96 Blazer 4x4 rough idle and backfire
 

Was driving down the highway last weekend and heard a single backfire. After that, it started rough idling and the service engine soon light began flashing about once per second. It backfires quite a bit too.

Yesterday I replaced the rotor and plugs. Distributor cap looked okay, but every plug that came out was nasty. That didn't cure my problem, but it gets weirder. I pulled the positive battery cable to allow the computer to reset. Once I hooked it back up and started the engine, it ran beautifully and I thought I was in business. About 30 seconds into it, the problem started again. It idles pretty rough in park, but when you put it in gear it really gets bad. Darn near dies. It starts like a dream, so I don't suspect timing. And why would it run beautifully for 30 seconds, and then mess up again?

I don't know where to go next. I hate throwing more money away on parts that don't need replacing.

Get the codes read. I am betting that one of them will be P0300, but there may be other codes that point you in the right direction. Sounds like there is a sensor problem, but guessing at it is sure to waste your time and money.

I knocked another thing off of the list today. Pulled the ignition control module and it checked fine at Autozone. Tomorrow, I'm gonna check out the MAF sensor and maybe pull the crank sensor. Haven't found a code reader yet to borrow, and can't afford the 200 smackers for a new one.

Bought a can of MAF sensor cleaner and that didn't do it either. Crank sensor is about a $70 part, so I don't think I'll throw any more money away until I know for sure what's going on. This is getting old real quick. Have had several people tell me that it probably jumped time, but I'm not sure.

I had a friend who came to the rescue today. A brother firefighter had a code reader and loaned it to me. It only showed one code: P0302 which is supposedly cylinder 2 misfire. I guess that helps narrow it down considerably. I'm gonna do a forums search for that code, but I'm assuming it's either gotta be plug, plug wire, or cap. I already changed all the plugs and the rotor.

Check this out:
P030x - Random/Multiple Cylinder Misfire Detected

Funny you mention that thread. I just read that whole thing right after I made my post. Went outside and pulled the #2 plug wire. It has a spot on it where its black and appears to have gotten hot. On the ohm meter in the 20k setting, it reads 2.83. I think that sounds way off. Too low I believe.

Welp, it just gets more and more aggravating. I put a new set of plug wires on, erased the codes using the reader, started it up, and same thing again. P0302. 70 bucks into this thing, and still haven't changed a thing.

You said that you replaced the wires, but did you replace the plugs as well? Or perhaps swap out the suspected bad plug and see if the code follows the plug to it's new cyl or not.
If it does then problem solved and you need new plugs. If it does not then you will need to move on to testing some other components and checking for a possible fuel leak in the intake manifold.

You don't need to just swap out parts, just about everything that can cause this problem can be easily tested first.

I put in new plugs and a new rotor as my first step, because I thought the cap looked good. I just got through pulling the brand new #2 plug to see if it had fuel on it. I started to think maybe a bad injector. It was slightly damp and had fuel on it, so I think that's a good sign. I went ahead and picked up a distributor cap. After this, I've hit the $100 mark. Getting ready to put it in now and see if that cures it. If not, I'm stuck. I mean, what else could cause only the #2 cylinder to misfire? If it was something else, wouldn't I be showing a lot more codes? This is the only code I've got.

It sounds like you may have a fuel leak in your intake that is leaking down into the #2 cyl.

It is probably coming from either the nut kit on the fuel line inside the intake or the fuel pressure regulator attached to the spider unit. If you take a flashlight and look inside your intake you will probably see some clean spots near the #2 intake port.

If you do then there is a leak and you will need to remove the upper intake manifold, find the exact source of the leak and get it fixed. If you open her up and see no puddling of fuel or clean spots then it is going to be the poppet valve on your #2 injector that is stuck open.

There is a great deal of information including pictures on both issues here as it is a very very common issue with these engines.