Hello all My first post...an electrical challenge, I hope someone can help figure this one out.
3400 Engine. This problem is intermittent. Sometimes (once every dozen or so starts) the engine turns over but doesn't start. When this happens, I have found out the following so far;- When I turn the ignition key to the 'run ' position (where the Powertrain Control Module is normally supposed to provide voltage to the fuel pump relay to cause the pump to run for about 2 seconds), the relay chatters very loudly and DOES NOT shut off. The pump keeps running even without turning the ignition swith all the way to start the engine. I did replace the relay with a new one and NO CHANGE in symptoms. I still have no idea why the relay would be getting this jittery signal voltage. It must be a 'jittery' supply from the computer because as the relay is chattering away, the fuel pump is chattering due to the fluctuating voltage as the relay goes off/on/off/on rapidly. I tried supplying a nice smooth 12 volts straight from the battery to the grey wire if the relay and voilla, the fuel pump immediately runs smooth. BUT, when I then turn the ignition key to try to start the engine, it doesn't start. Becuse the engine is actually starting for the time being, I cannot test fuel pressure, spark under no-start condition. I just read another post that mentions the oil pressure switch on some GM's supplies power to the fuel pump (relay?). Not sure what THAT'S all about, but maybe something wrong with this switch causing interfernce voltage? The other head-scratcher is, why wouldn't the engine start at these times? The pump is running smooth. Thing is, at the moment the engine is starting/ running fine. Actually, don't know if it's rlated, but when I hooked up a fuel pressure gage to check pressure in the rail, the problem ..magically went away again...coincidence? Maybe the computer is having problems and sends weird voltage to the fuel relay and NO VOLTAGE to actually run injectors/spark etc??
ANy ideas are greatly appreciated? PS; sorry for the long-windedness Ted