Just swapped engines/trannies in my truck. Now that I have things running, it is acting funny. It starts fine, and runs fine for approx. 20 seconds, then the idle will get rough. RPMs stay about the same, but it starts to shake noticeably. Not bad enough to kill it, but it's not pleasant. If I drive it, it will drive great for about a city block's length, then it will loose alot of power. I will have to give it about 3/4 throttle to drive around the block. It is shifting hard aswell. If I kill the engine, or stop and idle for a minute, it acts normal again, but only for that city block's length. Engine temp doesn't seem to make any difference. Only code being thrown is Random Misfire Detected...

Things new: Distributor, cap, rotor, wires, plugs, and basically every gasket short of HGs.

Things used from my old engine: All accessories except PS, crank pulley/balancer(old engine's was broken), crank sensor(old engine's was chewed up, causing no spark), fuel spider(just used this because it was newer), ignition coil(switched it over when troubleshooting the no-spark condition, since I knew mine was good).

It's a 96 4.3 Vortec.

 

No one?

 

Still looking for help, I'm out of ideas..

 

Very possible the fuel maybe giving you a problem. Check the fuel filter, crimped fuel line,
fuel pressure and the fuel regulator.

 

Possibly.. but I doubt it. I had an issue the first time I drove it with unburnt fuel getting into the cat, causing it to overheat and turn red. Actually ruined it, so I removed it yesterday.

So the misfiring cylinders are definitely getting fuel, but perhaps not enough to fire? Fuel filter is pretty new and I used the fuel spider out of the old engine, since it was newer and had been recently professionally cleaned. But a crimped line is definitely a possibility, I will look for that tomorrow. Thanks.

 

Noticed today that it runs significantly better with the front o2 sensors unplugged. I have replaced them before, so they are not bad.