Bad cat? Off road y-pipe?

[1-8-4-3-6-5-7-2]

1997 C1500 4.3 auto

If I run the engine hard like merging onto an interstate the engine will start to miss out real bad like it's running on only 2 cylinders. Imagine putting the power on a light switch and flipping it on and off real fast, that's what it feels like. It's like the power is on-off-on-off-on and so on. I let off the gas and keep the pedal at a normal cruise speed and it bucks for about 20 to 30 seconds and then it clears up.

Plugs, wires, cap, rotor were replaced about 2 years ago with all AC Delco stuff. (plug wires may have been another brand, but still decent. I don't buy crap wires lol)

Checked the fuel pressure a year or two ago when this problem started and it was a couple pounds under spec but that could be my cheap gauge not being accurate.

Now on a cold start I can hear a horrible loud rattle noise coming from the catalytic converter area. I don't think it's the transmission making the noise, but it's hard to tell for sure. After the engine warms up the noise gets much quieter and is only heard at a certain RPM when beginning to accelerate.

Codes are p0147 (rear o2 heater circuit) and p0304 (cylinder 4 misfire)

Based on that info, what are the chances I have a bad catalytic converter?

If the cat is bad, anybody know if there is an off-road y-pipe available for the 4.3 Vortec? I used one years ago on my 96 Burb 5.7 but can't find one for this truck.

 

[Schurkey]

You're right, fuel pressure gauges, like cranking compression tester gauges, and oil pressure gauges, need to be tested for accuracy/repeatability now 'n' then. You could connect a scan tool, and see if the short- and long-term fuel trims have been commanded "rich" which might indicate low fuel pressure. How old is the fuel filter? ('Course, it only takes one tank of contaminated fuel to plug a fuel filter.)

You may or may not have a "bad" catalytic converter. Misfire kills them--they get overheated and they melt or break into pieces internally.

More likely, you've got misfire problems which cause the bucking, which will eventually kill the converter. Fix the misfire, hope the catalyst survives. You might get lucky. I did, with my Trailblazer. Drove home from out-of-town with a terrible misfire, discovered later I had a failed ignition coil for #1 cylinder. Since your ignition system is relatively new, I kinda wonder about the "spider" injectors--a common failure point for the Vortec V-6 and Small-blocks.

When the converter plugs (which is NOT the only way a converter fails) you wouldn't have the bucking, you'd just have really low power, eventually the engine would run so bad you'd hardly be able to go three miles per hour. But there's no "bucking", just a smooth decrease in power.

"Rattle" under the vehicle is often a heat shield that's broken a spot-weld and is now loose at one end. "Chuffing" under acceleration can be an exhaust leak via a failed donut/gasket, or even a small rust-hole in the pipe.

Going without a catalyst is just irresponsible, and will eventually set a different code.

And the rear O2 sensor is going to have to be replaced even though it's probably not causing the bucking problem.

 

[1-8-4-3-6-5-7-2]

If I had a misfire problem, wouldn’t I have it more often? It bucks only when I run it above about 4000 rpm. It runs perfectly smooth all the time until I run it hard, then it bucks and doesn’t even have enough power to keep moving… I lose speed, then another 20 seconds later it returns to normal like nothing ever happened and it runs perfectly smooth again.

 

[1-8-4-3-6-5-7-2]

The truck has only 140,000 miles and runs great except for when I run it hard. It gets 19 to 20 mpg. But I’m thinking I may be better off just swapping to a 5.7 since these goofball problems are so hard to track down.

Another odd thing is that the “check engine” light doesn’t work. I’ve swapped the bulb with a known working bulb but the check engine light still doesn’t work. I need to look at a schematic and see what wrong there too.

 

[stutaeng]

I helped my nephew diagnose a strange issue with his s-10 4.3. His truck bogged down severely and started to overheat on the road. At idle everything seemed ok as far as fuel trims, O2 waveform, etc. He only had a MAF sensor code.

I had to get him on the road for the problems to be replicated. With foot all the way down it only went like 20 mph and coolant temperature climbing. Between my BIL and myself we concluded it was most likely the plugged cats. I never figured out why they got plugged, as I think his truck was running ok before.

 

[thegawd]

I bet your fuel pump is screwed. I only say this on passed experience with a delphi pump that was garbage right out of the box but I didn't know it, I could feel it though. I didn't and still dont have a fuel pressure tester otherwise I could have caught it, the pump was pulsing and it felt like a miss but I could never make it go away. I also just dismissed it as a problem of the fule pump as it was "new."

18 months later and while about to leave for a vacation the truck starts driving exactly like you describe. it drives fine under low power but if I actually push the pedal the truck starts bucking like someone is rapidly turning a light switch on and off.

oh my frustration exploded. I have a transmission jack thankfully as the fuel tank was completely full! I dropped the tank 3 times trying to fix it and on the 3rd time I replaced the "new delphi" with the "old AC DELCO" and surprisingly the problem was solved.

I should have been able to test this with a fuel pressure gauge if I had one.

 

[Schurkey]

Fuel pressure test gauges are often available as "loaner tools" at auto parts stores--O'Reilly's and AutoZone are known for this.

 

[1-8-4-3-6-5-7-2]

I checked the fuel pressure today. I turned on the key and it went to 62 psi as the pump ran and dropped immediately to 54 psi when the pump stopped. It held at 54 psi for 5 minutes. I stated the engine and the pressure stayed at 54 psi. If I revved the engine a little or put it in gear and gave it some throttle the psi would go up a little to about maybe 58 psi at the highest. The fuel filter was replaced previously and it had the same drivability symptoms before and after the filter. So that pressure is a bit low? Pump or regulator?

The source of my rattle noise was a blown out gasket at the big flange where the front and rear pipes connect together. I guess it was vibrating and sending the noise throughout the rest of the exhaust system.

Also fixed my ABS brakes today.

 

[El Tigre]

A fuel pressure gauge w/hose long enough to tape gauge to windshield can tell one what pressure is under load ,not just at idle. Install gauge ,tape to windshield ,and drive as you described earlier. Pressure should not fall.

 

[thegawd]

that's exactly how mine was failing, only under load. and like, constantly on/off.

let off the throttle and it would run perfectly.

it sure was a PITA driving the truck back home with that BS going on but I made it.