My 1999 4.3L has a couple of issues. Background... New GM crate motor about 3 years ago but only has 20K miles on it. New AC Delco Fuel pump and new fuel filter about 1 year old.

For a long time I could hear the cat rattling and new it needed to be replaced but it's my #4 vehicle so it wasn't a priority. The rattle went away and about a week later I noticed the engine running really rough at idle and missing on acceleration. I figured the cat had finally let go of a piece and lodged a broken piece and restricted the flow causing the rough idle.

Here is the upshot:
No codes other than P0300. The old cat was broken up pretty bad and a good shaking dumped a lot of crap on the floor. We checked for any pieces of the cat that could have made way into the muffler but blowing high pressure air from back of the tailpipe out through front inlet of muffler did not produce much out of the muffler. Replaced the cat with direct fit and new rear O2 sensor and still run rough.

New cat in place and new rear 02 sensor and driving shows a missfire at cylinder #2. Pulled the #2 plug and it's fouled. Next step is to replace all 6 plugs and start from there.

?'s Could the bad cat cause this missfire and fouled #2 plug? It looks to be more fuel related so maybe it's a separate issue. Does this point more towards the spider injector assy? Fuel regulator issue?

I'm afraid after putting in 6 new plugs I'm going to have the same issue. Any advice appreciated.

Thanks!

 

Problem solved: I thought I would post back with regards to the #2 plug and how it was running so rough. So the cat did need to be replaced but that did not contribute to the rough idle. So I put a new #2 plug in and had the same issue. After further inspection there was a small crack in the spark plug wire right near the boot at the distributor side!

I could actually hear a slight "ticking" sound where it was arcing. So, I bought a new set of plug wires and replaced the #2 wire and it now runs smooth as a baby's bottom. . I will wait for a bit warmer weather to replace the other 5 since it's running so well. Hope this helps someone in the future.

Thanks!

 

Rattling cat is fairly common. Over time it deteriorates and begins to break apart. When chunks of the substrate get caught in the pipes or muffler, it will cause driveability problems.

A poisoned, (restricted, melted, or plugged up) cat is caused by rich misfires. As it becomes more restricted, it can cause more misfires, stalling, hard starting, low power, hesitation on acceleration, etc etc. Whenever a poisoned cat is found, the root cause needs to be diagnosed and repaired to avoid destroying the replacement converter. Rule of thumb with a poisoned cat, is that something caused it to fail, it didn't fail by itself. It is a result of the root cause.

 

Thanks Captain! I appreciate the feedback and ideas to ponder.

A bit more to the story...The cat was rattling before the original engine threw a rod. I'm guessing the original engine was the curse of death to the Cat.
Installed New crate motor and the cat continued to rattle itself to death. Therefore, in this case, I think the bad plug wire is the culprit for #2 cylinder fouling and the new engine most likely did not cause the cat to go bad it was on it's way out before.

Thanks again,

 

Sounds perfectly logical. Good job.