DAF Euro 6 PACCAR MX-11/MX-13 Overheating and Coolant Expansion Tank Over-Pressurization on Uphill Loads

User question

Daf euro 6 raising temperature during uphill when loaded. Over pressuring coolant expansion tank

Artificial Intelligence Response

Direct answer to the question

• Most-probable root causes of “temperature rise uphill + expansion tank over‑pressurization” on DAF Euro 6 (PACCAR MX‑11/MX‑13):
  • Compressed gas intrusion into coolant: water‑cooled air compressor head leak and/or EGR cooler internal leak, secondarily head gasket/liner sealing fault.
  • Loss of cooling capacity under high thermal load: fan clutch not fully locking, restricted cooler pack, thermostat or water‑pump flow deficiency.
  • Less likely but must be checked: faulty pressure cap, air trapped from improper fill, retarder/aux heat exchanger issues.
• Prioritized triage: prove or eliminate gas intrusion first; then confirm airflow and coolant flow; finally check controls and peripherals.

Key points

• If system pressure climbs rapidly from cold or stays high after overnight rest with hard upper hose, suspect gas intrusion (compressor/EGR/head).
• If pressure tracks temperature and only exceeds cap rating on long grades, suspect cooling capacity (fan, radiator, flow).
• Do not continue heavy-load operation with pressure >1.8–2.0 bar or coolant loss: risk of tank rupture/engine damage.

Detailed problem analysis

Thermal and pressure behavior explains the fault tree:

• On long climbs, engine brake/retarder use and higher fueling raise heat rejection (coolant + CAC + EGR). The cooling stack and pump must dissipate tens of kilowatts continuously. Any shortfall (airflow/flow restriction) drives coolant near its saturation point at system pressure, causing expansion and venting. That pattern produces pressure that rises with temperature and falls as it cools.
• Over‑pressurization while temperature is only moderate, or very rapid pressure rise from a cold start, usually indicates foreign gas entering the cooling circuit (air/exhaust/combustion). Gas is far more compressible than liquid and will harden hoses quickly without proportional temperature rise.

DAF/PACCAR Euro 6 specifics that matter

• Water‑cooled air compressor: a cracked head or failed compressor head gasket can feed 10–12 bar air into the coolant jacket. This is a common HGV cause of “clean” over‑pressure with little/no oil contamination. Symptoms often worsen during frequent compressor loading (gearshifts, braking, suspension use) and long grades.
• EGR cooler: internal core cracking lets exhaust gas pressure enter coolant; may be load/temperature dependent. Soot traces in coolant are possible but not always visible.
• Engine sealing (head gasket/liner protrusion): marginal liner protrusion or micro‑cracks can leak combustion gas into coolant preferentially at high BMEP (long climbs).
• Fan system: electronically controlled viscous hub must achieve commanded lock; wiring or clutch degradation causes “spins but slips under load.”
• Cooling stack: CAC, A/C condenser, EGR and engine radiator pack easily accumulates debris between cores; restriction drastically reduces heat rejection right when you need it most.
• Flow hardware: thermostat sticking or water‑pump impeller erosion/slip limits circulation, causing localized boiling and pressure spikes.
• Aux heat exchangers: ZF Intarder/transmission/oil coolers dump additional heat into the coolant on downhills and during heavy braking.

Distinguishing patterns

• Gas intrusion signature
  • From stone‑cold start, upper hose firms up within minutes at idle, gauge still low.
  • Pressure climbs disproportionately to temperature, sometimes visible bubbling into tank.
  • Overnight cold hose still firm; pressure present when opening cap cold.
• Cooling shortfall signature
  • Pressure follows temperature; no abnormal pressure when cold.
  • Fan “roar” absent while ECT climbs; radiator ΔT abnormal; cabin heat goes cool at high load (flow loss).
• Mixed faults occur (e.g., marginal fan + small compressor leak).

Current information and trends

• In workshop practice on Euro 6 PACCAR, water‑cooled air compressor head leaks and EGR cooler leaks are frequent culprits for “mystery” over‑pressure with otherwise clean coolant. Many fleets proactively test/bench these before authorizing head removal.
• Electronically controlled viscous hubs age thermally; slippage under load without DTCs is common. Live data correlation (command vs. actual speed) catches this better than static checks.
• DPF active regenerations on long grades add thermal load; ensure regen strategy and fan control updates are current during diagnosis, but treat this as contributory, not primary.

Supporting explanations and details

Technical checks and expected observations

• Cap and pressure baselines
  • Typical cap rating: ~1.4–1.6 bar. At hot idle after a road test, a healthy system stabilizes below the cap rating and vents only if exceeded.
  • Install a 0–3 bar transducer tee’d at the tank and log pressure vs. ECT/engine load. A spike to >1.0 bar within 1–2 minutes from cold is strongly suggestive of gas ingress.
• Radiator ΔT
  • Under load with fans engaged, expect ~8–15 °C drop across radiator. Very small ΔT with high ECT implies poor airflow; very large ΔT with high ECT suggests inadequate coolant flow.
• Fan clutch
  • With 100% command, fan speed should approach pulley‑ratio‑scaled engine speed; significant shortfall indicates slippage. Audible “roar” should be obvious during engagement.
• Water pump/flow
  • Eroded or slipping impeller can produce good idle flow but collapse under load; cabin heater going tepid during a climb is a classic hint of flow starvation.
• Compressor/EGR isolation
  • Bypassing the compressor coolant loop or temporarily isolating the EGR cooler (bridging the engine-side hoses to maintain circulation) that immediately normalizes pressure strongly incriminates the isolated component.
• Combustion gas test
  • Block tester (CO2 chemical test) positive indicates combustion/exhaust gas in coolant (head/liner/EGR). Negative does not exclude compressor air leaks.

