98 Silverado losing power

I have a 98 Silverado, 2wd, 5 liter V-8, 180,000 miles. Occationally, my truck will lose power. Driving down the road I lose speed and cannot accelerate. When I depress the accelerator the motor does nothing. This happens at any speed from take off to highway speeds. When it begins to happen I lose speed slowly for maybe 30 seconds then the service engine light comes on and the problem gets much worse. Sometimes it will sound like it’s back firing but the sound is muffled. This seems to happen more often in damp weather but it can happen in dry weather too. And it doesn’t always happen in damp weather. It happens in the morning or afternoon. Auto Zone put their tester on it and a code comes back for O2 sensor but the guy didn’t think that would cause the trouble I’m having. Any ideas?

check the fuel pump?or the ignition parts?-Kevin

If the ignition parts and fuel pressure check out okay (might not hurt to check the CFI poppets for leaks at the same time), I would suspect the camshaft position sensor, which is located in the distributor. A failing camshaft position sensor will often give these symptoms and be very intermittent in nature, and difficult to diagnose since the engine computer will rarely throw a code that will lead you in that direction.

Since it happens when the weather is damp, that’s a pretty good sign of some kind of spark problem. And a faulty spark could cause an O2 sensor error being coughed up by the ECM diagnostics, with the O2 sensor still working. I concur w/AZ to not replace the O2 sensor at this point. If there’s no obvious problem w/the ignition system, consider the possibility of a plugged cat too. Mechanics can do a simple vacuum test to rule it out.

Might have a bad coil wire