after all that the plenum was bad, but still hasnt fixed that horrible sputtering and backfiring, after the plenum job sunday, i tried a map sensor, nothing, and another tps nothing? would the cat, or front o2 sensor cause this i think its running lean? or maybe the start of the death flash? boy if someone could figure this out they would be my hero!!! here is what all i have tried.
tps
iacs
front and back o2 sensors
plugs,wires,cap,rotor
maps
fuel pressure
i get codes 132 and 138 high voltage on both o2sensors and i think the cat is gutted. thats all i know?

 

tap on the side of the cat for rattling, if its not gutted, it may be going bad, and your o2 sensors may be going bad, and if the cats gutted, it may throw a cel for the rear o2 sensor, as it will be getting a dirty reading

 

just replace it with a magnaflow cat for 80 buck on ebay totally worth it

 

The death flash would only lower the performance but, in no way cause backfires, sputtering.

Remove the O2 sensor from the down-tube. Test.

If the sputtering and backfiring diminishes and it runs fine- you have a plugged CAT.

If the problem persist- check the ignition wires for a miswire. Check the coil wire for arcing to adjacent ignition wires causing premature detonation and misfire condition. Isolate coil wire from all cylinders and retest- if the condition improves- isolate the coil wire or, upgrade to heavier wire set- 8.5MM MSDs or similar.

If no miswires are present- check the distributor for loose, worn distributor shaft, cracked cap and rotor for any signs of damage. Replace as needed and retest. If condition improves that was it. If not- proceed to check plugs.

Check the plug condition. If they look bad, show signs of carbon traces n the insulator- replace the whole set and retest. If condition improves- that was it. If not, check EGR components.

EGR valve- although it is not likely to cause a backfire, it has been known to cause other erratic engine behaviors. Remove and clean- retest. If condition is the same- proceed to timing.

Timing: You may have jumped the timing chain or, the key may have worn thus allowing one of the sprockets to slip. Check the timing.

If all the above checks out, it may be the computer or, it could also be a mechanical failure in the engine.

Basically, if you getting sputtering and backfires, that means that the that the timing of the spark is out of sync.

Incorrect timing, incorrect ignition wiring, shorting ignition wires, crossfire due to arching coil wires and ignition wires will all cause the same symptoms as well as a plugged CAT. If the gases can't escape, they remain under pressure in the cylinder and eventually escape through the intake valves thus resulting in backfiring through the intake.

If your having backfiring out of the tailpipe, then it would most likely be a timing / miswire / misfire issue more than a CAT issue.

 

it sounds like its backfiring in the cator up closer to the engine, i tap on the cat ,sounds hallow and i noticed that it has about 15 nickle size weld spots on it? and when it sputters and backfires it doesnt matter how fast im going warm or cold, sometimes i can drive 20 miles before it does it others i can drive 1 mile and it does it? just randomly at anytime?when it does it i shut it off and start it right back up and it runs fine for a bit then does it again, seems to be gettin worse everyday. i put the best cap and rotor and wires the part store had on it. hopefully it will just go out whatever it is!!!!

 

how long ago you do your tune up ??

 

That's a hard one. I HATE intermittent issues. If it's running fine- we can rule out that a timing chain has slipped for sure.

If it was just sputtering I would say it was a clogged fuel injetor/s but, since you ALSO have backfiring near the motor this indicates it's intake related and not the exhaust cycle.

A plugged CAT will behave intermittently. Please remove your downtube 02 sensor and retest. NOTE: Do not do this for long> FIRE HAZARD. Short distance, or rev up in the yard etc.

Note results. If it goes away, your CAT is spent or, your muffler is clogged.

Try that first.

Q: What size wires are you running?

Q: What year is your truck?

Q: What is your engine?

Q: What plugs are you running?

 

ive tried running one sensor at a time unhooked, then both no changes. i did tune up about a month ago, my wires are.
BELDEN premium 7mm.
plugs are NGK V-power FR5-l
cap is ECHLIN mo29
i just went and looked at a spark plug and it looks like its real lean white not dark at all? 1998 1500 5.9 liter 360 gas 4x4 182,000 miles, new exguast from the cat back, got a flo pro muffler not sure how new ive only owned it for maybe 2 months

 

theres a sticker actually 2 stickers that say (pcm changed to 56046345AG) recall #831, dealer code 43508, date 3-8-01 authorized software update. does the oxygen sensor have a relay? dont know if this helps but i hope so. what about vss? vehicle speed sensor.

 

The O2 sensor has no relay. It's basically hard wired and has an internal heater inside to heat it apx 600F. This is the down tube sensor. The rear one - not sure if it has a heater inside. But, both have no relay.

Looks like your PCM was changed for sure.

Let's start at the beginning. Did your truck ALWAYS have this issue?

WHEN, did this issue appear?

Was there any changes made that caused a difference?

WHICH plug was white: the rear cyls or towards the front?

Don't worry about VSS- we need to fix this ignition issue first.