I have a 97 Blazer 4.3 vin W, few weeks ago it started running really rough. I did a tune up with no change. Its back firing, and firing on the compression stroke and stops the engine during cranking or can kill it if its running. So I put my timing light on plug 1, even though its non adjustable timing you can still see the timing compared to TDC, figuring at idle it should be 10 degrees BTDC and estimating that to be 1 inch before the timing mark my engine is firing 2 inches before the mark and would disappear under the water pump, now today its firing with the timing mark straight up, so like 4 inches before the TDC mark. I had the computer hooked up to a diagnostic scanner and the tech said the computer was functioning correctly and that cat was plugged, so I have removed the cat with no change in engine. Any suggestions on how this can be corrected?

 

What does the computer have to say about the timing advance? At idle, it can actually vary between 15* advanced all the way upto 30* advanced.

 

The computer may be overcompensating for a weak spark. When you did a tune up did you change the ignition coil? If not ohm it out to see if it's going bad. This happened in my truck at the beginning of winter and with trying to start it with ether it would try to run backwards.

 

I think I got it figured out. I did a full tune up expecting it to be the plug wire. I bought this a year ago for $450 cause it didn't run, the rotor came loose and it didn't spin on the shaft. After changing the cap and rotor I noted that the distributor probably needed changing. I had pulled the cap off when I changed the wires to check the cap and rotor and ended up losing both screws. The replacement screws I got was too big and ended up cracking the dis housing. So I got a New Dorman Distributor since it has an aluminum housing. I took off the exhaust to check for a plugged cat and muffler. Got it to start today and it idled better. Reved it up a few times and it sounded good. When I took the cap off I checked for play in the shaft, it moved quite a bit between the teeth, so I think the gears are worn out as well. I'll have to change the starter because the overrun clutch is slipping now so I'll post how it runs in a day or two once I get this figured out. And yes I changed the coil when I changed everything else. She now has a completely new ignition system. Here's what I changed; Plugs, wires, Ignition control module, coil pack, crank position sensor, distributor housing ( cam with shaft gear rotor cap and cam sensor0. Compression is good, tad low but ranges from 130-140. Cylinder 3 had a loose plug, figured last owner didn't tighten it due to the steering shaft in the way of a ratchet. All the replacement parts he put on are from a salvage yard.

 

Seems the Distributor was the culprit. The screw that I had test fit to hold the cap down was too big and broke the housing and allowed the cap to shift ever so slightly. And trying to hold it down just wasn't working with the engine vibrating from its rough idle due to fouled plugs. Took it out for a drive and it ran great, got a dorman distributor, came with everything, Stabbed it right the first time, ran good, and starts easy.