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13 GM korea (chevy optra 1.6) low gas mileage

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4 years 9 months ago #31042 by guafa
Hi guys,

This car was in my shop 4 weeks ago for excesive fuel consumption. I tought It was fixed by changing canister valve (which was stuck open). LTFT (-25%) STFT (-15%). Now customer came back with the same complain.

These are readings before/(after) Cat (HC 267/(42) ppm, CO2 14.2/(14.7)%, CO 0.6/(0.04)%, 1.5%)

LTFT is 1% and STFT is -3%

Calc. load (at idle) is 22% and (at idle + A.C) is 40%. Almost always A.C on.

Costumer has two twin cars (he says this one consumes more than the other).

No EGR, neither downstream O2 sensor.

Following parts were changed in the last shop
O2 sensor, throttle body, spark plugs, plug wires, cat.

The engine recently was rebuilt.

Any idea on how to face this, since PID's look fine?

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4 years 9 months ago #31049 by Andy.MacFadyen
Cat choked ?
Loss of compression ?
MAF reading ?

" We're trying to plug a hole in the universe, what are you doing ?. "
(Walter Bishop Fringe TV show)



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4 years 9 months ago #31105 by guafa
Hi Andy and thanks for your prompt response,

I am going to check compression and let you know.

It doesn´t have MAF and Cat is new. In fact we bypassed Cat with a pipe and customer still complaining. This is my list of possible causes:

Fuel tank leaking
fuel hoses leaking
High fuel pressure (which fuel trim numbers contradict)
fuel injectors leaking (which fuel trim numbers contradict)
high compression ratio vs low octane gasoline (10.5:1 vs 87 octane)(which owner´s twin car contradicts)
O2 sensor making engine running rich (which fuel trim numbers contradict)(it is a no heathed o2 sensor with a 0.5 hz frecuency)
variable length intake manifold valve stuck (now i remmerber customer complained about lack of power up hill)
Lack of spark power
Dirty injectors (inefficient combustion on roads that momentarily enriches the width of the injection pulse)
Engine oil (very viscous oil engine)
Sticky engine components
sticky suspension components

Regards.

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4 years 9 months ago #31123 by Tyler
Do the two cars have two different regular drivers? :blink: Or are they both regularly driven by the same person?

I've found that driving habits and conditions have more to do with fuel economy than almost anything else. IMO, your trims rule out most fuel control faults.

How is this customer measuring fuel economy, exactly?

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4 years 9 months ago #31127 by PDM
What are the expected and actual mileage numbers?

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4 years 9 months ago #31136 by guafa
Hi guys and thanks for your interest in this topic,

Do the two cars have two different regular drivers? :blink: Or are they both regularly driven by the same person?. Different drivers, which makes the things hard to compare.

I've found that driving habits and conditions have more to do with fuel economy than almost anything else. IMO, your trims rule out most fuel control faults. Yes, I have calculated habits as top one and second traffic (idle time of course worst with AC on).

How is this customer measuring fuel economy, exactly?. gallons required to fill a complete tank, divided by km traveled since last filling. Anyway, I´m constructing a "one galon tank" to measure again by myself.

What are the expected and actual mileage numbers?

The good one 35km/gal - 22miles/gal - 9.24km/liter - 10.8L/100km
The thirsty 22km/gal - 13.75miles/gal - 5.81km/liter - 17.2L/100km

Best regards.

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4 years 8 months ago #31202 by Tyler
Thanks for the info, sir. B)

Perhaps you should politely ask the drivers to switch cars? Top both tanks off, tell them to drive to 1/2 full and redo the math. If the poor mileage follows the driver, you have your answer. :lol:

Of course, as soon as you suggest this, they're almost certain to change their driving habits, which might invalidate the test...

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