sudden abnormal increase in fuel consuption

Your petro may have been stolen.

You may have a leak in the gas line.

Your driving habits may have changed (from highway to City).

Has the car been worked on recently?

You may need service. How old is this car? How many miles?

my petrol isnt being stolen, i drive the car alone

i have no leakages

i drive the same distance in the same location

the car hasnt been worked on lately

it is a 1998 make

The questions I raised were aimed at determining whether there was sound basis for the fuel consumption calculation that was given. Your answers seem to indicate that the calculation is accurate, i.e. no "missing" fuel. I always reset the trip odometer when I fill up then watch when the mileage gets up to around 290 miles then add around 11 U.S. gallons. (Thus 290/11=26mpg, you are using metric but same principle applies). Sometimes this will vary if I drive in traffic. Usually, the low fuel light goes on at this point.

Sometimes, I forget to reset the trip odometer. In this case, the "calculation" will be way off. The point is are you confident that the consumption was measured correctly?

I mentioned stolen gas because as the price rises, this has become a problem in some areas. Let's assume that is not true in your case.

So continuing along methododically, we learn that the car is 10 years old. This indicates that some mechanical failure may be more likely than were the car less "worn". Some posters have indicated various possiblities.

What is most interesting about the post is that the consumption change is dramatic. That is why I asked about recent maintenance and history. Did someone cross the plug wires or forget to connect the o2 sensor? Change the fuel mixture?

You did not give the location. If in the US, what does the emissions report look like for CO, HC, and so on? Has the car been tested recently? Does it have a carburator?

Thanks a lot,

Let me get the O2 sensor checked.

I am in Uganda East Africa. the car is EFI

I very much doubt oxygen sensor, and you're going to pay a lot to "have it checked out." You have reported a 100% increase (or doubling) in fuel consumption rate from 0.1 litre/km to 0.2 litre/km. I cannot think of any readily repairable engine component failure that would yield such a change...and still you did not observe a noticeable change in engine performance or vehicle driveability. For example: voluminous black smoke out the tail pipe. For another example: Rough idling and stalling, and sluggish acceleration if you are driving with a "dead" cylinder (because of disconnected plug wire or no spark).

If I had that large an increase in fuel use (amounting to up to half of my cylinders being dead), I would be looking for where that fuel might have been lost...out the other end of the tank! Could it have sloshed out going up a hill? Could it have been "borrowed" when I left the car for service in a shop? Could I have been cheated at the filling station, or by a station attendant?

And I would look at my km/litre log for evidence of a mistake. For example, could my trip odometer have been inadvertently reset between fill-ups?

To "fix" the "problem" (quotes because we're not sure we have a real problem), I would continue logging odometer readings at fill ups...even making fill-ups right away or sooner than normal....just to see if the results return to normal over the next fill-up or two. (This writer puts his money on "they will")

To be on the safe side (because I don't want under-hood fire), I would also look for "tells" pointing to undetected fuel spillage:

• Puddles or pavement discolorations under where the car is usually parked
• Fuel "track" near the filler tube or under the hood
• Fuel visibly leaking under the hood when the cold engine is first started and the engine is idling.

If these checks do not succeed in identifying possible raw fuel loss or logging error, then I would be wondering if it's not time to replace the vehicle. And, if it turns out to be a dead cylinder issue (or fuel passing through engine unburned), I would be preparing myself to soon be paying a big bill for replacing a catalytic converter or two.

Could be that your O₂ sensor is shot. Usually when that goes out, mileage drops drastically.

A failed thermostat can cause the engine to stay cold. The control system makes the engine run rich trying to warm it up.

This would only be a "likelihood" in very cold weather...and the car is only run for a very short time/distance (minutes/blocks) before the car is allowed to get fully cold again. Under normal conditions though, it's not likely...the engine will warm up, only a little bit slower, with a T'stat failed open. Such a failure mode could have some effect on fuel consumption, but not the huge amount.

Good insight, though.

Such a failure mode could have some effect on fuel consumption, but not the huge amount.

The early fuel injected Ford systems could do a lot more than you might think. I had changed a water pump on a 351 Cleavland. I broke the temp sensor that was mounted on the water pump area. When I started the engine it just rolled black smoke out of it. In about two min. it would not run. The early computer system did not have the limits on mixture that are now present. Without any resistance from the broken temp sensor, the computer thought the temp was cold enough to need a very rich idle mixture. Systems are much better now.

I think what we are all saying is how did you measure this fuel consumtion increase?

And is it real...?

everytime I re fill my tank, i rest the mileage meter to zero. I then observe when how far the refill takes me for the fuel meter to read the same value as when i did the last refill.

richoug,

It is just really hard to diagnose your problem from a distance. A couple of other posibilities, however, (1) air filter (had a lot of dust lately?), (2) cracked or broken vacuum lines (has your idle speed changed? engine running rough?)

