EGR off and DPF won’t burn: the +20% post-injection rule
Diesel · Methodology
EGR off and DPF won’t burn: the +20% post-injection rule
EGR delete is the easy part. Keeping the DPF burning afterward is the hard part. There is a specific post-injection adjustment that fixes the common “EGR off and DPF stops regenerating” failure. The concrete number comes from our 5 May 2026 Q&A. This article walks through the two failure modes behind the symptom and the workflow that compensates the physical one.
EGR delete fails the DPF on two separate levels. The control level depends on the manufacturer’s ecological-solution controller and has no universal fix. The physical level is predictable: less hot recirculated gas means cooler exhaust, and the DPF stops reaching burn temperature. The fix on the physical side is to raise the post-injection maps by about 20% until differential pressure across the DPF drops correctly after a regeneration cycle. Leave the air-flow maps alone — editing them breaks the MAF calculation chain on MD1 and EDC17.
This article is built from a Q&A session with our students on 5 May 2026. Aliaksandr asked about air-flow logic on MD1 after EGR off (~06:08). Adam asked how to disable EGR cleanly so the DPF continues to function (~19:24). The walk-through below cites the transcript directly — the timestamps map to that recording.
The two failure modes — control versus physical
The EGR system is not a stand-alone valve. It is one component in an ecological-solution controller that ties together DPF, EGR, SCR, and exhaust control. When you change EGR behaviour in the calibration, the controller does not stop reacting to the change. The strategy for the other components shifts to match. That is what makes EGR-off tunes act differently on different cars.
“They are two levels of problem. First level is the level of the non-physical behaviour of the system of EGR with DPF. These ecological systems work together, controlled by the Ecological Solution controller, which connects together DPF, EGR, SCR, whatever we have, even the exhaust control for ecology.”
“If you just change something in EGR, this influences directly the control of other elements. So this behaviour can be different between the manufacturers. There is no one good solution at the level of the control.”
Q&A 2026-05-05, ~20:00–20:45
Non-physical — strategy-side
Manufacturer-dependent. EGR off propagates through the ecological-solution controller. Behaviour you might see: one car keeps regenerating, another stops regenerating entirely, a third starts regenerating more often than before. There is no one fix at this level — reactions live in the DPF strategy maps and they are platform-specific.
Physical — flow and temperature
EGR off means more fresh air through the intake and less recirculated hot gas. Two consequences: less soot loading on the DPF, which is good, and lower exhaust temperature, which is the problem. The DPF can fail to reach the temperature it needs during regeneration. This failure mode has a predictable fix on the fuel side.
The reason this matters: before you reach for a calibration change, you need to know which level is producing the symptom. The +20% post-injection rule below addresses the physical level. Maybe the control level is what is actually failing on a given car — regeneration never triggers, or fires every 200 km. In that case, raising post-injection will not help. The fix lives in the DPF strategy maps on that platform.
Why DPF regeneration stops burning after EGR off
The physical mechanism is direct. Recirculated exhaust gas raises intake temperature. When you remove the recirculation path, intake temperature falls. The same amount of fuel burns at a lower combustion temperature. The exhaust stream that reaches the DPF is cooler than it was before the delete.
During a regeneration cycle, the calibration commands the DPF to hold burn temperature for a long enough window to consume the accumulated soot. If the exhaust enters the DPF cooler than the regeneration design point, the cycle can start, run its full duration, and end. The filter never reaches the threshold. The completion bit gets set in software, the cycle counter increments, and the soot remains in the filter.
The diagnostic indicator is the differential pressure across the DPF. After a successful regeneration, the pressure differential drops because the filter is cleaner. After a failed regeneration cycle on an EGR-off tune, the differential does not drop — the filter is no cleaner than before.
“If you just do that and you see that there is a problem with burning out the DPF, you will see that the differential pressure is not dropping huge after burning process — for example, the burning at the standing still.”
Q&A 2026-05-05, ~23:00
The phrase “burning at the standing still” refers to a stationary regeneration cycle — the controlled burn the ECU forces when the soot load is high and the car is parked at idle. If the differential pressure plot does not drop sharply at the end of that cycle, the burn did not complete its job. That is the signal to apply the post-injection compensation below.
The +20% post-injection rule — workflow
Post-injection is a fuel event delivered after the main combustion event in the same engine cycle. The fuel does not add torque. It enters a cylinder that is already past the power stroke. What it does is burn in the exhaust stream and raise the exhaust gas temperature heading into the DPF.
That makes post-injection the right calibration lever for restoring DPF burn temperature after an EGR delete. The air side is doing what it should — less EGR, more fresh air, cleaner combustion. The exhaust temperature deficit on the way to the DPF gets paid for by adding fuel after the main event.
The mechanic of the fix is simple. The number, from the live demonstration, is +20%.
“Then you need to increase by 20%, for example, the post-injection values, which are, as you remember, before the duration. And then you will have it burning properly.”
