Now that I am done with the trans, I am working on some issues that may or may not be related to the frankenmotor.

As the subject says, I have a hesitation with the engine when cold. When it warms up, it'* beautiful. But from dead cold, from idle when I snap the throttle to say, halfway, there is a distinct stutter that can be heard through the intake, and it'* driving me nuts.

Now, here'* the "constants" of the issue: New plugs, verified good wires, running S2 L67 coils, swapped to low-mileage injectors, rail, and FPR. Scantool shows no codes and nothing out of the normal in real-time monitoring. But a real interesting fact is this: when I restrict the BCM to it'* position at idle, and give it the throttle, stutter goes away.

I have yet to test the fuel pressure, and I am planning on testing that. But if there is anything else I am missing, clue me in please

Typically happens in the morning after the first cold start and rolling on the throttle?

One possibility (similar to something I had happen 3 months ago) is a fuel injector leaking down overnight. Fuel pools in the manifold. When you increase manifold flow by accellerating, it'* pulled in, flooding out one or two cylinders. Once the car warms up, the fuel is burnt off.

Pull your rail at night if you can, but leave it hooked up to the fuel lines. Put 6 identical small containers under each injector (shot glasses or small tumblers work great). Prime the system for 2 seconds three or four times to build pressure, and let it sit overnight.

See if you have any fuel dripping out by morning. 30 minute setup, 30 minute put back.

This can also cause hard starting on cold mornings, but it depends on where the fuel pools (this can depend on parking angle/slope, injector number, amount of leaking, etc). If its hard cold starts, you can preven the injectors from firing by holding the throttle to the floor when you start. If there'* fuel pooled there, it'll pull it in, and burn it. The car will actually start with the injector signal disabled by the WOT switch. This was my case, but again, yours may be similar with slightly different circumstances/amount of fuel/where it'* pooled/parked on a slope, etc.

Hope that made sense.

It makes sense to me

BUT....

Engine starts great when cold. Idles perfect. And if I go easy on the throttle, I get no hesitation. It'* when I really goose the throttle (and yes, I know I'm not supposed to be doing that anyways) that the hesitation is noticeable. And this happened with all the original injectors and FPR as well. Though I do suppose leakdown could happen with ANY used injector.

In regards to the pooling, wouldn't this show up on the O2 as a rich condition on startup? I should note that I do get a puff of smoke on cold start as well.

Leave it to Don to make a mountain-sized problem an anthill. He starts it up, thinks for a second or two, unplugs the MAF, and bam...car runs much better than it did before.

I've put on an APB on the sensor already to a certain parts supplier. Hopefully, the good doctor has one in stock

EDIT: It'* not the MAF. I swapped MAFs between the L27 and L67, and problem continued on the L67, L27 remained unfazed. So, an idea struck me..."what about the MAF screen?" I put the screen in the L67.....disconnected the computer to clear all faults and go stupid again, reconnected, and....PERFECTION.

SO, for my application, running with the screen is the best way to go

You had the screen out?

That could do it. The MAF was unable to read properly and due to being cold fuel trims etc were different than warm. Makes sense. Good find.