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Old 25th April 2013, 17:49   #1
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Unhappy Ford Fiesta: Sudden Drop in Fuel Economy

I have a Ford Fiesta 1.6S which has done about 38K on the ODO and close to completing 4 years on the road.

The car went for a premature service schedule as I bought this car used and wanted to do a basic check up and also change the required fluids. I completed the 50K service already. I drove the car for 1 full month before I sent it for service to observe anything that would require attention when the car goes for service.

I calculate the FE numbers by the tank-fill method and the FE was around 9.8 to 10 KMPL before the service. After about a week of service, the FE started dropping off pretty quickly and went down to about 8.2KMPL and its staying there for now. I took the car for a quick check and the following things were done.

1. The airfilter that was in the car was pretty old and full of dust, it was a bit hard as a cardboard piece. That was changed.

2. The Spark plugs were removed, the engineer did make the ground a bit more closer to the electrode and refitted.

3. Reset the ECM, no error codes were found either

4. The engine cooling thermostat was checked too, but the engineer mentioned he didnt find anything abnormal with it

5. There is no issues with the usual surge of the 1.6S, its very normal and there is no noticeable jerking too

I have an observation though, from the time the FE started dropping the AC cooling cuts off intermittently while I am on stop-go traffic. It chills when the car starts moving. If am in B2B traffic, the cooling cuts off quite frequently.

I still feel that the FE has not improved much, Is there anything else that I should look at?

Your inputs folks!
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Old 25th April 2013, 18:04   #2
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Default re: Ford Fiesta: Sudden Drop in Fuel Economy

Quote:
Originally Posted by anachronix View Post
The Spark plugs were removed, the engineer did make the ground a bit more closer to the electrode and refitted.

Reset the ECM, no error codes were found either

I have an observation though, from the time the FE started dropping the AC cooling cuts off intermittently while I am on stop-go traffic. It chills when the car starts moving. If am in B2B traffic, the cooling cuts off quite frequently.
Why was the spark plug gap closed? It would be advisable to replace the plugs rather than play around with their gap. Also while setting the gap did the engineer use a feeler gauge to set the gap accurately?

Why was the ECM reset? Normally after a ECM reset the mileage falls a little bit as the ECM is busy learning short term and long term fuel trims.

When was the coolant changed last? Also check if your radiator fan is working properly. To check this, disconnect the coolant temperature sensor when the engine is cold and start the car. The fan should start running immediately. If the fan doesn't run then there is a problem with the radiator fan and it might need to be replaced. This will also be a cause for low FE as the ECM goes into fail safe mode and starts running the engine on a rich air fuel mixture.

A defective fan also causes loss of AC cooling when the car is stationary.

Last edited by vikram_d : 25th April 2013 at 18:05.
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Old 25th April 2013, 18:15   #3
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Default re: Ford Fiesta: Sudden Drop in Fuel Economy

I concur with vikram_d's views, except that the coolant on the Fiesta is supposed to last for 1 lac kms.

If the AC fan motor is not working, the AC cooling will drop, and also cause the temperature to hit the max, which is not good. If you see that happening, please don't use AC and get the fan replaced. It costs around 4 K.Mine was replaced under warranty.
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Old 26th April 2013, 10:48   #4
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Default Re: Ford Fiesta: Sudden Drop in Fuel Economy

Quote:
Originally Posted by vikram_d View Post
Why was the spark plug gap closed? It would be advisable to replace the plugs rather than play around with their gap. Also while setting the gap did the engineer use a feeler gauge to set the gap accurately?
No. he did not use the feeler gauge and did the adjustments the jugaad way of tapping it on the floor. I will get them replaced!

Quote:
Why was the ECM reset? Normally after a ECM reset the mileage falls a little bit as the ECM is busy learning short term and long term fuel trims.
I wanted the ECM to be reset, the ECM learns the driving pattern but does it affect the FE?

Quote:
When was the coolant changed last? Also check if your radiator fan is working properly. To check this, disconnect the coolant temperature sensor when the engine is cold and start the car. The fan should start running immediately. If the fan doesn't run then there is a problem with the radiator fan and it might need to be replaced. This will also be a cause for low FE as the ECM goes into fail safe mode and starts running the engine on a rich air fuel mixture.

A defective fan also causes loss of AC cooling when the car is stationary.
Quote:
Originally Posted by vnabhi View Post
If the AC fan motor is not working, the AC cooling will drop, and also cause the temperature to hit the max, which is not good. If you see that happening, please don't use AC and get the fan replaced. It costs around 4 K.Mine was replaced under warranty.
I will get this checked, as of now I could hear the fan working in hi-speed mode.