Q&A 2026-05-05, ~23:15–23:25
Where the post-injection maps sit
Post-injection maps live before the duration map in the EDC17 layout. That positioning is consistent across the EDC17 family and carries into the MD1 generation. It matches the pre-injection / post-injection family pattern documented in our KB-01 four-tell identification method — the same map-family logic that lets you tell pre- from post-injection structurally without DAMOS.
The post-injection family typically ships as multiple group copies inside the calibration area, the same way other map families do on EDC17. The pattern of synchronous editing across all group copies of a family is documented across the Knowledge Base — the workflow is the same here.
Workflow in WinOLS®
- Open the calibration file in WinOLS.
- Locate the post-injection map family — positioned before the duration map. If you are not sure which maps are post-injection, the four-tell method walks through the identification.
- Apply +20% across all group copies of the post-injection family, synchronously. The synchronous edit is important — the ECU consults multiple copies depending on operating mode, and they need to track each other.
- Save the project and flash the file to the ECU.
- Confirm the fix using the diagnostic indicators below.
The +20% figure is a starting point validated for the case Adam asked about — a customer with a clean EGR delete whose DPF stopped completing regeneration cycles. It is not a tuning prescription for every platform. The verification step below tells you whether the number was right.
Air flow logic on EGR off — what NOT to change
This is the question Aliaksandr asked about on MD1 (specifically MD1CS006) at the start of the Q&A. The short answer: leave the air-flow maps alone. Editing them is a common mistake that introduces a second problem on top of the temperature deficit.
“With EGR off, or any modern ECU starting even from EDC17, we should just avoid changing the logic of air maps. The basic logic is that there are just the flow maps of the flow when the EGR is open and there are maps when the EGR is closed.”
Q&A 2026-05-05, ~06:25–06:55
How the air-flow logic is structured on MD1
The flow maps are paired by EGR state. One set when EGR is open. One set when EGR is closed. The pair operates as a hysteresis — the ECU picks which set to consult based on what EGR is doing in real time.
On top of the EGR-state pair, there is a regeneration-state branch. When the ECU is in active regeneration mode, the flow logic shifts to a slightly different set for the same EGR position. On MD1 the working pattern is two sets — one for normal running, one for regeneration mode — and the EGR-open/EGR-closed split applies inside each.
“There are just two sets typically in the MD1 for these two modes — normal work and the regeneration state. So this pair is used to calculate the flow.”
Q&A 2026-05-05, ~07:00–07:25
How the MAF flow is calculated
The flow calculation chain is:
- Start with intake air pressure multiplied by RPM — this is the air the engine should be pulling in given its current state.
- Subtract the flow value coming from the EGR maps — this is the share that came back as recirculated gas, not fresh air.
- What is left is the calculated MAF flow.
“The difference between the amount of air calculated by air pressure multiplied by RPM, and minus this flow which is just from the air maps — then we have the flow. This flow always bases on the pressure multiplied by RPM, not from the MAF sensor, not from the flow sensor.”
Q&A 2026-05-05, ~07:30–08:05
If you edit the air maps to “compensate” for the EGR being gone, you change the term being subtracted. The calculated MAF flow no longer matches what the engine is actually pulling. The diagnostic routines compare calculation against sensor reading, flag the gap as an air-system fault, and trigger a torque limit. The customer feels it as power loss and limp mode.
The correct disable point is the EGR valve control side — the maps that tell the EGR valve when and how far to open. With EGR commanded closed, the flow chain stays consistent. The air maps still describe what the system would do if EGR were active, and the calculation produces a sensible MAF flow.
Verifying the fix — diagnostic indicators
After the +20% post-injection edit is flashed, the question is whether the DPF is actually burning. There are four indicators that tell you that, listed in order of how directly they answer the question.
Differential pressure across the DPF
The primary indicator. During and after a regeneration cycle, watch the differential pressure across the DPF. A correct regeneration ends with the pressure dropping. A failed regeneration ends with the pressure essentially where it started. If the +20% adjustment worked, you will see the drop — the same diagnostic signal that flagged the problem in the first place, now reading the opposite way.
Regeneration frequency
Over a few hundred kilometres of driving, the regeneration cycle frequency should look reasonable for the platform — not every 100–200 km, not silent for thousands of kilometres. If frequency is wrong, the failure is more likely on the control level (DPF strategy maps), not the physical level you just fixed.
No DPF dashboard messages
Customer-side: no DPF warning lights, no “regeneration in progress” messages stuck on after a long highway run, no forced limp mode.
Soot loading model reset
The calibration carries a calculated soot loading model independent of the differential pressure reading. After a successful regeneration the modelled soot should reset toward zero. If the model does not reset even though the pressure dropped, the two diagnostic paths disagree — investigate the soot model logic before assuming the calibration is done.
