Fuel consumption rising when idle

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Fuel consumption rising when idle

Postby slatery » Sat Jun 26, 2021 11:08 pm

I noticed that the fuel consumption in my 08 3.0R 5EAT was a bit high over the last week, but traffic has been bad so chalked out up to lots of stop/start. After filling up the fuel tank and resetting the trip metre did I see it happening, while sitting at idle the average fuel consumption was rising fast, about 0.2l/100km every 30 seconds.

I haven't noticed a loss in power, but it was idling a bit tough after starting after sitting for a few minutes, and normal driving seems uneffected and lowers the average fuel consumption.

Does anyone have an idea on where to start looking? There's no check engine light and don't have an OBD2 reader, so I was going to check fuel injectors and spark plugs first
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Re: Fuel consumption rising when idle

Postby Yowie » Sun Jun 27, 2021 7:18 am

That all sounds pretty normal.

When idling and stationary, you are effectively using infinity litres per 100kms, so that situation is going to push your dashboard fuel economy readout in the "less efficient" direction.

"but it was idling a bit tough after starting after sitting for a few minutes,"

I get that on my E85-fuelled car. I assume (without having the means to test it) that the fuel in the fuel rails (sitting right on top of the engine) overheats because it doesn't get to circulate and the engine & fuel hasn't had a chance to cool down yet either.

Did you find that the roughness went away after a few seconds? I would put that down to the heated fuel being used and/or circulated back to the tank and being replaced with fresh cool fuel in the rails.
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Re: Fuel consumption rising when idle

Postby slatery » Sun Jun 27, 2021 8:51 am

It has never done this before, which is what's causing my concern, the average fuel consumption gauge normally cuts off so this doesn't happen, inner the course of the 10 km between home and work my fuel consumption has file from around 13l/100km to 15 and only goes up
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Re: Fuel consumption rising when idle

Postby bigBADbenny » Mon Jun 28, 2021 7:14 am

I’d be checking for an inlet tract leak.
The plenum before the throttlebody is noted to crack at the centreline near the clamp.

You can do a visual inspection, or use eg brake cleaner or unlit propane at idle (have an assistant watch the tacho for rpm change) or use inlet pressure testing with the engine off.

Cleaning the maf and checking its properly seated in its oring is another maintenance basic.
You can also check for exhaust leaks.
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Re: Fuel consumption rising when idle

Postby flatsix » Tue Sep 07, 2021 7:23 am

slatery wrote:I noticed that the fuel consumption in my 08 3.0R 5EAT was a bit high over the last week, but traffic has been bad so chalked out up to lots of stop/start. After filling up the fuel tank and resetting the trip metre did I see it happening, while sitting at idle the average fuel consumption was rising fast, about 0.2l/100km every 30 seconds


did you have any luck fixing this ?
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