Fuel consumption issue / problem

user_emile

Hi there, I'm hoping for some advice with a fuel consumption problem I have.

I have a '00 Golf Mk4 2.0l petrol model but am sure this could relate to any other car out there. my car has 80000mls on the clock with a full service history as I have had it for a number of years now.

It used to average 33mpg, even up to 36mpg on long open journeys. End of last year my Engine Management Light came on and the car dropped down to below 20mpg and one could literally smell the fuel from the exhaust system.

Diagnostics indicated a number of faults which was reset and the throttle body cleaned at same time. Diagnostics read again a week or so later which indicated the problem: intermittent / no activity fault detected on O2 sensor circ. Bank1-sensor1 (code 16518 p0134).

I therefore went ahead and had the pre-cat sensor replaced and throttle body cleaned again at same time.

I have been running the car for a couple of tanks and I only manage to get an average of 24/25mpg per tank. When cold, it only barely manages 20mpg, strange though, some journeys if i really drive it hard it might get 30mpg.

I had the car back at the garage today:
Diagnostic test shows no faults,
Emissions test shows everything to be normal

The car drives fine, plenty power as per normal, never stutters and no problem starting or turning off.

I thought maybe the the on-board computer might be at fault by showing incorrect mpg but low consumption confirmed by average miles per tank. I used to get in the order of 360 miles per tank but now only get about 260 miles per tank.

Anyone with with some ideas out there will be greatly appreciated as I am at a dead end with garages trouble finding the issue.

B11gt00e

Do you discern any pulsing in the revs when idling? ...

user_emile

Hi, thanks for replying.

No, it idles perfectly fine. There is no obvious observation faults running or idling the car.

Tefral

Does the car heat up properly? the reason im asking, if the thermostat is broken and is staying open not allowing the car to heat up, the ECU will keep the choke on and will reduce your fuel consumption.

It is also possible that you may have a fuel leak, have a look at the pipes around the fuel rail, over the years from the heat and the cold, the little rubber pipes can crack, you will have to check this when engine is running as fuel evaporates.

user_emile

Hi there, thanks for your reply.

According to my engine temp gauge, the car seems to be heating properly as the dial rises its normal position.

I have not considered a fuel leak and will definitely look into that tomorrow - not the simplest task I assume., although I have read in other posts that spraying WD40 onto pipes may indicate leaks.

Thanks again.

mickdw

I believe your MAF sensor can gradually fail over time causing a number of faults such as high fuel usege but will often not give a direct fault

user_emile

Thanks for the replies. I doubt its a MAF sensor problem as neither the diagnostic test nor the exhaust emission test indicate any irregularities. The car runs fine also.

I had a look for any fuel leaks but with my limited knowledge I doubt i have looked everywhere. However I could't see any dampness or spray from any of the pipes under the bonnet.

If anyone out there have any input I would really appreciate it, or if anyone knows of a garage / mechanic who would be able to troubleshoot this issue I would also really appreciate it.

galwaytt

...it being a 1.4 Golf, you might want to cast your net a bit wider, and do a compression test and leakdown test as well........

...is there any evidence of borewash, btw...........i.e., any petrol in the oil - if you drained the oil, and got a distinct petrol smell from the oil it'd point at the two tests above..... ?

Henry Ford III

galwaytt wrote: »

...it being a 1.4 Golf, you might want to cast your net a bit wider, and do a compression test and leakdown test as well........

...is there any evidence of borewash, btw...........i.e., any petrol in the oil - if you drained the oil, and got a distinct petrol smell from the oil it'd point at the two tests above..... ?

It's a 2.0 I think. See above.

peasant

Are you sure your air supply is clear?
Clean air filter, no obstructions, no collapsed ducting ?

cronin_j

cronin_j wrote: »

It is also possible that you may have a fuel leak, have a look at the pipes around the fuel rail, over the years from the heat and the cold, the little rubber pipes can crack, you will have to check this when engine is running as fuel evaporates.

I reckon if it was leaking from the fuel rail is may quite likely have gone on fire by now.

Tefral

RoverJames wrote: »

I reckon if it was leaking from the fuel rail is may quite likely have gone on fire by now.

not all the time, i had a fuel leak from the rail that was spraying on to the head, car didnt catch fire, now if it made it as far as the exhaust manifold, i suspect i may be the unfortunate owner of a burnt out Integra

Tea drinker

Could it be the front 02 sensor is faulty? Just putting it out there......

cronin_j

cronin_j wrote: »

not all the time,

that's why I said quite likely

Tefral

RoverJames wrote: »

that's why I said quite likely

AzureAuto

Hi there OP,
When you had the lambda sensor replaced, was it replaced with an OE spec bosch sensor. The reason I ask is that some spurious makes of sensor aren't as good quality as original spec ones. It is this sensor which provides the primary input for the air/fuel calcualtion when the engine reaches normal operating temp. This combined with the reading from your MAF will tell the ECU how much fuel to inject for how long (pulse width signal to the injectors). Now, if your ECU is getting less accurate information from either sensor could result in increased fuel consumption however it wont flag the engine management light because it is still within the acceptable spec determine by the self-diagnostic function of your ECU.
Secondly, as your engine mileage increases mpg will inevitably decrease.
have your MAF checked out and cleaned with a proprietory carb spray and check the make of the sensor fitted.

user_emile

Hi, thanks for all the replies and helpful points.

Here is what I am noting at the moment, according to my onboard computer the consumption on journeys vary and are not regular at all, some journeys might get 22mpg, most however are approx 25mpg, some 27mpg and some even 30mpg. It has nothing to do with whether the car is cold, warm, driven in the morning or the afternoon, or length of journey. I can see no pattern whatsoever.

However, the car used to average 33-36mpg no problem, pending on journey type.

At this stage, I have not ruled out a fuel leak. I need to do some research and see where the rail is that is being referred to.

I am not sure whether the air filter was replaced at the last service, I'll replace this in any case and check if there are any obstructions evident.

The front O2 sensor I assume is the same as front lambda sensor that I had replaced due to the fault on the diagnostics. This was a Bosch part that I ordered from the UK. (btw, its so much cheaper buying these parts of the web and having them replaced by your own mechanic. VW wanted €190 plus labour, Bosch part cost me less than a €100 and then €30 to fit)

I'm just wondering in the back of my head, could the post lambda sensor be intermittent faulty and not show up on the diagnostics?

Thanks again.

Tea drinker

But (correct me if I am wrong) the post cat lambda does not make any fuelling adjustments, it just checks the function of the cat.
Did you do a reset of the ECU when you replaced the O2 sensor? I think you need a reset after this.

Did you fit wider wheels / tyres?

Darsad

AzureAuto wrote: »

Hi there OP,
When you had the lambda sensor replaced, was it replaced with an OE spec bosch sensor. The reason I ask is that some spurious makes of sensor aren't as good quality as original spec ones. It is this sensor which provides the primary input for the air/fuel calcualtion when the engine reaches normal operating temp. This combined with the reading from your MAF will tell the ECU how much fuel to inject for how long (pulse width signal to the injectors). Now, if your ECU is getting less accurate information from either sensor could result in increased fuel consumption however it wont flag the engine management light because it is still within the acceptable spec determine by the self-diagnostic function of your ECU.
Secondly, as your engine mileage increases mpg will inevitably decrease.
have your MAF checked out and cleaned with a proprietory carb spray and check the make of the sensor fitted.

+ 1 nice to see a competent informative post ! You know just because the computer says no fault it doesnt mean there aint any . Find someone who knows how to read and check the relevant sensor parameters !.