Thanks for the pointers guys!
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Old 26th April 2013, 11:17   #5
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Default Re: Ford Fiesta: Sudden Drop in Fuel Economy

Quote:
Originally Posted by anachronix View Post
I will get this checked, as of now I could hear the fan working in hi-speed mode.
In all probability, the engine is running a little too hot, hence like Vikram mentioned, the ECM is running a richer air fuel ratio, and hence the drop in FE. That is also why, the compressor is cut off. Please do monitor the engine coolant temperature, and then the entire cooling system.
Quote:
Originally Posted by vikram_d View Post
To check this, disconnect the coolant temperature sensor when the engine is cold and start the car. The fan should start running immediately.
Vikram, IIRC in a Fiesta, if the coolant temperature sensor is disconnected, the temp guage will show max, and the engine will not start.

EDIT: Did you check the FE after cleaning the Air Filter?.

Last edited by dhanushs : 26th April 2013 at 11:18.
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Old 26th April 2013, 11:18   #6
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Default Re: Ford Fiesta: Sudden Drop in Fuel Economy

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Originally Posted by anachronix View Post
No. he did not use the feeler gauge and did the adjustments the jugaad way of tapping it on the floor. I will get them replaced!
That means there is a very high probability of the gaps being different amongst all the plugs. Please get them changed ASAP.

Quote:
Originally Posted by anachronix View Post
I wanted the ECM to be reset, the ECM learns the driving pattern but does it affect the FE?
The ECM does not learn any driving style. It only stores the short term and long fuel trim based on your driving style. The mileage during the first few days does fall when this is happening, but by how much depends on the car and ECM maps.

Quote:
Originally Posted by anachronix View Post
I will get this checked, as of now I could hear the fan working in hi-speed mode.

Thanks for the pointers guys!
Just to be safe get the fan checked and ensure that it is running properly at both speeds (high & low). Also, since summer is already here I would get recommend that you get the coolant changed. Better be safe than sorry.

Edit: @Dhanush - Thanks I was not aware of that, but I find this behavior rather strange. I have not seen this kind of thing on any of the petrol cars that I have owned including my Mondeo.

Last edited by vikram_d : 26th April 2013 at 11:22.
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Old 26th April 2013, 13:13   #7
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Red face Re: Ford Fiesta: Sudden Drop in Fuel Economy

@anchronix,

Since you have already considered many such factiors, have you considered bad fuel for the low FE or the fuel filter clogged. Normally resetting the ECM will change the DTE display but I would not look for DTE when calculating the FE.
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Old 26th April 2013, 14:03   #8
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Default Re: Ford Fiesta: Sudden Drop in Fuel Economy

I did a test run hooking up the OBD via Torque Pro and the engine temparature is around 82C, but I was not driving through the B2B traffic where the AC usually cuts off. I am leaving the car for another detailed check of the AC and cooling system tomorrow. I am hoping to have a positive update!

Quote:
Originally Posted by dhanushs View Post
In all probability, the engine is running a little too hot, hence like Vikram mentioned, the ECM is running a richer air fuel ratio, and hence the drop in FE. That is also why, the compressor is cut off. Please do monitor the engine coolant temperature, and then the entire cooling system.
Yes, its dropped quite a bit now and its below 8KMPL

Quote:
EDIT: Did you check the FE after cleaning the Air Filter?.
Yes, I am getting them changed tomorrow.

Quote:
Originally Posted by vikram_d View Post
That means there is a very high probability of the gaps being different amongst all the plugs. Please get them changed ASAP.
I am calculating FE by the tank-fill method and I dont think its the fuel as I havent seen any symptoms of that yet. The car is absolutely normal to drive, no drop in power or any signs of jerking.

Quote:
Originally Posted by ghodlur View Post
Since you have already considered many such factiors, have you considered bad fuel for the low FE or the fuel filter clogged. Normally resetting the ECM will change the DTE display but I would not look for DTE when calculating the FE.
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Old 26th April 2013, 14:22   #9
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Default Re: Ford Fiesta: Sudden Drop in Fuel Economy

Quote:
Originally Posted by anachronix View Post
I am calculating FE by the tank-fill method and I dont think its the fuel as I havent seen any symptoms of that yet. The car is absolutely normal to drive, no drop in power or any signs of jerking.
This reminds me, have you changed your filling station by any chance? Also, when the car is running richer than normal it becomes super smooth and feels a little bit quicker than normal. If the car is running at 82 deg.C. then the coolant and the cooling system seems fine.

One more thing to keep in mind is that during our Indian summers the mileage of any vehicle will drop, but 2kmpl is on the higher side.

In bumper to bumper traffic the AC efficiency goes down and that is normal. This happens as there is no airflow going over the radiator and AC condenser. This is where the radiator fan comes in.

