94 Bonneville either stalls while coasting or idles high

This problem crept up on me over the last week. First the check engine light would just come on while driving. Then I noticed the car was sometimes difficult to stop because it was idling high, sometimes as much as 2500 RPM! Momentarily cutting the ignition with the key will bring the idle back in line. Now the engine has started occasionally stalling when lifting off the accelerator while driving under 30 mph or so. It runs perfectly smooth even when stalling. I can tell when the car is in the mood to stall, because it won't shift into overdrive, weird. I lift off while watching the tach, and the RPM's just dive right below 1000 without even pausing. The engine always starts back up fine.

I haven't been able to get the codes out myself. It's a 94, so I believe it's smack between being able to use the paper clip trick and using a generic scanner. Can anyone confirm this?

Based on what I've read so far, I've narrowed the possible causes to the MAP sensor and EGR valve. I'm leaning toward the MAP sensor. Does anyone have a better sense of what could be going on based on the above symptoms?

I believe that is the early version of OBD II,some 1994 GM models were this way.Another thing to check is the fuel pressure,the fuel pump might be getting weak.

Hmmm. Well, I've never changed the fuel filter after owning the car for 5 years, and I just recently almost ran the tank empty, so maybe there's something to that fuel pressure theory.

General rule with a GM vehicle,will not run under the the specified fuel pressure specs.Get a Delphi pump if the fuel pump is bad.

The 94 Bonneville does not use a Map sensor.. The EGR will be a linear and will cause a very rough unstable idle and possibly cause it to stall..

This car is an OBD1 compliant car, it does use the OBD2 16 Pin Connector and OBD2 codes... But it is fully OBD1 Compliant.. You will need a scan tool capable of reading the OBD1 94-95 GM cars..

With the idling issues that you are having there is a very good chance that the TPS is failing..

Look for a code P0122 or P0123

It can be in the fuel system and fuel pumps can go weak.Had one 4 or 5 years ago do this,it was a bad fuel pump and it had very low fuel pressure.

I agree with Don (Jrs3800), check the spring return on the TPS to be sure it is resting on the throttle pin. If it is loose feeling and does not return home with the spring pressure, replace it!