I just replaced the intake manifold gaskets on my 97 Trans Am. Double checked all the vacuum lines and all connections. When I went to start it for the first time it idled at 2000 rpm then slowly lowered to just under 1000 rpm and died. I rechecked all connections and they all looked good. I unplugged the MAF sensor to see what would happen and it again idled up to 2000 rpm, but then lowered to just over 1000 rpm and held there and kept running. Could this be a symptom of a bad MAF sensor? The thing I don't understand though is if it is, why would it initially go to 2000 rpm and then slowly drop back down even with the sensor unplugged? That so far is the only thing I seem to be able to do to keep it running. Any insight would be appreciated.

Matt

 

On possible explanation....

It may be getting air from somewhere other than the air passing through the MAF. That would explain the initial elevated 2,000RPM idle, followed by the IAC shutting down to try and control the idle speed, but only getting to 1,000RPM.

When you use the MAF, and there is enough air leaking in AFTER the MAF (could be a hole in the inlet elbow, end of the elbow folded under, or a vacuum leak somewhere on the TB or intake manifol), the PCM bases the fuel quantity on the MAF reading, which may be close to zero, because of the leak. Not enough fuel = stall.

When you unplug the MAF, everything is the same, but the PCM defaults to speed density and calculates the mass air flow from RPM, VE tables, IAT and MAP, which takes vacuum leaks out of the equation. In speed density, it would be seeing actual air flow and supplying enough fuel to keep it idleing at 1,000rpm.

One possibility....

 

I haven't had time to trouble shoot any today, but that seems like a more probable explanation to me. I couldn't see how the MAF would suddenly be bad after 1 day. I know the intake elbow and all the connections prior to the throttle body are tight, so I can rule that out. I am going to have to look further upstream I guess. Would a leak at the actual intake manifold gasket at the head cause something like this? I am hoping it is anything but that as I don't want to tear it apart again...course I will if I have to. I have a few hoses I am going to try and replace one of which connects to the throttle body, in hopes of it being something there. Thanks for the help so far.

Matt