Things that you need to ask them check are as follows

Radiator fan
AC Compressor and its clutch
Gearbox clutch. This may be slipping a little and that can also cause drop in mileage.
Air intake system and throttle body. Air filter you are already changing. Ask them to use a throttle body cleaner and spray that into the throttle body.
Check intake pipes for any obstructions.
Check throttle position sensor.
Check idle air control valve and sensor.
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Old 26th April 2013, 14:48   #10
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Default Re: Ford Fiesta: Sudden Drop in Fuel Economy

No, I have been filling from a reputed bunk where most imported cars in the Chennai city fill fuel. I am almost certain its not the fuel. However, I am going to ask the service folks to check the quality of fuel in the tank.

Quote:
Originally Posted by vikram_d View Post
This reminds me, have you changed your filling station by any chance? Also, when the car is running richer than normal it becomes super smooth and feels a little bit quicker than normal.
The AC was working normal when I did this run which was on a fairly empty stretch of road. I will observe the engine temparature today on my way back home and see if there are any unusual spikes in engine temparature in B2B traffic.

Quote:
If the car is running at 82 deg.C. then the coolant and the cooling system seems fine.
The AC effeciency goes down in summer time thats normal, but I get hot air from the vents when the AC cuts off which I guess is not normal.

This reminds of one more thing, just a few weeks before I bought the car the owner had trouble with AC in the car and he has changed the AC condenser.

Quote:
In bumper to bumper traffic the AC efficiency goes down and that is normal. This happens as there is no airflow going over the radiator and AC condenser. This is where the radiator fan comes in.
Thanks, thats a comprehensive list to check when I drop the car tomorrow for service

Quote:
Things that you need to ask them check are as follows

Radiator fan
AC Compressor and its clutch
Gearbox clutch. This may be slipping a little and that can also cause drop in mileage.
Air intake system and throttle body. Air filter you are already changing. Ask them to use a throttle body cleaner and spray that into the throttle body.
Check intake pipes for any obstructions.
Check throttle position sensor.
Check idle air control valve and sensor.
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Old 26th April 2013, 14:56   #11
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Default Re: Ford Fiesta: Sudden Drop in Fuel Economy

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Originally Posted by anachronix View Post
Thanks, thats a comprehensive list to check when I drop the car tomorrow for service
Since the AC condenser has been changed also ask them to check for refrigerant leaks if any. If refrigerant levels are low then the compressor needs to work overtime to keep the cooling levels up. This can also cause drop in mileage.

On free open roads the refrigerant levels doesn't matter much as the engine is always at a good cruising RPM and in turn so is the compressor. But in bumper to bumper traffic, the compressor is depending on the battery to keeps it's RPM up and the cooling drops dramatically if the refrigerant is low.
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Old 26th April 2013, 19:09   #12
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Default Re: Ford Fiesta: Sudden Drop in Fuel Economy

Quote:
Originally Posted by vikram_d View Post
Things that you need to ask them check are as follows

Air intake system and throttle body. Air filter you are already changing. Ask them to use a throttle body cleaner and spray that into the throttle body.
Check throttle position sensor.
@anachronix (Ford Fiesta: Sudden Drop in Fuel Economy)

I am not sure if it is related to your issues. I had the dreaded engine warning light issue and I used to get my FE in the range of 7-8 at that time within city, but other than that no issues with the car's pickup or running. After the issue got severe and reproduce-able i got the throttle body changed under warranty @Chennai ford Arumbakkam and my FE moved to 10 and has been till date.
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Old 26th April 2013, 19:17   #13
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Default Re: Ford Fiesta: Sudden Drop in Fuel Economy

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Originally Posted by RajeswaranK7 View Post
@anachronix (Ford Fiesta: Sudden Drop in Fuel Economy)

I am not sure if it is related to your issues. I had the dreaded engine warning light issue and I used to get my FE in the range of 7-8 at that time within city, but other than that no issues with the car's pickup or running. After the issue got severe and reproduce-able i got the throttle body changed under warranty @Chennai ford Arumbakkam and my FE moved to 10 and has been till date.
What was wrong with the throttle body that it had to be replaced?
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Old 26th April 2013, 19:29   #14
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Default Re: Ford Fiesta: Sudden Drop in Fuel Economy

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Originally Posted by vikram_d View Post
What was wrong with the throttle body that it had to be replaced?
@Vikram

I did not get to know the exact technical analysis, they just confirmed that the throttle body is at fault( Team-Bhpians guided me on this, so asked the SA there to check it first) and Ford has asked to replace it under warranty ( i have posted the same in @vnabhi's thread http://www.team-bhp.com/forum/long-t...ml#post2597151). I had noted down the error code from the warranty sheet somewhere, will see if i can get it.

Last edited by RajeswaranK7 : 26th April 2013 at 19:33. Reason: Added more info
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Old 26th April 2013, 20:03   #15
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Default Re: Ford Fiesta: Sudden Drop in Fuel Economy

There is no warning lights or error codes in my case, I am hoping its not the same issue!

Quote:
Originally Posted by RajeswaranK7 View Post
i got the throttle body changed
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