Originally Posted by TheBlaster9001

Manifold air temp (MAT), also known as intake air temp(IAT), is a primary input sensor in MAP-style engines like yours. Without getting too in-depth, it is responsible for helping the computer determine how much air is actually coming in to the engine - the Manifold air pressure sensor can tell the computer how much pressure the air is at, and with the input from the IAT, it can know the temperature of the incoming air. Using the combined gas law, Pressure * Volume = k (a fixed amount of air, for example, the volume of your intake) * Temperature. So the computer knows the volume of air incoming based on the MAP and IAT sensors. I had mentioned above that I would suspect that the IAT/MAT is bad on your car, as without it, the computer can only guess about the amount of fuel to mix with the air.

Pressing on the brakes killing it would also make me wonder about a vacuum leak in the booster. Do you have vacuum boosted brakes? Any chance they are bad? You can try disconnecting the brake booster line, plugging it, and seeing if the car will die.

Edit: to make my answer to your question more clearly, yes, I do believe that a bad MAT could cause stalling/shutoff. But if the actual use of the brakes is what kills it, it may be a vacuum leak. But should it die regardless of if the brakes are pressed, then the MAT may be at fault.

Originally Posted by Bangkok Dean

Your comment on a vacuum leak at the brakes booster is interesting. But the stalling is not all the time and only after going from highway speeds and then going to stop and go traffic but only sometimes. To apply the brakes you lift off the gas pedal, I do notice when slowing down the engine sometimes drops rpms and almost stalls.

I will change the MAT sensor and see if that helps. This problem goes back 3 years off and on.

Thanks for the help.

Originally Posted by ihatebarkingdogs

I see what looks like an aftermarket harness on your swap. Cool.

Here's an off the wall observation based on studying the JTR "How to swap GM EFI engines". They spend some time discussing the Park Neutral input and providing an operational VSS input. The ECM uses these two inputs to know whether the vehicle is moving and how to position the IAC in anticipation of coming to a stop, and to prevent a stall.

When the trans IS in P or N, the IAC is controlled to a pre-programed RPM value. If the RPM increases, the IAC is commanded for less counts. Typical IAC counts are 15-20.

When the trans is NOT in P or N, and the vehicle is moving, the ECM positions the IAC at a pre-determined number of counts which is higher than the counts at idle. The idea is if the throttle is suddenly closed, and the car comes to a stop, the ECM may not be able to command the IAC to open quickly enough to prevent a stall; thus the IAC is already open more than is required to idle.

JTR recommends that a functional VSS be provided. Many aftermarket harnesses for the TPI permanently ground the P/N input wire to prevent a Code 24 (because no VSS is present). With the P/N input permanently grounded, the ECM does not know the vehicle is moving, so it will command the IAC to ZERO Counts (attempting to get the "idle" down), and may not be able to command the IAC to open quickly enough to prevent the stall.

Check your wiring diagrams for the pin-out for the P/N input to your ECM. Then check your harness to see if this pin is permanently grounded. Looking at DataMaster will also give you the P/N switch status, as well as IAC counts. If the IAC counts are zero while driving, watch them when coming to a stop, and particularly if the engine stalls, what the IAC counts do. If you can get DataMaster working, you don't need a scanner. The default screen has all the data you'll need. DM can be fussy to get it to link to the ECM however. Your test connector only has 3 wires, but those are the only ones you need for DM.

There's another idea.

Originally Posted by Bangkok Dean

Thanks for this info.
As I mentioned I have had this problem for over 3 years now and have run the DataMaster but I don't know how to read the data provided.

I plan to change the MAT sensor to see if that helps, Then do a drive with the DataMaster program to get a read out. Now to find someone that can read the data is another challenge.

Originally Posted by Bangkok Dean

Mostly applying the brakes, not hard stop but slowing or just lightly tapping the brakes. It dies but putting it in neutral and rolling turn the key off and it starts right up.

Originally Posted by car addict

Any chance lockup torque converter not unlocking? Had similar symptoms with an old Pontiac I had.

Originally Posted by TheBlaster9001

Thats not a bad idea, it could definently stall it out. A tranny fluid change might help, unless that's been done already.

Originally Posted by TheBlaster9001

Thats not a bad idea, it could definently stall it out. A tranny fluid change might help, unless that's been done already.

Originally Posted by aklim

What would cause the trans fluid change to help? If the fluid was that FUBAR, I wouldn't know but I would suspect that the trans has been so neglected that it is probably on the way out. Kinda why I think flushes have gotten a bad rep. Guy neglected it for eons after it was due. Suddenly, he has a hiccup and flushes it. Too little too late. A short while later, the trans dies. Sure, trans flush screwed it up.

Originally Posted by ihatebarkingdogs

And it's Right Hand Drive. So cool.


What are the two groups of four red-things in the valley with the electrical connectors on them?