Jonroe60

I have a 95 Chevy 1500 4wd 350 auto. 170000 miles, I've had for two years. After easily starting it misses badly for about 5 minutes, then smoothes out and runs fairly well. It seems to be getting worse as time goes by. It is only getting about 4 or 5 mpg, and doesn't have the power a 350 should. I changed the plugs, no help. I dont know how long they were in, but they were somewhat fouled. It doesn't use or burn oil that I can see. I had a mechanic check it for codes, and nothing showed, he also changed the cap, rotor and wires, and nothing has helped so far. What should I check next.

racerx55

a typical problem causing them to have a very rich cold start and horrible mileage is the coolant temp sensor for the ECU - NOT the gauge temp sender.
They always go bad, but not far enough out of whack to throw a code. it tells the ECU its -40* outside which throws a ton of fuel to the engine.
Best way to check is to throw a scanner back on it and read the datastream and see what the coolant temp is showing.
Another possibility could be a weak coil or ign module failing, check the spark and see how far of a gap it will jump, it should be blue / white. if it is orange and wont jump more than about 1/2" to 1" there is something weak in the ingition system. You can do with with a normal test light by grounding the clip and having someone crank it over while holding the tip of the test light about 1/2" from the coil post, it will jump an arc from the post to the tip of the test light. It doesnt have enough current to even light the bulb in the test light, but if your hands get too close you take the chance of getting zapped so you have to be cautious.