Is oil in the turbo hose normal? Telling a normal film from a fault

Yes, some is. A little oil inside the turbo hoses and the charge air cooler (intercooler) pipes is normal on an engine with closed crankcase ventilation. Service bulletins from Isuzu and Hino say so plainly. What points to a fault is the amount: oil pooled in a hose or in the cooler, rising oil consumption, blue smoke. Even then the turbocharger is not the first suspect.

Where does the oil in the turbo hose come from?

The thin film in the hose comes from the crankcase ventilation. Blow-by, the gas that gets past the piston rings into the crankcase, is not vented to the open air but fed back into the intake, and oil vapour goes with it. Isuzu gives the reason in a service bulletin for its diesel trucks:

“Unfortunately, blowby gas production and air intake demands do not always match which allows some oil vapor from the crankcase to enter the intake system.”

Isuzu, service bulletin IB14-J-002B, October 2021

The vapour joins the intake ahead of the turbo. BTN Turbo’s instruction for the VAG 1.9 TD places the breather pipework “between the turbo and air filter box”, and Isuzu’s warning about a restricted air filter points the same way. The compressor then carries the vapour into the pressure side: the turbo hoses to the intercooler and on to the inlet manifold.

How much oil counts as normal?

Neither bulletin gives a figure. The test is whether the amount is excessive. Hino’s tech tip on turbocharger replacement says:

“It is normal to find some engine oil at the turbocharger outlet, within the CAC (charge air cooler) piping, or inside of the CAC.”

Hino Trucks, Tech Tip TT-17-015, 3 October 2017

The next sentence sets the threshold: “Seeing oil in these areas is not a cause for concern unless the amount of oil is grossly excessive.” Hino shows the difference with photographs, not a measurement. As we read them, the pipes shown as normal are wet and dark inside; those captioned “heavy oil saturation” show a compressor inlet and a hose coated in black oil.

Isuzu makes the same point about diagnosis: “Oil in the intake system is common for a closed PCV system and is not a good indicator of a turbocharger failure.” Both bulletins were written for truck diesels: Isuzu’s N-Series and F-Series, and Hino trucks with J05 and J08 engines. We did not find an equivalent document from a car maker; where the vehicle’s own service information sets a limit, that limit applies.

Drivers say much the same. On a Turkish car forum, the owner of a diesel with more than 200,000 km on it asked about oil at the turbo hose. The replies called a light sweat normal on two conditions: no oil standing in the hose, no visible drop in the oil level. Two suggested cleaning the hose and seeing how soon it looks the same again.

When is the oil a sign of a fault?

Oil is a sign of a fault when it collects, or when other symptoms come with it. MAHLE’s turbocharger damage brochure lists the effects of an oil leak at the turbo: blue smoke from the exhaust, engine oil accumulated in the intake section and the charge air cooler, loss of power. With increased oil consumption and blue smoke, it says, the turbo belongs in the search for the cause.

It belongs there, but not first. Of the condition it calls heavy saturation, Hino writes: “This is not usually caused by a faulty turbocharger and further inspection is required.” The return pipe and crankcase pressure have their own article, and so does the oil left in the intercooler after a turbo failure.

What does oil on the outside of the hose mean?

Oil weeping from a hose joint points to that joint, not to the turbo. Hino’s checklist has the intercooler hoses inspected for looseness and leaks, and adds: “Oil leaking from an intercooler hose is not indicative of a failed turbocharger.”

The bulletins stop there; what follows is our conclusion from those two sentences. If a film inside the hose is normal, the oil outside is that film coming out past a loose clamp or through a split, and air may be escaping at the same spot. According to Isuzu, an air leak makes intake pressure unstable, and oil can then get past the seal on the turbo shaft.

Isuzu’s remedy is to listen for high-pitched noises, then repair or replace loose or damaged hoses and clamps. Under the manufacturers’ instructions, the sweat that drivers call normal still gets a check: of the joint, not of the turbo.

Where do you look first when there is too much oil?

When oil has pooled, the checks start away from the turbo. The items come from the Isuzu and Hino bulletins, easiest first. Isuzu’s note on its own list: if one of these conditions is found, the turbocharger “most likely is OK and should NOT be replaced”.

1. Oil level and condition. Oil above the maximum mark raises crankcase pressure and pushes oil through the ventilation into the intake. Overdue or fuel-diluted oil aerates, with the same result; Isuzu’s sign is a milky colour. Hino adds that thinner oil means more oil in the charge air pipes.
2. Air filter. A restricted filter creates a vacuum in the intake and draws excess oil through the ventilation. Replace it.
3. Crankcase ventilation. Check the hose and the oil separator for blockage. BTN Turbo names some BMW and Land Rover diesels whose breather clogs over time, and recommends a new breather filter with every replacement turbo.
4. Hoses and clamps. Loose clamps, split or cracked hoses.
5. How the engine runs. According to Hino, a misfire, uneven injection or low compression can make the turbo shaft shudder and carry over more oil. Its instruction: “Do not replace the turbo for this condition.”
6. The turbo, last. Hino’s rule: no replacement unless liquid oil is confirmed together with a mechanical failure of the turbo. Its inspection looks for a wheel rubbing on the housing, damaged blades, and a shaft that binds or has excessive play.

What should you do?

• A thin film, no other symptom: wipe the hose, check the clamps, watch the oil level. If the film is soon back or the engine uses oil, go through the checks.
• Oil weeping at a joint: check the clamp and the hose; replace whichever is damaged.
• Oil pooled in a hose or in the intercooler: drain the intercooler, then work through the checks in order.
• Pooled oil with blue smoke or oil consumption: add the return pipe and crankcase pressure to the checks, and judge the turbo by measurement.