I have two monte carlos.(2004) The engine will idle for a little bit and then shoot up in rpms. If the mass air flow sensor is plugged in, the car wont start. I have swapped mass air flows from my other monte carlo but the same thing. Also swapped throttle bodys, and cleaned them. I dont know what to do.

 

Which engine? It could be a mix of two or more sensors on the TB. I had a similar issue where it was the MAF and IAC valve. The "almost dying then reving high" was caused by both of those being bad.

 

Both 3.4

 

First thing I would do is clean the IAC valve and MAF. Other than that, youre going to have to do some diagnostics. Check live data and compare to factory ideal readings. Check all the sensors with a multimeter to make sure everything is functioning correctly. All specs and ranges should be easily found on google. It can be a process but its worth it to learn the stuff.

 

IMO it sounds like a post TB vacuum leak to me. With the MAF plugged in, the thing won't run because it says X air is coming in when its really getting X + the leak (which is likely substantial - too far out of adjustment range to compensate). With the MAF unplugged it defaults to the MAP sensor which likely has a more accurate reading with the leak occurring - but that added air past the IAC is driving the rpm way up as air is getting into the engine that the pcm isn't commanding.

 

I would definitely check that capped off nipple in the (back?) of the intake manifold. That cap likes to dry rot.