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JKH
Dodge Dakota
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6/01/2004
14:00:40

Subject: Air/Fuel Ratio
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if i install new intake, headers, and exhaust, will the stock PCM auto adjust my air/fuel ratio to keep the engine from going lean, or will i have to get it reflashed?

what about the Mopar Performance PCM?



hmmm
Dodge Dakota
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6/01/2004
15:59:35

RE: Air/Fuel Ratio
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It's hard to tell. Our FI systems use thousands of programmed maps to adjust a/f depending on input from various sensors. As long as the changes you make allow the engine to operate within the parameters specified on any one of the maps then you're OK.

You can trick the PCM by moving sensors around (like the IAT) or use a FPR & bigger injectors to run rich if you have probs. The easiest way to tell is to hook up a secondary a/f meter and watch it for a while.



ticats
Dodge Dakota
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6/01/2004
17:16:24

RE: Air/Fuel Ratio
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For the mods that you mentioned, the forward O2 sensor will constantly adjust the air-fuel mixture towards stoich (14.7:1) as long as the computer is in closed loop operation. In open loop mode, the computer takes the air-fuel mixture and ignition timing from pre-programmed tables based upon the historical operation of the particular motor, selecting from the table based upon inputs from the MAP sensor, coolant temp. sensor, tps, etc. The O2 sensor is ignored in open loop.

ticats, ontario, canada.
97 Dak CC Sport, 5.2L, 44RE, 4.56SG, race weight 4,120 lb, 13.98 @ 99.85



irish
Dodge Dakota
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6/01/2004
19:05:35

RE: Air/Fuel Ratio
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Where have you been? Havent seen you on the r/t board...

'99rc r/t
13.51@99
N/A



hmmm
Dodge Dakota
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6/02/2004
12:34:56

RE: Air/Fuel Ratio
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ticats,

Sorry, bud, you got that one a little confused. Here's a link to a good explanation. Our systems are Speed Density systems and you'll see the explanation under that heading.



ticats
Dodge Dakota
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6/02/2004
14:46:23

RE: Air/Fuel Ratio
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hmmm ... not quite sure where I'm in disagreement with that article. Unless it's in the last paragraph of the Speed Density section where what I wrote might suggest that there were no preprogrammed fuel mapping points for closed loop operations.

"Because a Speed Density system still has no sensors that directly measure engine airflow, all the fuel mapping points must be preprogrammed, so any significant change to the engine that alters its VE requires reprogramming the computer."

Bear in mind that my response was made in the context of JKH's original message where he asked if a new intake, headers, and exhaust would prevent the stock PCM auto from adjusting his air/fuel ratio to keep the engine from going lean. For these mods, a reflash is not needed. He might not gain the full performance benefits for these mods as he might under a MAF system but that's another type of question.



S Dave
Dodge Dakota
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6/02/2004
15:08:57

RE: Air/Fuel Ratio
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If you limit the changes to just headers and a cat-back exhaust system, then your computer will compensate by adjusting the fuel pulse width to maintain the proper air/fuel ratio. If you remove the cat in the exhaust system or install the M1 intake manifold, the computer can't compensate and you will run lean. Not trying to start an argument. But, for those that don't believe me have your engine hooked up to diagnostic scanner and see for yourself. You would think that Mopar Performance wouldn't sell the M1 intake if it wasn't going to perform like it should because of the way the fuel tables are programmed in the stock or MP computer. But, they do and it doesn't. If I find that article written in Mopar Now about the M1, I will try to upload it. Or at least inform those of you what issue it happens to be in.



ticats
Dodge Dakota
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6/02/2004
15:58:36

RE: Air/Fuel Ratio
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S Dave ... that's interesting. I've hooked up my OTC scanner both on the street (part throttle) and on the track (wot) and recorded pulse width and O2 sensor outputs. I have the M1 4bbl (plus internal engine mods) and I have not noticed it running lean with the stock injectors and oem fuel pressure. As a matter of fact, when I tested 24# injectors, I could see how rich it ran at part throttle and why it bogged badly when I suddenly would go wot. Just my experiences.



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