Poor fuel efficiency (MPG) 2005 Montana sv6 3.5 fwd 265K km

[cityRaccoon]

Hi folks!
Another trouble with a car.
Now I found out that MPG is really poor. Car indicates that it is 16.3 l per 100km, which is 14.4 US MPG. But I feel it actually even worse.

Tank was almost empty and we pour about 55 liters (14.5 US Gal), reset odo. It was around 250 km (155 mi), when 'low fuel' light was on again. It seems we are getting 19l per 100 km (12.3 US MPG).

Weird... engine runs smooth, starting fine even in a bitter cold, good acceleration. I cleaned MAF and throttle body recently. Added Seafoam to the gas couple of times. Engine burns oil a bit, like 1 liter (1 quart) between oil changes. "Check engine' light on sometimes mostly when it is cold outside...

Maybe it is a leak, but I can not smell any gasoline inside or outside. Breaks not seized...

Puzzled with it, my neighbor has got f-150 (2004) and MPG is much better then ours

 

[EaOutlaw]

To start with find out what code is causing the check engine light.

If one or more of the O2 sensors are not allowing closed loop your van will get bad mileage.

A simple thing like a clogged air or fuel filter could cause all sorts of problems.

I worked on a Honda many years ago, It was at many shops, they all took their turn hanging parts on the car trying to get it to pass emission inspection.

The car ran well had power yet the emissions mainly HC and CO were off the chart. Running very rich.

All that was wrong was a clogged fuel filter, Once I replaced the filter and drove it down the interstate for a 10 mile round trip and retested the computer for codes none found retested the emissions and it would pass the inspection with no problem the readings were now normal.

If you haven't already check the basics. You can remove the fuel filter and drain it.
Wipe it off and try to blow through the filter the normal direction of fuel flow.

If you cannot blow through it easily replace it.

If the air of fuel filter is good, let us know what codes you have.

The fuel injectors may be too old and no longer have a good spray pattern.

With the mileage on your engine 1 qt of oil between oil changes is not excessive

But if your worried about the mechanical condition of the engine pull a compression test and a cylinder leak down test if you find low compression on any or all of the cylinders.

BTW a computer will sense a lean mixture from a clogged fuel filter and dump what fuel it is getting to keep the vehicle running.

Normally this will trigger a check engine light. which is why it is important to keep the vehicle in good running condition and never allow a check engine light to go ignored a new serious problem that arises will get ignored as well

 

[erndog72]

I didn't see anything about the most common cause of bad mileage, proper tire pressure.

 

[melsg5]

Tire pressure may contribute but if the numbers they posted are correct you don't get single digit mpg from low tire pressure.

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[cityRaccoon]

Guys.
Thanks for the reply. We are going to replace catalytic convertor and oxygen sensor. Can not read errors "check engine" isnt on anymore.

 

[EaOutlaw]

Guys.
Thanks for the reply. We are going to replace catalytic convertor and oxygen sensor. Can not read errors "check engine" isnt on anymore.

I would advise saying your money and diagnose the problem.

The 02 sensor and catalytic converter are parts that can go bad yet more often show symptoms because of other problems.

A simple vacuum leak, bad fuel, clogged fuel filter clogged air filter, low fuel pressure and or volume plus many other things can all cause codes for the 02 sensor and catalytic converter.

If you have other reasons for replacing these parts that you have not mentioned, for example High mileage and or preventive maintenance that is fine yet make sure to have to check the codes in the computer and have the emissions tested at idle and off idle.

If you have high emissions readings the new parts you installed will go bad prematurely. and you will still have poor fuel economy

 

[melsg5]

why wouldn't the codes be in history? Did a shop with a Tech II or equivalent actually check the vehicle?

 

[cityRaccoon]

EGR passage

Update on the issue.
I was reading some articles dont remember who posted those links melsg5, EaOutlaw or sergeimoskva. That specific article was about EGR system.
So, 3 indicators:
1. Really bad MPG
2. Low idle RPM (hot engine)
3. Car runs just fine

So we have all 3, Idle speed is around 500, which is too low. And car drives like new, starts in a bitter cold with no hesitation, good acceleration.

So it seems like faulty EGR/ insufficient flow.
Than I read another story by some mechanic, he says that for GM 3.5l engines most times valve is functioning properly but EGR passage getting clogged with carbon. They sorta known for this thing.

Car is 10 yo and in last 7 years nothing really was done about the engine.
I'll post some photos if I ever venture to do the cleaning.

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