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IDC not 0 during engine braking


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OK, this is maybe not the right forum, but I figured the folks here would know the best how the computer works & understand my question.


I monitor various parameters with the BTSSM from time to time, and I notice that during a descent down a hill, engine braking, foot off the gas, IDC drops to NEAR zero, but not ZERO. At 3k rpm it's usually 1.7 or so. If I downshift, from say, 4th to 3rd, raising the RPM, the IDC rises to around 2.x or so. (I assume a little bit of air is getting by the throttle plate (well, I 'know' it is from monitoring the MAF at the same time)


I was under the impression from reading various things that 'modern car ECU's will shut off gas to the engine while engine braking, using no fuel'. I don't understand how that can be the case, if the IDC isn't 0, and does change slightly with RPM.


It's not a LOT of fuel, granted, but I thought it'd be no fuel.


Am I missing something?


Not a big deal, but it's been itching at my curiosity for some time now, and I can't find anything that talks about it, so I figured I'd ask some folks that know more than me. :)




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It might just be that there's another switch somewhere in the ECU that shuts off fueling when there's no accelerator pedal input, and it overrides the IDC (so even though it shows a duty cycle, it isn't injecting fuel). A good indication is to keep an eye on your AFRs. If it's pegged at full-lean (20 or so), then you can be sure there's no fuel being injected. If, in this condition, you give just a tiny bit of throttle input, you should see the AFRs jump back down to stoichiometric (14.7ish), but the IDC should stay about the same.
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Yes, its as solidxsnake explains. On an H6 the minimum IDC, MAFv or g/sec also show non-zero values even with the engine off and only ignition on. Same is true when engine breaking at 30 or 60 mph. And it doesn't matter if I use standard params, 1, 2 or 4-byte fields - all register non-zero values.


Depending on how your initial start-up DBW TPS calibration goes (the ECU/BIU repeat it each ignition cycle, especially right after a reset or reflash), closed or near-closed throttle AFR will register as anywhere between 20.x:1 and 23.x:1.... varies from car to car, year to year, even ignition cycle on the same car. So that's one tell tale filter to check when reviewing your IPW, IDC, MAFv, MAF g/sec.

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