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Poor Fuel Economy

Need technical advice ?
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Joined: Thu Jan 25, 2018

Poor Fuel Economy

Postby dinobravo » Tue Sep 25, 2018 3:48 pm

Hi Guys, need help. Anyone has encountered similar situation?
Appreciate if anyone has any insights to this issue.
Tried searching the forum for half a day and couldn't find something similar.

2006 Honda Civic 1.6 (FD4) AT

I usually get 390KM - 410KM per full tank (40litres at the pump)
About a mont ago, all of a sudden, the mileage has dropped to 290KM - 300KM per full tank (40 litres at the pump).
Normal driving, no changes in driving habits or route.

There are no codes or whatsoever, car seems to run fine.
No EO consumption, no white/ blue / black smoke coming from tailpipe.
But it just gulps fuel.

I have tried to check the pre-cat upstream O2 sensor. Heater circuit seems fine with 3.3 ohms resistance.
Not sure if the resistant value is off limits.
Not too sure how to check EGR valve leakage.
Idle RPM is fine, no fluctuations.
But car does vibrate / rumble a little at idle. (Could be engine mounts though)

It is fustrating as there are no codes. No noticeable symptoms. But it just gulps fuel.
running short of 100Km per full tank is a lot.

My assumption could be either or a combination of these few,
1) Sluggish O2 sensor (not performing well for fuel mix, but still within treeshold not to set a code to the ECU)
2) Leaking EGR valve, slightly stuck open plunger, allowing unmetered air into the system causing a vacuum leak
3) EVAP canister gone bad.
4) There was a temporary code P0746 (pressure control solenoid stuck OFF), but the code has been cleared and not returned since. But I believe that this part may be going bad and I will still replace this part nevertheless.
Not sure if P0746 will put the car into limp mode. But it drives fine through all gears. No issues with shifting or speeds above 100km/h. Maybe limp mode on this car will put the car in "default" map setting? But then the mileage drop is too much to consider.

Visited a few workshops but they are reluctant to investigate or check the car as there are no codes being set.
So I guess I will have to solve this myself.

The obvious is to change suspecting parts that may be bad.
I'd like to avoid throwing money on parts that may not need changing.
As these parts are in no way cheap. The O2 Sensor (AFR) is $260.

Any help insights is very much appreciated!

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