Parts for your 1991 Mitsubishi Pajero-Knock sensor

1991 Mitsubishi Pajero knock sensor — do you have one and what does it do?

Based on Mitsubishi’s factory Workshop Manual for 1991 Pajero/Montero (Engine Electrical, Group 13A) and the MMC ASA/EPC parts catalogue, a knock sensor is fitted to 1991 Pajero models running the 3.0‑litre 6G72 EFI petrol V6 (early V20 series). Carburetted petrol models (such as many 4G54 2.6) and the diesel variants (4D56 2.5) were not equipped with a knock sensor in that era. Diesels don’t use a conventional spark‑ignition strategy, and the carb’d petrol setups relied on mechanical/vacuum advance without ECU knock control, so a sensor wasn’t part of those systems.

For owners of a 6G72‑equipped 1991 Pajero, the knock sensor is a small piezoelectric microphone threaded into the top of the engine block (beneath the intake plenum, in the “valley”). Its job is to “listen” for detonation (pinging) and let the ECU trim ignition timing to protect the engine while keeping performance and fuel economy tidy on Aussie and Kiwi fuels.

What it does day‑to‑day: the sensor feeds a signal to the ECU, if knock is detected under load or on hot days, timing is pulled back momentarily. If the sensor or its wiring fails, the ECU usually plays it safe, runs retarded timing and you’ll notice doughy throttle response, poor fuel economy, or a check‑engine light.

Signs it’s worth a look:

• Audible pinging under load or climbing hills
• Sudden loss of grunt, higher fuel use, or a CEL with a knock‑sensor code
• Intermittent misfire or hesitation after hot soaks

Servicing and replacement tips for the 6G72:

• Inspect the harness: the valley gets hot and brittle, cracked conduit or oil‑soaked plugs are common culprits.
• Access requires removing the upper intake plenum on most models. Have new plenum gaskets on hand and label vacuum hoses before you start.
• Fit the new sensor to a clean, dry boss, no sealant on the mating face. Torque to the factory spec (typically around the high‑teens to mid‑20s N·m, check your manual for your exact spec).
• Route the lead away from ignition coils and high‑tension leads to avoid interference.
• After replacement, clear ECU memory and take it for a gentle adaptation drive on decent‑quality 95–98 RON petrol.

Why some 1991 Pajeros don’t have one: diesel models manage combustion differently and the classic diesel “knock” isn’t the detonation a petrol ECU can correct with timing, so there’s no benefit. Older carb petrol setups use mechanical/vacuum advance only, without closed‑loop ignition control, a knock sensor wouldn’t have a controller to talk to.

Popular questions

Where is the knock sensor on a 1991 Pajero 3.0 V6?

On the 6G72 EFI V6 it’s mounted in the top of the engine block, under the intake plenum in the V of the engine. You’ll usually need to remove the upper plenum to see and reach it. Look for a single sensor with a short lead heading into the main engine harness.

Do diesel 1991 Pajeros have a knock sensor?

No. The 4D56 2.5‑litre diesel doesn’t use a knock sensor. Diesel combustion isn’t controlled by spark, so the ECU can’t trim timing the way it does on petrol engines, Mitsubishi didn’t fit a knock sensor to those models in 1991.

Can a bad knock sensor harm the engine?

Indirectly, yes. The ECU will usually run conservative timing when the sensor fails, which protects the engine but makes it feel sluggish. If the sensor is giving a weak or noisy signal and the ECU doesn’t catch detonation, persistent pinging can stress pistons and bearings. If you hear pinging, fix the cause before racking up more kilometres.