My water pump in my 96 dodge neon went bad, so we replaced the water pump, and timing belt and the tensioner. When we got it all put back together we started the car, and it started right up! It ran for about 20 minutes, then started making a knocking sound in the drivers side of the engine. The car shut itself off, and will not restart. The pistons are moving up and down, the valves are opening and closing, we have fuel and spark, but the car will not start. It turns over quickly but will not catch. When we crank it it seems to be blowing compression out of the trottle body. WE are wondering if a blown head gasket would cause this? The oil is not milky it is stll black. If not what are the signs of a blown head gasket? If it is not a blown head gasket, does anyone have a suggestion of what could cause this? Please help!

 

It sounds like your timing belt may have slipped and the engine could be out of time, this would explain having air being blown out of the throttle body, as valves would be opening and closing, but at the wrong time. Turn the crank back to TDC and verify that your cam, or cams if you have a DOHC are also at TDC. I had this happen to me on a DOHC V6 engine that I did a head gasket job on once, one of the heads jumped time, but the car still ran, but rough with no power. I verified the problem this way as well.