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Discussion Starter · #21 ·
Have not set ohm reading on signal wire from ecm, and did indeed try the power down on both, no result
 

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Discussion Starter · #22 ·
The timing offset on the 99 is -4.4,degrees the. 96 was 0.1 degrees
 

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Discussion Starter · #23 ·
The fuel economy on the 99 has been progressively deteriorating for a year or so from about 14 plus to 11 plus mpg
 

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Discussion Starter · #24 ·
I understand the boost sensor can play a part in fuel economy also
 

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Have you checked the other sensors that use the grey 5v wire and ground ? Dont have wiring diagram in front of me.
But IIRC the CPS and 1 other sensor use that power and ground circuit also. Should show them in the shop manual.
ALSO
With the CTS reading full cold. That will kill your fuel mileage. It may be bad also. There should be a table that gives resistance/temp values for the CTS in the manual.
 

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CPS and MAP are on the 5 volt circuit.
The ECT reading indicates an open circuit, bad ground or bad sensor.
Short the 2 ECT wires and see if you get a high temp reading.
ECT, IAT, MAP, and CPS grounds are spliced in the harness under the intake.
Are the engine grounds at the rear of the pass. side head in good shape?
 
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Discussion Starter · #27 ·
All intake grounds were removed and renewed
 

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Discussion Starter · #28 ·
The grey sensing wire is dedicated to the ecm runs thru no other devices
 

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The grey wire at the CPS and MAP sensor is spliced under the intake and runs to the PCM thru the 8pin connector at the rear of the engine. It is a 5v. feed from the PCM for the CPS and MAP sensors on your 1999.
 

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Discussion Starter · #30 ·
According to the wiring schematic gm sent me the voltage I detected was at 4.97volts at feed wire, green, the device (map sensor) shows a direct short with resistances across all poles as identical at 1.57 ohms so it spiked the ecm, not certain why it permitted that as the ecm typically has some level of protection, there is a few service info pieces that indicate that I can indeed fault the ecm, and requires replacement
 

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Doubt the sensor damaged the PCM. Normal sensor function is from a very low resistance to a full short.
A short in the wiring harness can cause problems and damage the PCM.
If you think the PCM is bad you could try the PCM from your 96 or get your hands on a PCM from a 97.
96 PCM in a 99 will work except for the WTS light.
97 PCM in a 99 will work perfectly and does not have security. An easy and quick way to narrow down the problem.
98-up PCM will require PASSLOCK reprogramming which makes diagnosis a bigger PITA.
 

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Discussion Starter · #32 ·
I tried the map sensor in the 96, and no start, then returned the good known 96 map sensor back and now no start on 96 either!
 

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Did you recharge the batteries? They will go down fast when you are trying to troubleshoot.
They have to pass load test also not voltage test.
 

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Discussion Starter · #34 ·
Both battery sets are fully charged, the common denominator is both trucks show service engine indicators, both have under 125k miles, both were exceptional runners, would start in 15 degree weather without block heat, until this
 

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Is that means, it has DTC? aka service engine indicator?
You need to see if you can read the code, hopefully, it gives us direction.

Another thing to try is a putting a KNOWN WORKING PMD.
I hope you have a spare PMD, with 2 trucks, hopefully, one of the PMD is still good.
 

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Hate to say it but. Looks to me the shorted Boost sensor took out the PCM on both trucks. Yes. A shorted boost sensor can fry the PCM.
 
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ECMs are about a buckthirtyfive, replace the badboy!
Where do you buy ECM for $1.35? Inquiry minds want to know.
It is not just the ECM, is the data inside the ECM that is important.
 
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