Previa (1996) - Poor Fuel Economy

[stjoolzz]

I know that Previas have not been the best at fuel economy but mine is decreasing especially over the last few weeks. The car is labouring a little when setting off and on the road and the MPG has decreased from 16mpg to 14mpg round town over a couple of weeks. I could not believe that I have to fil up so quickly.

Anyone any ideas of what to check before I give it to the garage to look at?

[peterc]

Try whacking some STP fuel cleaner in (Silver bottle one) the petrol tank. Could be your Lambda sensor starting to fail as well. Had a MOT recently ? just thinking if your emmissions was high, that points to a crapping lambda.

Good luck.

Pete

[stjoolzz]

Try whacking some STP fuel cleaner in (Silver bottle one) the petrol tank. Could be your Lambda sensor starting to fail as well. Had a MOT recently ? just thinking if your emmissions was high, that points to a crapping lambda.

Good luck.

Pete

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Thanks

It had its MOT 6 months ago - I will give the STP a go.

[PREVIACAR1]

I know that Previas have not been the best at fuel economy but mine is decreasing especially over the last few weeks. The car is labouring a little when setting off and on the road and the MPG has decreased from 16mpg to 14mpg round town over a couple of weeks. I could not believe that I have to fil up so quickly.

Anyone any ideas of what to check before I give it to the garage to look at?

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PERSONALLY I'D GO A LITTLE UP MARKET FROM STP AND TRY THE SLICK 50 INJECTOR CLEANER OR IF YOU CAN FIND IT.FORTE ADDITIVES/CLEANERS ARE VERY VERY GOOD.I ASSUME AIRFILTER AND FUEL FILTER ARE BOTH OK????

[stjoolzz]

PERSONALLY I'D GO A LITTLE UP MARKET FROM STP AND TRY THE SLICK 50 INJECTOR CLEANER OR IF YOU CAN FIND IT.FORTE ADDITIVES/CLEANERS ARE VERY VERY GOOD.I ASSUME AIRFILTER AND FUEL FILTER ARE BOTH OK????

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Thanks. I did ry STP but it has made no difference. the air filter is OK but I will admit that I have not looked at the fuel filter.

It also feels like it is 'dragging'. I drove from London to Southampton at Christmas and when I got there ther was a perculiar hot metal smell (not the usual hot smell that you get when the car has been driven)

[holtgizmo]

if you replce the fuel filter as it may be clogged and then try a product called active8.you will find your fuel consumption will decrease and mpg get better.i have a 2,5 turbo lucida luxury and get an average of 37 mpg.

search the net using active8 to find site as you cant buy in the shops.

it works

[karllangston]

PERSONALLY I'D GO A LITTLE UP MARKET FROM STP AND TRY THE SLICK 50 INJECTOR CLEANER OR IF YOU CAN FIND IT.FORTE ADDITIVES/CLEANERS ARE VERY VERY GOOD.I ASSUME AIRFILTER AND FUEL FILTER ARE BOTH OK????

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Thanks. I did ry STP but it has made no difference. the air filter is OK but I will admit that I have not looked at the fuel filter.

It also feels like it is 'dragging'. I drove from London to Southampton at Christmas and when I got there ther was a perculiar hot metal smell (not the usual hot smell that you get when the car has been driven)

if it feels like its dragging and you have a hot metal smell you need to look at the brakes. if the caliper slider pins have rusted the calipers dont release fully and the pads drag causing higher consumption hot smell etc.

[stjoolzz]

Thanks for the advice - it does not seem like the brakes. My dad reckoned on a problem with the differential.

I have not been able to find activ8 anywhere.

I have booked it in for a service next week - they will change the platinum plugs (due as the car has done 124000 miles. Fuel, Air and Oil filters will be done also.

[stjoolzz]

Had the service done (£600 in total)

Change of plugs (platinum), air filter, fuel filter and oil filter, brake pads and oil (after a flush).

They discovered that one of the HT leads was arcing and had nearly burnt out one of the plugs so these were changed as well.

They did not find anything binding

Acceleration is definately better but it always is after a service. Maybe a more frequent oil change ??

We went on holiday so was doing long drives a lot of the time (40-80mph) and seemed to get 22mpg and 20mpg for the two times that I filled up. Not using it much at the moment as have just got an old run around (a micra). Next time I fill up it will be after town driving so I will be able to see how much difference there is. I am not expecting anything dramatic but anything better than 14-15mpg would do!!!!

[peterc]

You might want to think about getting the Lambda sensor changed.

Cut and pasted:

OXYGEN (O2) SENSOR. Used on both carbureted and fuel injected engines since 1981, the oxygen (O2) sensor is the key sensor in the fuel mixture feedback control loop.

Mounted in the exhaust manifold, the O2 sensor monitors the amount of unburned oxygen in the exhaust. On many V6 and V8 engines, there are two such sensors (one for each bank of cylinders).

The O2 sensor generates a voltage signal that is proportional to the amount of unburned oxygen in the exhaust. When the fuel mixture is rich, most of the oxygen is consumed during combustion so there is little unburned oxygen in the exhaust. The difference in oxygen levels between the exhaust inside the manifold and the air outside creates an electrical potential across the sensor’s platinum and zirconium tip. This causes the sensor to generate a voltage signal. The sensor’s output is high (up to 0.9v) when the fuel mixture is rich (low oxygen), and low (down to 0.1v) when the mixture is lean (high oxygen).

The sensor’s output is monitored by the computer and is used to rebalance the fuel mixture for lowest emissions. When the sensor reads "lean" the PCM increases the on-time of the injectors to make the fuel mixture go rich. Conversely, when the sensor reads "rich" the PCM shortens the on-time of the injectors to make the fuel mixture go lean. This causes a rapid back-and-forth switching from rich to lean and back again as the engine is running. These even waves result in an "average" mixture that is almost perfectly balanced for clean combustion. The switching rate is slowest in older feedback carburetors, faster is throttle body injection systems and fastest in multiport sequential fuel injection.

If the O2 sensor’s output is monitored on an oscilloscope, it will produce a zigzagging line that dances back and forth from rich to lean. Take a look at the waveform on the opposite page - that’s what a technician wants to see when he checks the O2 - think of it as a kind of heart monitor for the engine’s air/fuel mixture.

O2 Sensor Strategies: Unheated one- or two-wire O2 sensors on 1976 through early 1990s applications should be replaced every 30,000 to 50,000 miles to assure reliable performance. Heated 3 and 4-wire O2 sensors on mid-1980s through mid-1990s applications should be changed every 60,000 miles. On OBD II equipped vehicles, the recommended replacement interval is 100,000 miles. The O2 sensor’s responsiveness and voltage output can diminish with age and exposure to certain contaminants in the exhaust such as lead, sulfur, silicone (coolant leaks) and phosphorus (oil burning). If the sensor becomes contaminated, it may not respond very quickly to changes in the air/fuel mixture causing a lag in the PCM’s ability to control the air/fuel mixture.

The sensor’s voltage output may decline giving a lower than normal reading. This may cause the PCM to react as if the fuel mixture were leaner than it really is resulting in an overly rich fuel mixture.

How common is this problem? One EPA study found that 70 percent of the vehicles that failed an I/M 240 emissions test needed a new O2 sensor

Pete

[stjoolzz]

thanks - I will have a look at that.

Julian

[stjoolzz]

The car is still only doing 13 mpg. It had its MOT today and I spoke to the mechanic. It passed the emissions with flying colours so he cannot understand why the economy is so poor. With the emissions as they are he felt that it would not be worth it to replace the sensor.

Any other suggestions or is it just age - 1996 and done 125,000 miles?