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1999 Chevy k2500 7.4, codes p0107, p1107. Engine chugs under acceleration, won't rev up. At idle engine revs up just fine. Unplug map sensor engine drives fine. Changed map sensor, still acts up. Now here is interesting part. Look at voltage at idle on scan tool, right around 1 volt. Reverse, 1.2 volts. neutral, 1volt. Drive, .01 volt. In reverse truck drives fine. Put it in drive and engine falls on its face and you lose map voltage at pcm. I'm pulling my hair out on this one. Maybe another sensor sharing 5v ref or ground causing this?
Measure the 5 volt reference at the map sensor when the map signal voltage drops....
Maybe a short to ground on the signal wire just when the engine is put into D ??
What is the map voltage with koeo? If you got a vacuum pump then pull the map sensor, connect it to the pump and apply vacuum while koeo and look on your scanner if the voltage follows the vacuum.
Also make sure your not getting substituted values from thescanner,do all these readings in generic OBD2, NOT in GM propietary protocol....
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Have you done a wiggle test on the wiring harness?
The MAP Sensor shares 5V ref (gray wire) with the EGR valve, the Fuel Pump Balance Module, and the Fuel Tank Pressure sensor (in tank).
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This is just my initial thoughts, in the pcm there is a resistor that changes 12v to 5v for a ref.i believe there is only 1 resistor that supply all the 5v refs. ( I could be wrong on that) since the issues change with the gear change I would look at the trans range switch. What if it shorts the 5v ref. If it is on on 1 resistor that also supplies the map sensor? Or is a wire to the range switch crossed with the map wires in some way? I might be thinking to hard.just a thought.
This is my ideas on it, to me the difference between forward and reverse is in direction the engine block moves in due to torque from the transmission.
I would guess in forward gear the wiring harness between the engine and body could be getting pulled resulting in an open in the 5v supply.
I would put a digital voltmeter or better a scope on the 5v terminal at the map and push-pull wiggle looking for drop outs and glitches.
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I'm also thinking about a wiring issue caused by the movement of the engine. Knowing if you're losing the 5V reference or the signal to the sensor when it goes to zero would be key to knowing which wire to trace.
For example, if you found that the signal wire was going open, then you could temporarily connect a jumper between the sensor and the correct PCM pin. If the problem goes away, then you're down to finding the open in the wire.
Well I finally figured it out after weeks! The heat wrap around the EGR tube unraveled and either melted or rubed on main harness causing signal loss to ground out when engine torqued in drive. Resecured it away from tube for now and I will have to go replacing wires soon. Thank you all for the great ideas and fast response!
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