Coolant Loss and a Hot Grade With No External Leak

The oily film plus unexplained coolant loss with no external leak points to an internal leak. Candidates: EGR cooler, cylinder head gasket or cracked head, injector sleeve/cup, engine oil cooler, and (less likely here) a cab heater core, which would show as a sweet smell and a wet carpet.

What the evidence does and does not rule out:

• Cold 15 psi pressure test holding rules out a large open leak, but tells you little about a leak that only opens when hot and pressurised - coolant passages and EGR coolers frequently seal cold and weep hot.
• Block tester blue at idle is weak evidence. At idle there is little cylinder pressure and lots of EGR flow; a combustion-gas test at idle is a classic false negative. It needs to be done at operating temperature, under load, or replaced with a better method.
• Oil level normal and oil not milky makes the oil cooler less likely but does not clear it - a small cooler leak can push coolant into the cooling system rather than into the oil.
• Reman EGR cooler 60 000 km ago raises suspicion but is not proof; reman units do fail, and so do the gaskets and housings around them.

So the ranking is EGR cooler first on history and oily film, head gasket/cracked head next, oil cooler third - but all three must be tested, not guessed.

Run it hot, and make each test decisive:

1. Hot pressure test. Bring the engine to operating temperature, shut down, pressurise to cap rating and watch. A system that now falls where it held cold confirms a temperature-dependent leak.
2. Running bubble test. Cap off, engine at fast idle, watch the surge tank for continuous fine bubbles and a rising level. Steady bubbling is combustion gas; intermittent burping can be an air pocket.
3. Repeat the combustion-gas test hot and under load, or use a coolant sample analysed for combustion by-products. Hot and loaded is the condition that matters.
4. Isolate the EGR cooler. Loop the coolant hoses so the cooler is out of circuit, refill, run hot, and repeat steps 1–3. If the symptom disappears, the cooler is implicated.
5. Bench-test the cooler with air under water at low pressure. Any bubble stream is a fail - do not pressure it beyond its rating.
6. If the cooler proves good, pressure-test each cylinder with the cooling system full and the rocker cover off, looking for coolant pushed out at an injector cup, and check for a head gasket breach between cylinders with a compression/leak-down test.

Where a cylinder is suspected, stop running the engine - coolant in a cylinder can hydrolock it on the next start.

Authorisation is not diagnosis, and the two have to line up before you turn a wrench.

• Do the isolation test first. If looping the cooler out clears the symptom and the bench test bubbles, the authorisation matches the evidence and you replace it - plus gaskets, coolant, and a full system refill and bleed.
• If the cooler proves good, an EGR cooler replacement will not fix anything. Say so plainly, in writing: what you tested, what the results were, what remains to be tested, and the risk of running it - coolant entering a cylinder and hydrolocking the engine, or a warped head if it keeps running hot.
• Do not sell a 'while we're in there' guess. Offer the next diagnostic step as a priced line item and get a signature either way.
• If pressed to just replace the cooler, record on the repair order that the replacement was customer-directed against the technician's recommendation, so the comeback is not yours.

If the cooler is replaced, road-test under load afterward and confirm the coolant level holds and temperature stays normal - the original complaint has to be gone, not just the part.