Question about 1996 Chevrolet Tahoe

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Tahoe won't start unless warm out

I have a 1996 Tahoe that will only start and run smooth if the outside temperature is over 75f. Whaen cool out, it cranks uneven and no fire-up I have taken it to 3 places including the Dealership and they all could not fix it. Anyone with the correct solution will be rewarded.
Thanks, Colin

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The Manifold Absolute Pressure (MAP) sensor signal is electrically used in a similar way to the use of Mass Air Flow (MAF) sensor signal (although internally it is built differently). 
It takes a 5 volt signal from the computer, and returns a lower direct current signal in accordance with the vacuum in the engine. A higher output voltage means lower engine vacuum (which means more air flow is occurring), which is then calculated as “more fuel is needed”. Lower output signal indicates higher engine vacuum (which means lower air flow), which requires less fuel. 

It's not just fuel control though. The MAP sensor signal gives the computer a dynamic indication of engine load. The computer then uses this data to control not only fuel injection, but also gear shift and cylinder ignition timing. In some cases it is even used to calculate changes in barometric pressure, to automatically adjust for different altitudes.

The MAP or (Manifold Absolute Pressure Sensor) is a little though expensive device installed in your air intake manifold. It has 5 or 12 Volts coming in, and it simply senses the vacuum in the manifold and attenuates (reduces, weakens) this incoming voltage by a certain factor. In other words it reduces the voltage in the range of 15% to 60% of the supply voltage (depending on the car's design these numbers will vary), and this varying (but non-pulsing) signal goes to the computer.

Too much attenuation kills the engine, it will simply shut off or not start. So if the MAP is bad it can lean down the mixture from the stoichiometric (a big word that simply means “balance of ingredients”) which is factory set at 14.7:1 (14.7 parts of air to 1 part gasoline) – down to 20:1, maybe even 50:1 or 100:1. 

Next time it rains,discontect the MAP sensor wire harness and see if it's starts. If the engine starts, with the wire harness disconnected. Replace the MAP sensor. Good luck and hope this helps, keep me posted. Be glad to answer any questions you may have, and let me know if this works. If not then we can try something else. Be safe friend. 

Posted on Jul 06, 2009

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The cold start valve needs to be replaced.
joelmitchell1@hotmail.com
512 576-0856 cell
512 704-6925 shop

Posted on Jul 06, 2009

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