89 2500 - Severe Cutting Out - MAP Sensor Issues

[Terbro]

I just bought an '89 C2500HD with the 350 motor (276K miles). It was running fine during the test drive. After taking it for the first drive after getting it home, it started cutting our hard...could barely get it home.

At first, I was convinced it was fuel related. I thought maybe fuel pump. First changed the fuel filter, not it. Looking at the TBI, I wasn't then yet convinced the FP was bad. The main symptom was that when giving it gas, it would feel like it either wasn't getting fuel or too much fuel.

Long story short, discovered that the IAC was broken, changed that (all u pull & pay parts). Started disconnecting electrical harnesses at the TPS, the MAP, and the ignition control. None would cause it to run any worse. Then suspected TPS; changed that...still nothing.

Then...pulled the vacuum line on the MAP sensor while running. It instantly idled much better, and revved fine! However, while initially running phenomenal while cold, it does cut out still after it's warmed up. I'm almost positive the issue is related to the MAP sensor. However, I just installed a u pull & pay MAP sensor, reconnected the vacuum line, and it's back to the original problem.

Any suggestions would be great...as I'm now at a loss.

 

[Terbro]

I doubt this is related, because it is running incredibly strong while cold (tons of torque, and proving it can run smoothly through the rpm range), but this motor has a ton of blow-by). The PCV lines smoke like a chimney when disconnected, so the oil rings are shot. I thought maybe all the smoke had clogged up the MAP sensor, but obviously such was not the case.