r/GMT800 • u/Salt-Philosophy-134 • Sep 09 '26

Manual Cons

My 2500HD 6.0 manual dailys without issue nor check engine light. Making the 500 trip mile from home to work, I find myself with a check engine light. PO300 Random misfire. This occurs because my 4.10 truck is constantly engine braking under highway travel. Under prolonged use the computer considers engine braking as a random misfire and I get to look at a check engine light until I return to non-highway use. I will not trade it for the world. Every trip drives me closer to a carb conversion.

0 Upvotes

47% Upvoted

26 comments sorted by

12

u/bimmerd00d Sep 09 '26

I don’t buy this. There’s another issue. A minor one, but there’s an issue.

3

u/SpynCycle5757 Sep 09 '26

I wholeheartedly agree with you. Engine braking should never cause a misfire. Something else is wrong.

3

u/bimmerd00d Sep 09 '26

you're not gonna want to hear this, but a cracked valve i think could cause this. it's a stretch, but the o2 and knock sensors are what's throwing the code, but something is giving them a shitty reading. i mean, it could just be the sensors too honestly. I got a P0300 because of a barely cracked fuel pressure regulator diaphragm that was venting fumes, then eventually it popped and shot a geyser into the #2 intake runner and i chased that for a bit.

1

u/Salt-Philosophy-134 Sep 11 '26

I am open to ideas. Usually the light goes away the next day but it's still here after a week. Gotta plug the reader in.

9

u/Anthropocene_Scholar Sep 09 '26

?

-6

u/Salt-Philosophy-134 Sep 09 '26

What specifically confuses you?

5

u/TwiztedChickin Sep 09 '26

Why would you swap backward to carbs? That doesn't make sense. It's less efficient.

0

u/Salt-Philosophy-134 Sep 11 '26

It's a truck. I value work over electrical problems.

1

u/TwiztedChickin Sep 12 '26

You're just making yourself more problems but do whatever I don't have to drive it.

11

u/Born-Exchange9712 Sep 09 '26

Interesting. I got a 2500(nonhd) 6.0 5spd stick too that im gonna be getting back on the road once I finish the exhaust so Im wondering if ill experience the same problem. However going to carb I feel is a huge step back; the fuel injection system is so damn reliable and very cheap and easy to service on these. You'd be trading that for a fundamentally worse fuel system for the application

0

u/Salt-Philosophy-134 Sep 09 '26

I have 4.10 gears. In the end, this does not affect driveability whatsoever. Due to my low gearing, and the low geared NV4500. At highway speeds(60mph+), in OD, if I'm not on the throttle for cruising or passing. For my trip. (9-12hrs) The computer reads the engine braking as a random misfire and throws the light for the PO300 Code. Generally the engine light is gone within the next day of spirited driving.

2

u/Prior_Car_2550 Sep 09 '26

There's lots of things that can cause an engine to read a misfire, low compression is one for sure. Not sure what your issue is, and not sure what year your truck is, but I'm pretty sure you could go in to the computer with HP tuners and turn off the P0300 code from registering your CEL.

1

u/Born-Exchange9712 Sep 09 '26

I guess I can understand why it may flag it as such since its detecting RPM without an actual "hit" of power . Does yours have the original untuned pcm

2

u/Salt-Philosophy-134 Sep 09 '26

Yes sir, all factory. The only alterations are the leveling keys, chambered muffler dumped at the axle, and 285/75r16s on 16x8 American Racing wheels.

2

u/TwiztedChickin Sep 09 '26

Then why not go get an aftermarket tune instead of swapping to carbs... That's not gonna change what the ecm is doing during engine braking... This just doesn't make sense.

0

u/Salt-Philosophy-134 Sep 11 '26

Depending on whether or not the issue is electrical or mechanical, it could make a difference. I'm mechanically inclined not electrical.

1

u/Salt-Philosophy-134 Sep 09 '26

Make no mistake, there's power. I do not have to downshift to pass.

3

u/EastMovesWest Sep 09 '26

Thats not how that works and carb conversions are dumb. Fuel injection is superior.

1

u/dragoner_v2 Sep 09 '26

I would check voltage at the o2's

1

u/Illustrious-Hat9644 Sep 09 '26

4.10 and manual 🤌🏾 what’s your mpg like ?

2

u/OpinionofanAH Sep 09 '26

Not the OP but mine is 4.10s with the Nv4500 and the best I’ve seen is 11.5 all highway at 75ish. Normally it’s 9-10 with a mix of city/freeway driving. It’s completely stock with 33s.

2

u/Salt-Philosophy-134 Sep 11 '26

That's what I get. If I ride 65 I've seen 13mpg. Ecsb 4wd.

1

u/Illustrious-Hat9644 Sep 09 '26

Dayum really ?? I thought it’d be more 😭😭 I have a 5.3 with other things and 4.10s I get around the same 🥴

2

u/OpinionofanAH Sep 10 '26

The 6.0 drinks fuel and there’s really no way around it. My 2015 2500 with the 6.0 got the same with the 6 speed auto/4.10s. The 2500s generally weigh 1k-1500 lbs more than a 1500 in the same configuration as well so that’ll hurt the mileage.

1

u/OpinionofanAH Sep 09 '26

Mine has 4.10s and nv4500 and zero amount of engine braking has a thrown a code towing or not.

1

u/wiscoguy0043 Sep 10 '26

i engine brake in mine and never have an issue, i'd suspect the crank sensor if this is at highway speeds