Ethical and legal aspects

• Emissions: Any EGR cooler bypass for diagnosis is temporary and will set emissions DTCs; restore to stock immediately after testing. Do not operate on public roads with emissions systems disabled beyond a brief controlled diagnostic drive.
• Safety: Never open the cap hot; use PPE. Over‑pressurization can rupture plastic tanks and hoses; vent slowly when cold. Dispose of coolant per local environmental regulations.

Practical guidelines

60–90 minute, no‑parts triage (prioritized) 1) Instrument quickly

• Fit a 0–3 bar pressure gauge/transducer at expansion tank; connect scan tool (DAVIE) to log ECT, fan command, actual fan speed, pump speed (if available), vehicle speed, load, DPF status.
• Cold start, cap on. Observe pressure for first 3–5 minutes:
  • Rapid rise >0.8–1.0 bar while ECT <60 °C → go to gas‑intrusion path.
  • Minimal rise → proceed to load test.

2) Gas‑intrusion path

• With engine cool, bridge the compressor’s coolant inlet/outlet to bypass the compressor jacket. Repeat cold‑start pressure observation and a short drive:
  • Pressure behavior normalizes → replace/repair compressor head/gasket.
• If still abnormal, temporarily bridge/isolates EGR cooler coolant circuit (expect a MIL); repeat:
  • Normalizes → EGR cooler internal leak.
• If still positive, perform CO2 block test at expansion tank neck; if positive, proceed to cylinder leakage investigation (head gasket/liner/head crack).
• Overnight check: with cap on, cool engine; next morning, if hose is hard before start, gas ingress is highly likely.

3) Cooling‑capacity path (if gas intrusion ruled out)

• Cooler pack inspection: remove front grille/seal strips, separate CAC/AC/radiator as feasible; clean out debris between cores (not just front face). Check shutter assemblies if fitted.
• Fan operation: command 100% with DAVIE; verify audible roar and speed correlation. Inspect wiring and hub for silicone leakage.
• Radiator ΔT under steady climb; if poor airflow suspected, repeat after thorough cleaning or with fan commanded on.
• Flow checks:
  • Thermostat operation: verify opening temperature and bypass function via ECT trends and hose temperatures.
  • Water pump: look for seepage, bearings noise; correlate cabin heater output under load; consider flow test or replacement if mileage high and evidence of erosion.
• Pressure cap: bench test; replace if it vents early or holds excessive pressure.

4) Aux heat sources

• If equipped with ZF Intarder or engine brake, monitor coolant temp behavior during sustained use; inspect associated heat exchangers for blockage or cross‑leak (oil in coolant/coolant in oil).

Parts/material notes

• Use PACCAR‑approved OAT coolant at correct concentration; vacuum‑fill to prevent air locks. Follow DAF bleed points and sequence for high/low temp circuits.

Thresholds to stop testing

• ECT trends above manufacturer red zone, or coolant pressure >2.0 bar, or rapid coolant loss → stop, allow to cool, tow if needed.

Possible disclaimers or additional notes

• Exact thermostat opening points, cap ratings, and fan ratios vary with VIN and cooling package; consult the vehicle-specific service data.
• A negative CO2 test does not exclude a compressed‑air leak from the compressor; treat tests as complementary, not definitive in isolation.

Suggestions for further research

• Check with your DAF dealer for any VIN‑specific service bulletins on cooling fan control updates or compressor head revisions.
• Review DAVIE guided diagnostics for MX‑11/MX‑13 cooling system and EGR cooler leak tests.
• If head/liner suspected, measure liner protrusion and inspect fire rings per PACCAR spec.

Brief summary

• Your symptom set most often traces to gas entering the coolant (water‑cooled air compressor or EGR cooler) or, secondarily, inadequate cooling capacity (fan/stack/flow) under peak load. Prove or eliminate gas intrusion first using a cold‑start pressure log, compressor/EGR isolation, and a CO2 block test. If negative, verify airflow (fan engagement, stack cleanliness) and coolant flow (thermostat, pump, radiator ΔT). Replace a marginal cap and vacuum‑fill with the correct coolant after any intervention.

If you can, please share:

• Exact coolant temp and measured pressure during the event, whether hoses harden quickly from cold, any coolant loss, presence of a retarder, recent cooling system work, mileage, and any fault codes from DAVIE. This will let me tailor the next steps to your truck.

Disclaimer: The responses provided by artificial intelligence (language model) may be inaccurate and misleading. Elektroda is not responsible for the accuracy, reliability, or completeness of the presented information. All responses should be verified by the user.