU has lost it's learnt fuel trim values and is starting from a new datum point  b) perhaps the faulty Battery was not allowing the various engine sensors to be working to thei  r optimum performance. It is well known that a failing Battery can  sometimes cause spurious warning lights to illuminate on the dash board.

Regarding the above, I stand to be corrected as I am not a qualified vehicle technician. If a professional garage technician wants to dispute the above- please be gentle !


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