This ones gonna stump ya!
I recently purchased a beautiful 04 gtp for a really good price but the check engine light was on and I asked the gentleman selling it and he stated it had code U0155 ( lost comm with instrument cluster) so I had looked it up and it was a pretty common problem for the internal chip on the board in the cluster to go bad so I thought it was no big deal. He hasn’t replaced the cluster so I’d start there. It drove amazing the entire ride home and had plenty of power, my first suspension was when the stock boost gauge wouldn’t work. It always stayed at 0 boost the whole ride home even when in boost. I got it home and checked the codes and it had po108 and p2229. over the next few days cleared the engine codes and that’s when it all started happening. It stumbles and hesitates when accelerating normally. It also tries to die when slowing down and has stalled and wouldn’t start back up until I unplugged the map. With the map sensor unplugged it will run fine. I haven’t drove it much with it unplugged though I wouldn’t want to cause detention. I thought it could have been the sensor so I purchased two new map sensors and replaced the Baro and map sensors with no luck, p2229 comes right back within 10 seconds of starting the car. I have checked the 5v reference and the low reference with the key on engine off on both the map and Baro. They checked fine. I took the car to my brother-in-law’s because he works at the gm proving grounds so he knows his way around vehicles and his gm diagnostics from his computer had us measure the voltage drop and the resistance of the wires and they all checked out. After testing multiple things with the wiring it lead us to replacing the ECM. I picked one up from the junkyard from a 04 gtp comp g and did the security relearn, apron firing up and going for a drive it still messes up. I took my multimeter out and connected it to the signal wire on both the map and Baro one at a time with key on engine off and I had 4.76-4.78 volts on both sensors, I then proceeded to fire it up and Baro stayed within the 4.7s. The map went down to 1.5? It was down around one and a half volts. Then as I gradually pressed the pedal (with it hooked to the signal wire on the map) the voltage started to rise (like normal) but then as soon as it started to rise it would fall down to like .52 volts. On my scanner I can read the map in inches of mercury and it starts off fine at around 21” but as I press the pedal (gradually) it will slowly rise to 58.6-60”. Ive never really measured it at wot but I feel like it’s doing the opposite of what it should somehow. Ive torn apart most of the wire harness from the Baro back to the pcm looking for a short but I can’t find anything. The Baro sensor voltage remains the same regardless of throttle position. Thank you for reading this I’d be happy to answer anything you need.
