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Weird misfire / no codes

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Old 01-10-2018, 10:06 PM   #1  
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Location: Charleston, SC
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Weird misfire / no codes

So I've been chasing an intermittent misfire for about three months now. It's been present far longer than that, but I've finally decided to do something about it. Truck has 195k miles, in-tank lift pump, II +60hp injectors that are less than 5k miles old, and a Smarty Jr.

At light throttle, with boost below 10psi and rpm between 1700 and 2000, there's a very, very slight "hitch" in propulsion. It goes away immediately when I add throttle. At first I thought it was actually suspension related, like tight shocks on a wavy road, but it's now painfully obvious that it happens on all roads, and my KYB's are no longer tight. It was present before I had new injectors, and it's present now.

I'm leaning toward a sag in rail pressure, whether it's a glitchy FCA, or a failing rail pressure sensor. Could also be a Smarty programming issue, but I've changed the tune on the Smarty, varying the torque management and timing, with no change.

I've run a quart of 2-stroke with a tank of fuel to see if it could be a sticky pump or FCA, but noticed no difference other than a stumble at first start up and a fair amount of smoke when cold. That's long since past now and it starts great, and doesn't smoke.

Not sure if it's relevant, but the truck idles fine unless I'm coasting to a stop with the tranny in neutral, in which case it hunts between 750 and 1000 rpm until I get below 10mph, when it settles out at 750.

Anybody have any thoughts?
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