Desperate Trucker Owner.... SCR Melted

[hhow55]

Common Causes Leading to High Exhaust Temperatures and SCR Melting
Several upstream issues can cause these elevated temperatures:

Problems with the DPF — Such as cracking (common in systems with 90-degree outlets), breaching, clogging, or failure to regenerate properly. This leads to incomplete soot burnout, hydrocarbon buildup, and uncontrolled high-heat events during regeneration.
Overfueling or rich running conditions — Often from faulty injectors, turbo issues, or tuning problems, which increase exhaust hydrocarbons and raise temperatures during regen.
Restricted airflow — Due to clogged air filters, intake blockages, or other restrictions, causing incomplete combustion and higher exhaust heat.
Failed regenerations or aftertreatment issues — Including problems in the Diesel Oxidation Catalyst (DOC) or dosing system that allow soot/hydrocarbons to accumulate and burn uncontrollably.
Contaminated DEF (Diesel Exhaust Fluid) — Less directly linked to melting but can contribute to system inefficiencies that indirectly raise temperatures or cause other failures.

Melting is a severe form of damage often discovered during diagnostics for low SCR efficiency codes (e.g., SPN 3582 or P20EE), where the catalyst no longer functions properly. In some cases, earlier Cummins SCR designs had higher failure rates, including recalls for catalyst issues (primarily efficiency, not explicitly melting).
To diagnose, use Cummins INSITE software for SCR efficiency tests, forced regens, and temperature logging. Inspect the DPF/SCR assembly if high temps are suspected. Prevention involves regular maintenance (air filters, DEF quality), monitoring regen frequency, and addressing fault codes promptly. If melting has occurred, the SCR catalyst typically needs replacement (costly, often $4,000+ for OEM).

[DVT873]

hhow55, Rawze. What causes DPF outlet temps to go higher, sometimes several hundred degrees higher than inlet temps? I think I know few reasons, but they don't all make sense to me. Can this happen without the DPF having cracks?

[hhow55]

However, outlet temperatures can sometimes exceed inlet temperatures ("temperature inversion") for these main reasons:

Normal during certain regeneration phases:
Soot oxidation inside the DPF is exothermic (releases heat). With high soot loading, burning soot generates additional heat, especially toward the filter's rear. This can briefly cause the outlet to read higher (e.g., 50–100°F difference), depending on soot/ash load. Sources note this as potentially normal if not excessive.

Faulty or inefficient Diesel Oxidation Catalyst (DOC):
The DOC should oxidize dosed/post-injected fuel to raise temperature before the DPF. If the DOC is clogged (e.g., face-plugged), degraded, or low-efficiency, unburned hydrocarbons pass into the DPF.
These then burn inside the DPF, creating extra heat and elevating outlet temperatures (sometimes >1200°F). Mechanics frequently diagnose this when outlet > inlet, as it indicates "raw fuel through DPF," risking damage to DPF and downstream SCR.

Excess hydrocarbons or combustibles in exhaust:
Leaking fuel injectors, overfueling, oil/coolant consumption (e.g., bad turbo seals), or dosing issues introduce extra fuel/oil into the exhaust.
This burns in the DPF, spiking outlet temperatures.

Sensor or wiring faults:
Swapped/misconnected sensors, faulty DPF inlet/outlet temperature sensors, or wiring problems can cause false readings showing inverted temperatures.

Other factors:
Partial DPF blockage increases backpressure and localized heat.
High soot loads without proper DOC function amplify exotherm in the DPF.


Normally, during active regen:

DOC outlet (DPF inlet) targets ~1000–1100°F (550–600°C).
{bad info removed by Rawze}

Persistent or extreme inversion (e.g., outlet >> inlet) usually signals a problem, often requiring DOC inspection/replacement, DPF cleaning, or diagnostics for fuel/oil entry. Monitor via scan tools during forced regen for accurate assessment.
Sourced from AI............

[Rawze]

(12-20-2025 )hhow55 Wrote:  ...
DOC outlet (DPF inlet) targets ~1000–1100°F (550–600°C).
DPF outlet is similar or slightly lower/cooler as heat transfers and gas expands.
...

This is an incorrect statement. That AI trash result is hallucinating ...

It is ABSOLUTELY NORMAL for the outlet temps to be higher than the inlet temps, especially as it gets slightly cleaner during the regen. This is because the fuel (and soot) ignites inside the DPF element, and the exhaust gasses are pushing the heat in only one direction ... towards the outlet. The flame and heat does not go forwards towards the face.