When +20% isn’t enough — manufacturer variability
The +20% number is the starting point validated for the case demonstrated on the Q&A. It is not universal. Different manufacturers run different ecological-solution controllers, different regeneration strategies, and different post-injection mechanics — the strategy responses to EGR off can be wildly different car to car.
If +20% does not produce a differential-pressure drop after a regeneration cycle, the failure has probably moved away from the physical level and into the control level. The path then is to look at the DPF strategy maps directly:
- If regeneration is firing too often or too rarely after the EGR change, adjust the strategy maps in the DPF area to bring the cadence back to normal.
- If the regeneration fires but does not deliver enough fuel for the burn, adjust the post-injection family further — but first check the duration maps to confirm the issue is amount, not time window.
“If we see that the regeneration is too often or too rare, we can just reduce or increase the values in the strategy maps which are in the DPF area. If we see that the amount of fuel is too big or too small for the regeneration, we can just change the post-injection maps which are before the duration.”
Q&A 2026-05-05, ~21:00–21:30
The other possibility, before reaching for more calibration changes, is that the DPF is physically compromised. A cracked monolith from earlier failed regenerations, oil contamination from the engine running with a stuck EGR valve before the delete, or sensor issues with the differential pressure circuit itself. A diagnostic on the part replaces calibration time wasted on a tune that cannot fix the symptom.
The full diesel calibration workflow
EGR off and DPF correction is one workflow inside a larger diesel calibration method — torque modelling, combustion physics, and how the ECU’s protection layers interact with the changes you make. Diesel Practice covers the full method on EDC17 and MD1.
See Diesel Practice →Frequently asked questions
Why does my DPF stop burning after EGR delete?
EGR delete sends more fresh air through the intake. Combustion runs cooler at the same fuelling. Exhaust reaches the DPF at a lower temperature. Regeneration starts, but the filter does not reach burn temperature. You see the symptom on diagnostics: differential pressure across the DPF stays high after a regeneration cycle. The fix is on the fuel side, not the air side. Increase post-injection values to lift exhaust temperature back to where the DPF burns properly.
What is the +20% post-injection rule?
After an EGR delete, if the DPF differential pressure does not drop after a regeneration cycle, increase the post-injection maps by about 20%. Post-injection is fuel injected after the main combustion event. It does not add power. It adds heat to the exhaust stream. The 20% figure is the working starting point demonstrated in our 5 May 2026 Q&A. The post-injection maps sit just before the duration map in the EDC17 layout — the same family pattern shown in KB-01’s pre/post-injection identification method.
Should I change air-flow maps when disabling EGR?
No. On MD1 and any modern ECU starting from EDC17, leaving the air maps alone is critical. The flow logic is built on two flow maps as a hysteresis pair (EGR open / EGR closed), with a second pair for regeneration mode. The MAF flow is calculated from pressure multiplied by RPM minus the flow value from these maps. If you edit the air maps to compensate for EGR off, you break the calculation chain. The ECU enters fault mode and applies torque limits. Disable EGR via the EGR valve control maps and compensate exhaust temperature via post-injection.
Why do some EGR-off tunes work and others don’t?
Two failure modes operate at the same time. The first is control-level: EGR is one component inside an ecological-solution controller that ties DPF, EGR, SCR, and exhaust control together. When you change EGR behaviour, the controller’s strategy propagates the change. Different manufacturers respond differently — one car keeps regenerating, another stops, a third regenerates more often. The second is physical: cooler exhaust temperature. The +20% post-injection rule fixes the physical mode. The control-level mode needs strategy-map work inside the DPF area and is platform-specific.
Is +20% always the right number?
It is the working starting point demonstrated in the Q&A. If the differential pressure drops after a regeneration cycle, the calibration is correct. If the pressure still does not drop, the failure is more likely on the control level — DPF strategy maps, regeneration frequency, or a physical condition such as a cracked monolith or oil contamination. Push the post-injection further only after you have ruled out those failure modes, not before.
Related reading
- Throttle closes at WOT after tuning: the MAF overflow trap — The other downstream effect of the same EGR delete: all flow through the MAF walks the airflow signal into the overflow threshold.
- Bosch EDC17 tuning guide: maps through physics — The map structure this article assumes you already know.
- KB-01 — Pre/post-injection identification: the four-tell method — How to locate the post-injection family in the file before applying +20%.
- KB-04 — AirDesire on EDC17C74 for Stage 1 — Related air-side calibration on the EGR-open branch.
- KB-12 — EDC17 CP44 pre-injection 8-bit encoding — The same family pattern on the pre-injection side.
- Related discussion — Why did my DPF stop regenerating after EGR off? — The forum thread for this article: post your ECU, what the differential pressure did, and what fixed it.
- Tuners Guild Knowledge Base — Q&A archive with Thomas: monitoring traps, torque structure, calibration workflows.
- Tuners Guild pricing — All diesel courses, bundles, and upgrade paths.
