98 454 Missing When Warm

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618 Syndicate

618 Syndicate

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98 C3500 454/4L80 stock other than cats gone, and downstream O2 sensors are turned off. HP tune on the stock PCM.
Ran great for about 50 miles of a 60 mile trip, then started missing hard (like WHAM) backfiring and wouldn't take throttle input. Temp climbed to about 190° with no thermostat, ambient temp was about 85. Sat for about 5 hours, drove the same distance home, ambient temp was about 65°, coolant never got above 175° and it ran great the whole way.
Any thoughts?
 
454cid

454cid

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That's warm for no thermostat. When I ran my truck without a thermostat while trying to flush the radiator it might have hit 150F, while on the highway.

I would get live data, on it, and start looking at sensor readings.... coolant sensor, o2's, TPS, MAF.

How old are the cap and rotor?

More humid during the mis-firing?
 
618 Syndicate

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454cid said:
That's warm for no thermostat. When I ran my truck without a thermostat while trying to flush the radiator it might have hit 150F, while on the highway.

I would get live data, on it, and start looking at sensor readings.... coolant sensor, o2's, TPS, MAF.

How old are the cap and rotor?

More humid during the mis-firing?
Click to expand...
Cap and rotor are less than a year old, yesterday wasn't humid at all for around here, less than 20%
Live data is an option, I'll get that sorted out.
 
stutaeng

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Look at fuel trims. Lean or rich are hints. It seems these ECU are not too picky when it comes to throwing a code even if trims are out of range, not sure why.

Are your injectors stock?

Hopefully you can replicate the symptoms and find the culprit easily. Intermittent misfires are sometimes hard to find.

Edit: check fuel pressure as well.
 
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The stock injectors are junk, but I've not heard of any implicated in the symptoms you describe. I once tracked a long history of random misfires (with nowhere near as serious side effects as yours) to a broken ground strap. Watching the engine run on a moonless night with the hood light unplugged there was a lightning show going on in the engine compartment.
 
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Fuel pressure/volume and spark are both things I've considered, but I can't for the life of me figure out why it's only happening after I drive it for a while...
 
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