AI Mechanic

Symptom · Turbo diesels, any age

Judders under load with black smoke

Black smoke is unburnt fuel. Either there is too much fuel, or not enough air to burn what is there. Both sides are measurable in under an hour.

01Possible causes, none of them ranked yet

  • Boost leak on the pressure side

    A split hose or loose clamp lets charge air escape, so fuelling runs rich under load.

  • Sticking variable-vane turbo

    Carboned vanes cannot swing, so boost never reaches target when the engine asks for it.

  • Blocked air filter or intake

    A restricted intake starves the engine of air at exactly the moment it needs most.

  • EGR valve stuck open

    Exhaust gas dilutes the charge under load and the engine stumbles like a misfire.

  • One injector over-fuelling

    A worn nozzle sprays badly, which shows up as smoke and a shake under load.

  • Restricted diesel particulate filter

    Back pressure chokes the exhaust, boost falls and the engine judders at the top of the load range.

Nothing on that list is the answer yet. A measurement decides which ones survive.

02The tests, in the order a senior tech runs them

Test 1

Log actual boost against requested boost on a road test, pulling in a high gear from 1800 rpm.

tool
Scan tool with live data
expect
Actual within about 150 mbar of requested. A shortfall bigger than that is an air-side fault.

Boost on target moves the fault to fuelling or the exhaust. Boost short keeps the air side in front of you.

Test 2

Pressurise the intake system to 1 bar with the engine off and listen for the leak.

tool
Boost leak tester and a compressor
expect
Pressure holds for 30 s or more. A hiss locates the split hose or clamp.

A holding system rules out a plain boost leak and moves the turbo and EGR up.

Test 3

Clamp or blank the EGR pipe and repeat the same road test in the same gear.

tool
Blanking plate or line clamp
expect
Judder and smoke clearly reduced if EGR is the cause. No change means it is not.

This one test rules the EGR in or out for the price of a blanking plate.

Test 4

Read exhaust back pressure, or compare DPF differential pressure at idle and at 2500 rpm.

tool
Back pressure gauge or scan tool
expect
Under about 150 mbar at idle. A steep climb with revs points at a blocked filter.

High back pressure explains lost boost without any turbo fault at all.

Log boost before you touch a spanner. Half of these cases end at a split hose that costs less than an hour of labour.

Your car is not this car.

Open a Case with the vehicle and the symptom in front of you. It asks the two or three questions that narrow yours, then names one test with the value to expect, and re-ranks the list on what you measure.

Open your first Case

Free and complete. No account, no card. A Bay Pass after that is $12.00 for 90 days.

Other symptoms