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VW MKIII-A3/B4 TDIs This is a discussion about MKIII-A3/MkIII Jetta/Golf (<99.5) and B4 Passats (96,97) TDI's. Non TDI related postings will be moved or removed.

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Old February 26th, 2005, 11:28   #1
greenskeeper
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Default engine idle rpm and fuel consumption (VAG-COM)


Is there any harm to lowering the engine idle rpm via vag-com? I can get mine down to 840rpm or as high as 1040rpm. The default value does not give me 903rpm. This has occured since my long road trip...prior to that the idle was always dead on 903 via vag-com with the default value. Also should I be able to read a fuel consumption reading in vag-com with the car at idle? It seems to go from reading 0.00 to 0.37 l/hr with nothing in between...it is either 0.00 or 0.37 and is not consistent.

I am wondering if timing belt stretch would have anything to do with this? The timing is right at the upper part of the graph however.

Reason is I've got about 41,000 miles on this timing belt, and plan to change at 50,000 just to be safe.

Words of advice? Or am I just rambling?

Thanks either way!
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Old February 27th, 2005, 11:44   #2
uponblocks
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Default engine idle rpm and fuel consumption (VAG-COM)

All of the fields in Vag-Com are digital, and therefor come in “quanta” that vary depending on the field you are looking at. RPM, for instance, changes by 21 (903=43*21). You will never get your engine speed to indicate exactly 900, for instance. 882 is the next RPM you will see as you adapt the RPM down. This is just the resolution of the OBD data line, NOT the internal resolution of the ECU. Similarly, fuel consumption seems to come in units of 0.37 l/hr, which is rather coarse. You can calculate this value yourself with greater precision using the actual injection quantity. [l/hr] = [mg/R] * [RPM] * 4 [injectors] * 0.5 [power stroke/rev (4stroke)] * 60 [min/hr] * 1e-6 [kg/mg] * 1/0.883 [liter/kg]. Here’s a graph that shows this correlation.

Here’s a graph to give you a sense of fuel consumption as a function of RPM:


I can’t really speak to the issue of whether adapting your RPM is good or bad. If it idles smoothly at the lower RPM, and you are satisfied with the throttle response, you will save a tiny bit of fuel. The fuel curve flattens out at 900 RPM, so you may not be saving as much as you anticipate.

Edited to add: In cold weather, the ECU bumps the idle RPM up until the engine warms a bit. Maybe a faulty temperature sensor is responsible for your idle drift. Just a thought.
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Old February 27th, 2005, 21:51   #3
RiceEater
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Default engine idle rpm and fuel consumption (VAG-COM)

You really don't want to idle as low as possible. You could increase the induction coking rate by idling too low. Of course this doesn't mean you should idle &gt; 1k rpm either as this could mean excess fuel to idle. 903 (at standard conditions) happens to be the balance VWAG engineers agreed upon.
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Old May 5th, 2011, 17:01   #4
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Anyone noticing an improvement by lowering idle speed? I lowered mine to the lowest something like 882 rpm just for the heck of it as I do a lot of city driving. I also have 5*BTDC timing so that hopefully creates less soot in exhaust and counters the intake clogging Did both adaptation changes together and noticing a good increase in fuel economy. Mostly probably due to the the timing change though.

Last edited by dezeljunky; May 5th, 2011 at 17:03.
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Old May 5th, 2011, 17:17   #5
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Quote:
Originally Posted by dezeljunky View Post
Anyone noticing an improvement by lowering idle speed? I lowered mine to the lowest something like 882 rpm just for the heck of it as I do a lot of city driving. I also have 5*BTDC timing so that hopefully creates less soot in exhaust and counters the intake clogging Did both adaptation changes together and noticing a good increase in fuel economy. Mostly probably due to the the timing change though.
How long have you had your timing at 5 deg BTDC?
have you noticed a change in consumption overall and power ?
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Old May 5th, 2011, 17:27   #6
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02-27-05, 11:51 PM

holy old thread re-birth!
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Old May 5th, 2011, 17:32   #7
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Oh ya... for sure! The only thing is on colder morning just need to keep it still at lower load for the first few miles as it's a little more noisier(dieseling).

I've had it like this for about 2 months and about 2500kms(I don't drive as much anymore)... Once warmed up it's noticeably more power and rev-happy. Fuel economy is easily 3-4 mpg more. Use to get about 42mpg consistently and now I'm at 47mpg consistently. Check out this thread http://forums.tdiclub.com/showthread.php?t=216420 I also lowered idle rpm to the lowest as I drive in the city with a lot of red lights. I sometimes stop the car on longer lights but mostly I let it idle. I figured less energy to maintain a lower rpm so might be a tiny bit of savings haha.... we need all the help we can get. I don't know how to quantify the tiny amount due to the lower idle speed but mostly it's the 5*BTDC I would say. The power increase is also significant and like others who've done it, it's hard to go back to stock!
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Old May 5th, 2011, 17:34   #8
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I also did the cage mod and that smoothed out the power band some too...

I love recycling threads
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Old May 5th, 2011, 17:58   #9
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Well the question is, did the cage mod give you the increased fuel economy, or the lowered rpms at idle? Did you do both or one, test it, then the other.

I'm running a 1100rpm idle and my fuel economy has for the most part, not changed.
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Old May 6th, 2011, 02:00   #10
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Just realized this post is in the MK3 forum... sorry for cross posting but honestly, I don't know if the lowered idle rpm helped. I was hoping to get thoughts from others *sigh*... maybe one day I'll set it back up and see if it makes a diff. Likely the diff is too small to measure.
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Old May 6th, 2011, 08:52   #11
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If you drop the idle speed 10% your idle fuel consumption should drop a similar amount but the idle fuel consumption is so low compared to driving I doubt you'll notice it.
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