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Old 04-16-2013, 08:14 PM
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400 tbi

I have taken the top end of a 350 tbi and bolted it onto a 400 sbc. It now runs but it won't run right yet. It only runs somewhat ok when the map sensor is unplugged. When the map sensor is plugged in it starts fine, runs for a bit then gradually stalls out. Even if I hold the throttle open a bit, the rpms eventually fall and it stalls. I have to constantly rev it up to keep it going. Map sensor is good, 20 in of vacuum at idle, coolant sensor is good, timing is advanced 4 degrees with est bypass unplugged, throttle position sensor is good, set at .58 volts at idle, got spark, 13 psi fuel pressure, good spray pattern, iac motor moves in and out. I've been through the wiring countless times. I don't know what to be looking for anymore. Any help would be greatly appreciated.

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Old 04-17-2013, 08:13 PM
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I am not an expert by any means but when I was trying to decide if I would go TBI or carburetor on my 400 I was told that TBI is very sensitive to vacuum. Because my 400 has a modified cam that I anticipate will have low vacuum I decided to go carburetor. It could be the intkae vacuum on the 400 is low relative to the 350 and by disconnecting the MAP sensor you are tricking the system for a time but eventually the lack of vacuum reduces fuel delivery to the point it stalls. Not sure what the fix would be but perhaps this bump will get you a better answer.
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Old 04-17-2013, 08:21 PM
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it stalls out when it's plugged in and runs fine when it's unplugged. The block still has a stock cam in it and the vacuum gauge reads just over 20 at idle. all the sensors readings are within the normal specs. but at this point any feedback I can get will be a big help
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Old 04-27-2013, 08:48 AM
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What injectors are you running? The Cop Car Caprice injectors flow more than the stock 350 TBI injectors.

Sounds like you simply arent fueling correctly as you ahve added 50 cubes to an ECm that thinks its fueling a 350.

TBI is a Speed/density system and is extermley sensitive to changes. It uses the TPS(throttle position sensor) and MAP(Manifold Absolute Pressure) sensor to calculate the a/f charge.
Disconnecting the MAP sensor may jujst throw the system into a sort of limp mode and supply a rich mixture, which may be why it runs better in taht state.

Try raising teh fuel pressure and see if it helps. Stock is 11-13 psi on the 88-93 systems, the 94-96 systems were 32 psi IIRC.

peace
Hog
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Old 04-27-2013, 05:36 PM
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What you need to do is ajust the minimum air rate.
You have more cubes as pointed out by Hogg. It needs more Idle air to idle correctly, consequently the mixture ratio will be off as well...necessitating the Min air rate adjustment.
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Old 04-29-2013, 05:02 AM
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How do I go about resetting the min air when the engine won't stay running?
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Old 04-29-2013, 04:43 PM
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Warm it up, shut it off. Jump aldl with key on engine off. Check to be sure IAC is seated.Then unplug it.
Crank Throttle stop screw in until it will run on its own after restarting it. Get it to idle at about 475-500 , then set the tps to .58
Shut it off , plug the IAC back in and check your work.
Perhaps you may also have an intake leak ? Have you checked?
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