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Unread 08-01-2013, 11:17 PM   #1
hearld5000
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More Power Before Warm-up? MDS?

The last few weeks I've notice my 2006 5.7 Hemi feels like it has a lot more power in the mornings before the engine warms up...any thoughts on why this might be?

Is it true that MDS doesn't enable until it warms up to a certain point? Could this be related?

On a side tangent, I have a SuperChips tuner...what is the point of disabling MDS outside of the drone some people get with aftermarket exhaust?

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Unread 08-02-2013, 12:33 AM   #2
trax95008
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Every modern car will have more power at first start up of the day. Two reasons, 1: it's sucking in cooler more dense air, and 2: the computer is running it richer on fuel which mimics a choke on a carburetor. So there it is. More fuel, more air, MORE POWER!!!!
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Unread 08-02-2013, 04:58 PM   #3
90grandoneer
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X2^^^. Until the engine warms to a certain point, the fuel injection is running in open loop (same as it would at WOT), and instead of a 14.7:1 air/fuel mixture, it's probably running ~11 or 12:1 a/f ratio. Maximum economy is the former, maximum power is usually 12.8-13.2:1. Our stock Hemi's run ~10.8-11:1 at WOT. I've not monitored the a/f from start to warmup. I'll have to do that??

Also, on mine, the MDS doesn't start working until the engine temp. is ~115=125 degrees. Usually takes less than a half mile, depending on the ambient temp. The MDS doesn't have anything to do with more power right after startup. You're sure right though, performance is a lot better when cold.
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Unread 08-02-2013, 09:04 PM   #4
quicksilverado
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The transmission also holds gears longer to rev the engine higher. This helps warm the engine up faster.
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Unread 08-02-2013, 11:12 PM   #5
hearld5000
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I can certainly understand that, but these gains really feel more than anything else I've ever driven. Could this point to a bad EGR perhaps? I'm half tempted to do a 0-60 in the morning comparison to at full temp, it really seems too significant to be normal.
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Unread 08-03-2013, 01:01 AM   #6
JRLT
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My buddy said a lot of computers usually run a "cold" program until the engine warms up and then the computer will start the "warm" program. The cold is setting things in line at certain levels until the engine gets warm and then it'll kick over to a more active program. Basically, if something's wrong, it wont act like it till the engine is warm. A friend's Ram 1500 with the 5.7L Hemi runs great until it warms up. He needs new O2 sensors(and he knows it) so it runs like crap after the engine warms up. So, you more than likely have an underlying condition.
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Unread 08-03-2013, 09:54 AM   #7
90grandoneer
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Quote:
Originally Posted by JRLT View Post
A friend's Ram 1500 with the 5.7L Hemi runs great until it warms up. He needs new O2 sensors(and he knows it) so it runs like crap after the engine warms up. So, you more than likely have an underlying condition.
That's probably because it's (fuel injection) running in "open loop" mode, which doesn't use the O2 Sensors for mixture control. In this mode mixture is controlled by pre-programmed fuel tables in the PCM that are utilized based on other engine sensor inputs. Once warm, and in "closed loop" operation, the front O2's are the primary input for proper (14.7:1) mixture control. If the front O2's are bad/going bad, they can go rich, lean, or become lazy and not respond as they should. For a little more info. on this, check this thread out:

http://www.jeepforum.com/forum/f67/o...rimer-1182446/
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