The idle speed is the same about 2000 rpm.

Could be a leaking exhaust valve not allowing full combustion and leaking pressue during the compression stroke.

Is the motor oil ok, water ok? the inlet valves could be burned.

Go to a trustworthy mechanic and let him check the car.

Too many things can increase the gas usage. without the specifics it is difficult to discern what the cause is.

To the list I would like to add my standard question; What has changed since the car worked correctly? New tires, low air in one or more tires(or tyres depending on your location). Is the automatic transmission slipping? Is the barking brake dragging? Is either of your calipers dragging. How about the temperature of the engine? If there is additional drag on a normal performing engine, it should generate additional heat. Is the engine temperature cooler than before? A cooler engine causes the engine computer to think the engine is still cold, requiring a richer mixture. If the engine is rich, you should be able to see it in the plugs. The plugs will also be rich looking if the air filter is restrictive. How about the plugs, they could be ready for replacement if they are not platinum. If the engine is hotter, you will need a good eye to spot it on the factory temp gauge (if it has one). Try driving the standard route that you drive and when you stop, check the wheels and tires to see if there are any noticeably warmer ones. That would signal a dragging brake, or bad tire.

Lots of choices here. All the things that I have mentioned (except the clogged air filter) will not show up on the engine codes. Probably worth it to check for codes first. Let us know. Good luck

are you running ac or defrost now?defrost runs ac comp. yea check for codes.I don't know where you are ,but here car parts stores will run a diagnostic on your car for free.check around.sounds like cold start system malfunction or o2 sensor

Another idea is to check your wheel alignment. You may have hit a pothole/kerb/brick that has damaged your alignment and with the tyres scrubbing it's like having the brakes on all the time.

If it's as bad as you indicate, look at the tyres and see if they seem to be wearing heavily. (That's a good indication.) Also "touch" your tyres after a drive and see if they are "hot".

If you're burning 25% more fuel, the energy must be going somewhere.

Scrubbing tyres can be such a drag!

Based on fuel purchase logging? Probably just an inadvertent logging or calculation error. Keep logging to make sure the problem is really a problem--that it does not go away. Based on experience, these usually do.

IF problem proves genuine, come back and can look for "culprits." Also, when you report back, describe prevailing weather conditions that preceded the anomaly. (A false positive for engine defect could be the result of excessive running in choked (with carb) or WOT enrichment/throttle bypass (with EFI)...which could go unnoticed in terms of "palpable" engine performance.

And finally, if you are using the odometer and amount of fuel needed to refil the tank...

1) are you sure nobody has fiddled with the trip odometer?

2) Have you been running the engine while stationary?!!?

3) have you been driving in reverse a lot recently ?

Noone mentioned check brakes?

Brake could be stuck. Hand brake especially.

Not my area of expertise, but this happened to my daughter on a ford escort wagon. all of a sudden she was filling up twice a week and doing same driving (while at school) Hand brake was in the center between the seats, and her hand bag strap had pulled the lever up two clicks... not enough to set the indicator light on the dash, but enough to engage the damn brake.

That was a difficult one to handle over the phone...

milo

I have had this happen to me several times in the past on several different cars. My experience has been the following:

Distributor cap has gone bad and car was running on 6 cylinders rather than 8. There was a noticable drop off in engine performance.

A wire has come loose from the spark plug, not as noticable change in performance, but worse fuel mileage.

The speedometer cable came loose from the speedometer causing fewer miles to register on the odometer. The needle did dance a bit on the speedo providing the clue to the problem.

The gear in the odometer wore out causing it to slip and not turn over properly. There would be a slightly audible clicking noise as the nylon gears slip by each other which you may not be able to hear when driving.

My guess is that your odometer may have gone bad if you find nothing wrong with the engine mechanicals. Drive a known distance and see if your odometer matches that distance, OR have another car follow you and check your odometer reading against that car's.

I have a 1996 Geo Prism, which, for all intents and purposes, is a corolla. This thread is the closest thread to my own "problem." Over the past 2 months, my car has had a sudden INCREASED in fuel economy. from 29mpg (12.3K/L) to over 50mpg (21K/L) !!! I have re checked my math, and rechecked everything. I have not had any maintenance done on the vehicle in that time. I have no idea what is going on? Any ideas? I am completely baffled. I would LOVE to replicate this phenomena in my 92 Geo prism, but have no idea what happened.

This is the classic example of someone playing a practical joke on you. Someone is secretly adding fuel to your tank. Pretty soon, you will experience poor gas mileage as gas is secretly being syphoned from your tank.