Off Idle Lean Out


Off Idle Lean Out

Postby Topfun99 » Fri Sep 14, 2007 9:34 pm

I am having a problem with the engine leaning out momentarily just when I open the throttle up at idle (900-1000 rpm). The A/F meter shoots to lean momentarily and the rpm's drop 100-200 rpm until the engine recovers. This only seems a problem when opening the throttle right at idle. I've tried lots of variation with the throttle pump settings (I'm using firmware 32, Halrun V1.71) including increasing the injector times and number of pulses. These changes help somewhat, but the engine still does not rev cleanly off idle. As a "bandaid" I radically increased the injector pulse times at the idle rpm ranges adjacent to and at the "0" load bars (I spiked a few bars around these ranges) and have got it to respond better. The car runs and revs clean otherwise, it's just this one spot off idle that I have this problem. This seems to happen no matter if I open either very fast or just moderately fast.

I am using an E11v2 on a bored/stroked (363CI) 5.0 Ford small block that right now is naturally aspirated. Nothing is extremely radical, the hydraulic roller camshaft is moderate size (228I/224E @.050 lift), the intake is a street/strip Holley Performer II and the headers are 1-5/8" primary tube shorties. The exhaust system is big (dual 3") because the car will have turbos in a few months. The throttle body (75mm) is one of the largest they sell for this application and is several steps larger than the stock one for the 5.0, but not really huge for this size engine.

I suspect that the engine may be "gulping" the air momentarily faster than the ECU can make the throttle pumps react due to factors causing a small lag in the system, perhaps from the MAP sensor lagging the actual engine manifold pressure. Eventually, I can probably tune the off idle lean out by continuing to treak the bars where the lean out is but I was wondering if anyone had any other suggestions.



your so close

Postby matt510 » Sat Sep 15, 2007 2:40 am

you are very close with your description of whats happening, your engine is getting that large "gulp" of air but its not so much to do with the ECU not being able to keep up its to do with the physical properties of air and fuel. Basically it goes like this, fuel is much heavier than air so when you get a large gulp of air, the MAP sensor notices this instantly and corrects instantly - the problem is because air is lighter that fuel (weight) it accelerates much faster than the fuel does.
The idea of additional fuel in the throttle pumps is that you try injecting a whole bunch more fuel in the hope that at least some of it can keep up with the air and get to the combustion chamber. This can be very difficult however if your injectors are not very close to the valve.
Try adding only 1 or 2 additional pulses but make them long (say 5 ms if its just 1 pulse or 3ms if its 2 pulses) see if that helps you out !

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