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 Post subject: fuel use issues
PostPosted: Tue Mar 06, 2018 8:56 pm 
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Joined: Wed Apr 05, 2006 12:47 pm
Posts: 28
Location: south wales
Car Model: None
wondered if anyone could give some advise on a problem I've notice recently

basically I think my car maybe using to much fuel, I usually get around 40 mile to
£10 fuel/quarter of a tank (driven sensibly with no high speed boosting) but lately the car
(seems) to be doing less

I always use the mileage at ive found this to be more accurate than the needle (another issue)
the car has run out of petrol twice! having filled the car up with set amounts of fuel, usually £80
and allowing for low level light, the car (seems) to be coming up around 20 mile short!
which is a huge difference

few points, the car does get serviced regular, but its due a fuel filter on the next service
could this be enough to affect this?

it has had a new fuel pump plus sender so there (shouldn't be an issue there)
so id be grateful if anyone could shed some light on this

on another note, we all know the fuel gauge isn't great on a gt4, but what actually controls it
is it the sender in the tank, or is it to do with the actual dash needle too


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 Post subject: Re: fuel use issues
PostPosted: Fri Mar 09, 2018 3:38 pm 
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Joined: Wed Apr 05, 2006 12:47 pm
Posts: 28
Location: south wales
Car Model: None
any thoughts at all??


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 Post subject: Re: fuel use issues
PostPosted: Fri Mar 09, 2018 7:05 pm 
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Joined: Wed Dec 14, 2016 8:25 am
Posts: 94
Location: Russia, Izhevsk
Car Model: ST205
temperature sensor
Oxygen Sensor

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 Post subject: Re: fuel use issues
PostPosted: Fri Mar 09, 2018 8:53 pm 
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Joined: Sat Sep 02, 2006 3:13 pm
Posts: 3602
Location: Bournemouth
Car Model: None
There are a number of things that affect ecconomy, one clue is whether the ecconomy dropped after having work done on the car.

Ignition timing - sometimes timing gets set at 10 degrees without the diagnostics link in place leaving thetiming retarded by 5-10 degrees. Unlikely to change on it's own unless the distributer is loose. Some ECU error codes also result in 'safe mode' retarding timing.

Fuel mixture - Poor ecconomy would usually mean too rich, unless it's so massively lean it's misfiring. Fuel filter would only make the mixture weaker so unlikely.
Rich mixture would usually be a failing lambda (O2) sensor, but could also be a faulty engine temperature sensor (ECU one is separate to the gauge one) or a leak in the exhaust system between engine and sensor.
If the new fuel pump is a high capacity aftermarket one, this will raise the fuel rail pressure slightly which will richen the mixture.

A few other possibilities:
Broken up cat or silencer baffles restricting the exhaust.
Vac pipe from manifold to fuel pressure regulator blocked / cracked / come off.
Vac pipe from manifold to turbo pressure senor (MAP sensor) blocked / cracked / come off.

If it's a 185 or 165 then splits / leaks in any of the intake pipework will cause mixture errors.

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 Post subject: Re: fuel use issues
PostPosted: Sat Mar 10, 2018 11:19 am 
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Joined: Sat May 16, 2015 8:21 pm
Posts: 169
Location: Richmond, the southern one
Car Model: ST205
Pumped your tyres up after the cold spell?!

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 Post subject: Re: fuel use issues
PostPosted: Sat Mar 10, 2018 7:01 pm 
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Joined: Wed Apr 05, 2006 12:47 pm
Posts: 28
Location: south wales
Car Model: None
thanks for al the reply's I will try and address a few of your questions

O2 sensor is less than a year old, so it shouldn't be a issue
now with regards to the temperature sensor, not 100% sure as to what one you mean exactly
but im waiting to change the Rad temperature switch, having had a read about and (believe) this can cause issues??
but surely not enough make the fuelling bad??
ive had no recent work on the car, so I assume that nothing would have changer there
except for the Rad switch the can is 100%
having a look around the engine bay, there are no obvious loose pipework, also all of the original hoses
have been replace, so shouldn't be a breaking/perishing issue
no cat on car, has been like this for years with no issues at all

ive yet to run a errorcode check, so should probably be first port of call


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 Post subject: Re: fuel use issues
PostPosted: Sat Mar 10, 2018 7:28 pm 
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Joined: Sat Sep 02, 2006 3:13 pm
Posts: 3602
Location: Bournemouth
Car Model: None
Error code check is quick & easy, though normally the light is on if there are current issues.

The rad switch just controls the fans,so unless fans are running full time or not running at all I don't see any benefit to changing it.

The water elbow on the side of the head is where the main temperature sensors are located. The one with a single connection drives the gauge and the the one with 2 wires is for the ECU.
If the ECU thinks the engine is cold, it will inject more fuel (also a point - is the engine reaching full temperature in a reasonable time ? Could be thermostat sticking and not shutting fully)
If you do a lot of short runs, then at this time of year the engine will spend much longer cold

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 Post subject: Re: fuel use issues
PostPosted: Sat Mar 10, 2018 7:38 pm 
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Joined: Wed Apr 05, 2006 12:47 pm
Posts: 28
Location: south wales
Car Model: None
thanks, fan switch does need doing as the rads don't run at all

no stored error codes

newish thermostat, car does gets to temperature as normal
thanks for the location of the water sensor, no way of checking I suppose
don't have a multi meter but could borrow on, if instructed on what to check for

yes currently the car is doing short runs, but as mentioned, it does get up to normal temps
reasonably quickly


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