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Dc46
New User
May 17, 2018, 9:05 AM
Post #1 of 4
(3555 views)
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92 GMC Sierra k2500 5.7ltr longbed 196000 miles. Was sitting for 10 months, replaced battery and altinator(altinator was still original part), passed smog. immediately after passing service engine soon light came on reading code 33. Checked vacuum hoses where do I go from here
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Discretesignals
Ultimate Carjunky
/ Moderator
May 17, 2018, 9:21 AM
Post #2 of 4
(3543 views)
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DTC 33 is MAP sensor circuit
How does the engine run? Does this have an electronically shifted transmission?
Have you inspected the map sensor, wiring and the connector? Look for a disconnected connector, damage, or wiring issues near the connector.
Do you have a volt meter? If so, and everything looks good physically, what is the signal voltage on the light green wire while the engine is running? You'll have to back probe the wire at the connector or use a wire a piercing clip to make contact with the circuit. The circuit has to be operation, so don't unplug the connector when making a measurement.
The signal voltage at idle should be close to 1.5 volts at idle.
The MAP sensor voltage signal is a differential signal. The higher the manifold vacuum the lower the voltage. The lower the manifold vacuum the higher the voltage. The MAP sensor ranges from .5 volts to 4.5 volts.
If the engine has mechanical issues or you have vacuum leaks, that will affect the MAP sensor signal performance. The MAP sensor is used for timing control, load calculation, and electronic transmission pressure control.
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
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Dc46
New User
May 17, 2018, 12:07 PM
Post #3 of 4
(3525 views)
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Yes transmission is electronically shifted and runs great at start up will even drive smooth at 70mph on freeway, after its good and warm it produces an erratic idol and dies out,and black smoke when revved to keep running. Fuel as been drained and replaced, air filter cleaned and all connections and hoses are 👍 what should be checked before replacing sensor
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Discretesignals
Ultimate Carjunky
/ Moderator
May 17, 2018, 12:45 PM
Post #4 of 4
(3519 views)
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An incorrect MAF signal can cause those symptoms.
You should reread the post above. It tells you what to check out before condemning the sensor.
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
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