10 replies to this topic

[aussiematt]

Gday Everyone.

I have a 2003 Chevrolet Suburban K1500 LT with a 5.3L 'Z' type engine. For the last 3-4 years it has consistantly used 13-14L of fuel per 100 Km. It is always regularly serviced. For no apparent reason it is now using 21-22L per 100 km. It has been scanned and the error code PO178 came up, a new costly fuel composition sensor has been installed and still no let up with high fuel consumption. Instantanios figures at idle go to 99L / . even under extremely gentle excelleration it goes to well over 35L/p/100km. when im just coasting there is no drag at all, it does'nt smell of fuel.
Can anyone help. I live in the Northern Territory of Australia and no one wants to go anywhere near it.

[Jsdirt]

In my past experience, high fuel consumption is usually the result of a faulty O2 sensor. But these days, it could be anything -- the O2 sensors, map sensor, clogged air filter, or just worn out spark plugs, or a fault in the ignition system. Usually these things will throw a code, but they could be on the ragged edge of the threshold that sets the check engine light.

Wish I could be more specific, but these things are tough to diagnose over the internet.

Hope this helps somewhat. Good luck.

Edited by Jsdirt, 26 September 2009 - 05:36 AM.

[aussiematt]

Thankyou for your reply, i will have a look at MAF and ignition circuit as all other items mentioned have been replaced to try and correct this problem. i do realise the difficulty in diagnosing this fault, but all imputs are welcome. Cheers Matthew.

[thearborbarber]

Do you have a flex fuel vehicle?

If so try these links

http://www.automotiv...topic488158.htm

http://www.automotiv...om/t359623.html

[Nytemare]

I would try cleaning the intake and throttle body and changing the fuel filter, than run a couple tanks of injector cleaner.

new plugs and wires may be a good help too if you haven't changed them before. They are due to be changed at 166,000km but can go bad earlier. Replace the plugs with NGK TR55, platinum.

At idle you will get a reading of 99L/100km always..because you aren't going anywhere really you are burning infinite fuel /km but the system is set to show 99L

another reason for this could be from the fuel you use. if you buy fuel with higher amounts of ethanol as fillers you will reduce your fuel economy.


Hope this helps

Cheers;
Aiden

[aussiematt]

Hi Aiden
Thankyou for clarifying the instantaneos figures on idle, i never really took much notice of those numbers, as i have always been very happy with the average fuel figures. New plugs and filters hae just been Changed. I always run premium fuel 95 or 98 but the finance manager likes the look ($$$) of the standard unleaded. I have been reading up about a "Seafoam" product that is designed for upper engine cleaner. Have you or any one else had any experience with this cleaner and if so can you tell me how it went.
Cheers Matthew.

[Nytemare]

I have heard of guys using Seafoam...I have heard lots of good about this product...I have used similar products with equally good reviews.
Just disconnect the vacuum line from your brake booster and siphon the Seafoam in to the intake from there...probably the easiest way to get it in there.
Just follow the directions on the bottle.

[coloradoSkiCountry]

Use only the Octane level that the truck calls for. Here in the US the 5.3L calls for 87 unless your above 4000 ft then you can get away with using 85 Octane just fine. There is no advantage to running premium fuel in the engine unless you have a performance tune done. I used to notice that my fuel economy started to go down when I ran 91 Octane instead of the 85.

My guess would be the following:

The fuel companies are putting more ethanol into fuels now due to strict environmental/pollution standards. My fuel economy definitly goes down when they switch to the winter blend of gas as well.

What type of Spark Plugs did you put in? I've found that I get the best performance/fuel economy with stock AC/Delco plugs. None of that fancy Shancy aftermarket spark plug nonsense. Change the plugs around 75,000 miles. I don't buy that 100,000 mile plug interval nonsense.

Change the Plug Wires if your truck is between 85,000 - 100,000 miles. (not sure what the conversion is for your neck of the woods)

Running Seafoam won't hurt at this point. It is useless to run the stuff in your vehicle if it has less then I say 80,000 miles.

I'd run a bottle or two of a good fuel injector cleaner thru the truck as well, (I prefer Chevron Techron fuel injector cleaner and run 2 bottles a year thru my truck, in the summer, around July, and again in the winter, around December)

What kind of oil are you using? These gas engines do well with standard 5W30, and you can get away with 10W30, but don't use anything else. 5W30 will work great, either Synthetic, Synthetic Blend, or standard conventional. You'll not notice any difference in fuel economy by using Synthetic over conventional, contrary to popular belief.

Do as others have suggested, clean the throttle body, and air filter. Unless you drive in dusty conditions (dirt roads) you should really only need to replace that air filter once a year. If your truck has a fuel filter outside of the fuel tank, go ahead and replace it with a good quality part. AC/Delco or NAPA will work good.

How are your tires? Did you increase the tire size lately? are all properly inflated? You'd be surprised how much under or over inflated tires can mess with the fuel economy!

The O2 sensors may be starting to get old. Most 02 Sensors now a days have a heating element inside of them, they can start to go bad as early as 85,000 miles, however it may not always throw a check engine code/light.

Are there any problems with the exhaust system? a Cat Converter which is starting to plug up can cause problems as well.

[aussiematt]

Thankyou Ryan for your reply, i have just returned from a quick trip upto Darwin and back and are happy to report that @ 130km/ph over 300 km one way, the truck managed to fall to a much appreciated 13.9L per 100 km with 4 kids and no luggage. On the way back with the finance manager and No5 + luggage and little bit more right foot 130-150 Km /per h it managed 15.8. So im thinking that the fuel composition sensor may have needed a couple of start cycles to get its act together, as these are the figures we have been familiar with.
As far as the octane rating of the fuel, i am confident that our minimum down here is (91-enleaded)
- ( 95-Premium) & (98- ultimate). So do you recommend that i choose a fuel type and have it tuned to suit, and can this be done via the latest Snap on diagnostic epuiptment?.

[aussiematt]

Thanks Aiden for your reply. I will source the product over here,give it a try,and post my results
Cheers Matthew.

[Nytemare]

Your truck has sensors that allow auto correction of timing and fuel maps for the octane you use. All be it a crude system with a stock tune. Your engine has a 10.8:1 compression ratio and you should use mid grade with anything above 10:1 up to 11:1 and preimum for higher than that, so that's 95 for you...89 for us in NA. The factory computer is tunnned, however, to allow use of regular grade fuel your 91 our 87. A reflashed computer has better maps to make more power and consume less fuel for all fuel grades and can switch on the fly as it gets readings from the knock sensor...and various other sensors. This is done by buying a hand held programmer, actual software to tune the computer, a dyno tune or you can buy a preprogrammed "mail order tune" from various different places (jennabear on here I beleive does them Blackbear tuning)

my truck has a preprogrammed computer and I run 94 octane fuel that would be 99 for you guys. I get better economy with that fuel than the low or mid grade stuff. Actually 70-100km per tank enough to make up the difference in cost and than some.

Edited by Nytemare, 01 October 2009 - 10:35 AM.