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Discussion Starter #1
This is on a full 3.0l swap. Only does it when sitting overnight really. Once driven it'll start fine all day after sitting for hours. Chasing down this problem and so far I haven't found a vac leak source. Have tried a smoke test (DIY method) and spraying carb cleaner around various leak points.

On a cold start it cranks fine, fires up and immediately dies within a second. Repeated efforts get better and better until it finally does start w/a little help from some throttle. It's spitting out black on ground from being rich.

I've cleaned the IAC, tapped on it, unplugged it (won't start at all obviously), cleaned the MAF and temp sensors, coolant temp sensors show it being correct (48* in the garage). Plugs are new, wires are fine. The car runs and idles normally once it's going and only seems to be limited to a cold start. Only things I can think of are a vac leak I'm not able to detect (gonna try the propane method in case carb spray wasn't cutting it) or something w/the IAC not opening fully.

I'm getting no codes.
 

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Do you have access to a smoke machine? I've seen a few videos on youtube for a how to make one. When we run across a suspected vacuum leak at work that's normally where we go.
 

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Discussion Starter #3
Not a real one. I used my food smoker to find a small leak by the EGR and we replaced the pipe and the EGR itself. But the smoker doesn't have the pressure to push too far into the motor.
 

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Im not real sure how you accomplished it with a food smoker but I bet that helped the exhaust smell amazing . To get deeper in the engine we either use the line off the brake booster or through the throttle body itself. We pump alot of smoke it in though when we do it.
With no codes or anything like that it cant be too serious.
 

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Sounds like IACV.
Seems like an air leak should run lean, not rich.
I'm pretty sure the windings in the solenoid degrade and fail but since current still flows, the PCM doesnt necessarily detect the error.
I made a pigtail last year for testing them and discovered one that was only opening a tiny bit when voltage was applied. That was just a connector cutoff a junkyard car that i put some alligator clips on.
I could probably send it to you if I can find it, but I'm sure you could come up with something just as easily.
 

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Fuel pressure issue?

After it's sat for a bit poke the schrader valve on the fuel rail and see if there is still pressure. You can turn the key on and off to prime the pump a few times and build some pressure back up.
 

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Fuel pressure issue?

After it's sat for a bit poke the schrader valve on the fuel rail and see if there is still pressure. You can turn the key on and off to prime the pump a few times and build some pressure back up.
I'm with Wade, fuel pressure was the first thing I thought of.
 

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Discussion Starter #9
Fuel pump is brand new as well as the filter. But I’ll check the valve. Swapped a bad injector (SVT Cobra rebuilds) and found plenty of pressure at the rail when that was done last year.
 

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Discussion Starter #10
Forgot to check the fuel pressure at the rail, but did do a propane test and heard no change in RPM while running. Popped a new IAC on and did find it helped smooth out the idle overall a little once running.

Here's a video from this morning after sitting overnight:
 

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Discussion Starter #16
@wadespencer99 I honestly can't remember as it's been a year of just fixing little things here and there over that time along w/vac and exhaust leaks that would possibly contribute to it. When we first got the motor in we test fired it w/out the entire UIM on and it started right up (spitting fire too). It's an e-tune and my next step this Summer was to get it to an actual dyno once these issues were sorted.

@B3NN3TT that's the intake filter. It's always done that (even the old K&N setup on my 2.5, this is their KAI kit) when you shut the car off it has an air through a straw sound.
 

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Yeah, mine does it too but the way it comes and goes is weird. Maybe it's just the IACV opening and closing - I assume there's no throttle inputs during this video.
 

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Could this be an O2 problem? Or perhaps the MAF?
O2 failure - It thinks its lean so it sends more fuel and cuts air
MAF failure - It thinks its getting more air than it actually is so it sends more fuel to prevent being lean while also cutting air until RPM's drop and its forced to send more air again.
 
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