spark timing and map sensor signal)1

01 pt cruiser 2.4 non turbo no eng. trouble codes symptoms are engine knocking and chattering during acceleration only. Engine starts fine. Scantool data shows spark spiking rapidly 0-5 0-7 degrees at idle. Map sensor signal is 0.20 volt to 0.25 volt when checked with scan tool . Map signal wire shows a normal 1.5 volts at idle when tested with digital volt meter near map sensor and near engine computer. I tested a normal running pt cruiser and the scan tool and volt meter readings of map signal both show 1.5 volts at idle. I also watched the spark advance data while engine noise is present 2000 - 2500 rpm and spark is at 0 degrees. My question is does this seem like a failing engine computer ? I would appreciate any ideas or suggestions thank you

Hey red67! I love the testing you've done so far. Not looking good for the PCM, IMO. Does the MAP signal indicated on the scanner ever change during manifold pressure changes? Do any other 5V sensors misbehave?

I think my only suggestions would be to check the PCM connector for corrosion/green crusties, and possibly do a battery disconnect to rule out computer logic lock.

Thank you for your response Tyler. The map signal does change with manifold pressure changes when observed with the digital volt meter . When you watch it with the scan tool it switches back and forth between 0.20 and 0.25 volts . The connections are nice and clean . I was suspicious of my scan tool until 3 other nice scan tools were used showing the same results. I wonder if a bad ground somewhere could cause this ? thanks again

It's possible? But I'd really doubt it. If you wanted to check PCM powers and grounds before making the call, I wouldn't blame you one bit! I can provide a diagram if needed.

I'd be more suspicious of a resistive connection inside the PCM, permanently causing the reading to be low, despite the sensor working perfectly.

I bet you you're right about what's going on inside the computer. I have the factory service manual for the car so I may investigate the power and ground situation but I think I need another computer too.

Let us know how it goes.

This reminds me of another PT Cruiser with PCM weirdness. Check it out:

www.scannerdanner.com/forum/post-your-re...-width-60-80ms.html#

EDIT: Just thought of another test on the MAP circuit. This sensor should show 5V on the signal wire at the MAP sensor with the sensor disconnected, and the data PID should reflect that. Seeing 5V on the DMM at the MAP sensor, but still showing .2 - .25V on the scanner would be another nail in the PCM coffin.

thanks for the map sensor unplugging suggestion. I have unplugged it and it shows 5 volts on the digital volt meter but I didn't think about checking the scan tool reading with it unplugged. I'm definitely going to check that. If it ends up needing a computer do you know a good place to order one ? thanks again