I have a '90 240 that is suffering symptoms of a failed air mass meter when my engine is cold after it rains. Basically, when I go to start the car in the morning after a good rain, the engine is hard to start. Sometimes it backfires and you have a smell of gas in the engine compartment.

Once the engine starts it idles smoothly and drives fine - at first. When the engine gets up to temp, the car starts having different symptoms. Upon initial acceleration from a stop, the engine will stumble and die. I can restart and it idles smoothly but as soon as I put it into gear and try to accelerate, it stumbles and dies. Again, sometimes it will backfire. When failing like this, I can disconnect the air mass meter and it starts fine, idles smoothly and accelerates with no problem.

After driving for 15 minutes, all the symptoms go away and the car runs and drives great - even with the air mass meter plugged in.

All of this sounds like a failed air mass meter but I have tried 3 different AMMs and they all will behave like this.

Any ideas what this may be? It's awful hard to debug since the failure only occurs when its cold and wet.

Other info that may or may not be related:

- I have been getting poor gas mileage for the past 1.5 years that I have been chasing down. I have replaced both temp coolant sensor and the oxygen sensor plus fixed all exhaust leaks.

- One of the injectors seems oily which makes me think that perhaps it needs new o-rings.

- All tune up items have been performed (wires, plugs, cap & rotor) in the past year.

- There are no codes on ignition or fuel injection

Any ideas on what the problem could be or how to chase it would be greatly appreciated.

Thanks in advance