Electronic Diagnostics: Toyota Prius II 1.5

Hybrid vehicles are all the rage today, but Toyota has been producing its Prius hybrid since 1997.

Kim Henson and Edward Haggar check out the diagnostic aspects of a second-generation example.

The concept of combining an electric motor with an internal combustion engine to minimise exhaust tailpipe emissions and fuel consumption was pioneered by Toyota in its Prius, introduced in Japan 21 years ago. This was the world’s first mass-produced hybrid vehicle and, since 2000, the Prius has been sold worldwide.

The first-generation (NHW11) models were produced from 2000 to 2003, with longer, more spacious, more powerful and more efficient second-generation (XW20) cars following. Third-generation (XW30) Prius models were produced from 2009 to 2015, eventually ousted by the fourth-generation (XW50) version.

Propelling the five-door hatchback is a conventional 1.5-litre four-cylinder petrol engine with twin overhead camshafts, working in conjunction with a three-phase brushless electric traction motor. The two motors complement each other to produce optimum efficiency in all situations. In normal driving with fairly light throttle openings, the petrol engine provides most of the power required, but is assisted when needed by the high voltage (HV) power produced by the electric system.

The Prius is as easy to drive as any conventional automatic and provides plenty of space for passengers and luggage, although boot space is a little restricted due to the battery pack behind the rear seats.

Older examples tend to be cheap to buy, although it should be noted that, as these cars age, battery life and performance deteriorates. Having said that, the petrol engine and the electric motor, plus the drivetrain, show typical attention to detail, and almost always provide excellent reliability and the longevity for which Toyotas are renowned.

The subject of this feature is a 2005 example fitted with Toyota’s 1NZ-FXE 14 type 1.5-litre petrol engine and TCCS management system. Edward Haggar is our guide to this model’s diagnostics.

The petrol engine and its management system can be tackled in similar ways to those applied to more traditional vehicles. However, the electric propulsion system does require specialist knowledge and equipment. For safety reasons, as well as to avoid damaging the vehicle, the battery and electrics should only be touched by mechanics who are technically qualified and experienced in these areas. In fact, the electric motor, cables and high-voltage battery pack (273 volts) seldom require attention.

Preparation

There is a common misconception that the Prius is a complex car that is difficult to look after. In fact, most servicing aspects are straightforward, at least in respect of the petrol engine.

Before carrying out any diagnostic work, find out when the vehicle was last serviced and, if in any doubt, carry out a full service, renewing the spark plugs, filters and engine oil. Officially, the car should then be serviced at least every 10,000 miles or annually, whichever comes first.

Be aware that, by design, the petrol engine starts up automatically to recharge the battery, and this can be alarming if you are not expecting it. It’s essential that you ensure that the engine cannot start up on its own after you have, say, drained the engine oil.

There is no timing belt to worry about as the Prius engine employs a chain. However, it pays to inspect the external auxiliary drivebelt at each service.

The Prius has two independent/ divided cooling systems, and both need to be carefully looked after in terms of coolant level and antifreeze mixture.

There’s a traditional cooling system serving the petrol engine, which uses a conventional water pump to circulate the coolant. In addition there’s a smaller capacity system for the gearbox/electric drive system, which is equipped with an electric water pump. The coolant should be renewed every 100,000 miles or 10 years, whichever comes first.

Cooling system efficiency is crucial to prevent overheating of the electric motor/generator/inverter, which could effectively write-off the car.

The car’s high voltage motive battery pack (smaller than in the first-generation

cars) is situated behind the rear seats, and there is an additional 12-volt battery in the boot. The high-voltage system cannot be used for jump-starting, however the standard 12-volt battery is fine for this.

The Prius does not have a conventional starter motor. Instead, the engine is turned over using the generator/highvoltage battery and at a much higher cranking speed than normal systems. The standard 12-volt battery supplies all the various modules with adequate operating voltage during the starting and running phases. It is kept charged by the generator as there’s no conventional alternator.

IGNITION AILMENTS

Our first fault with this Toyota manifests itself as an engine misfire, especially when the engine is pulling hard. Initially, the problem may take the form of just occasionally missing a beat, but this can develop into a continuing misfire.

The distributorless ignition system employed on this Prius incorporates separate pencil-type coils and, in each case, the firing stage is built into the coil. As time goes on, secondary spark weakness can arise, with attendant loss of ignition energy at the spark plug tip.

It’s essential that the correct specification iridium-type spark plugs are fitted, and they must be clean and in good condition. Toyota’s mileage recommendation for renewal of the plugs is 60,000 miles, but if the car is used often for short runs, and especially if misfiring has already taken place, it will be necessary to renew all four plugs much sooner.

The management system continually monitors the ignition system for misfires, and by assessing the speed of rotation of the crankshaft, the individual contribution from each cylinder can be established.

An easy and handy check is to swap the suspect coil with one from another cylinder, to see if the problem then affects the cylinder to which the dodgy coil has been moved.

Internal switching ailments can also affect the coils; to assess the signals to and from each coil, an oscilloscope is needed.

Note that, in this set-up, an ignition feedback signal is used to confirm that the coil has fired successfully. If there are problems with this signal, the engine may refuse to start.

DEFECTIVE/DIRTY MASS AIRFLOW SENSOR

Our next fault with the Prius is poor/ hesitant performance, accompanied by illumination of the system warning lamp. Fault codes may be stored relating to a

random misfire or possibly to a diagnostic trouble code (DTC) ‘P0101’, indicating a performance problem with the mass airflow sensor. It may be possible to use a generic scan tool to provide fault code information and real-time data.

If high fuel trim percentages are shown, this indicates that the engine ECU is incorrectly assessing the amount of fuel required by the engine. If codes are registered relating to circuit faults, carry out routine tests covering aspects such as supply voltages and signal voltage levels.

It is worth bearing in mind that often the root cause of such problems can be traced to contamination of the mass airflow sensor, resulting in the precision resistors employed miscalculating the signals being sent to the ECU. The engine then has to run with an over-lean fuel mixture, causing the below-par performance issues. Careful cleaning of the sensor will usually cure the trouble.

With the air filter casing released and removed, gently apply carburettor cleaner fluid to remove oily dirt and possible fragments of the air filter element that have become attached to the sensor. Note that it is always wise to use OE filter elements to minimise the possibility of them breaking up and contaminating the system.

THROTTLE BODY DIFFICULTIES

One particularly frustrating fault is the stalling of the engine just after starting and/or when the car slows in traffic. This may simply be due to a build-up of dirt within the motorised throttle body assembly. In particular, a heavy coating of carbon around the butterfly flap can reduce the passage of air that is required to keep the engine idling.

Remove the housing, then very carefully clean the butterfly flap and the throat of the housing to restore normal idling performance. When carrying out the cleaning operations, use a soft, clean, lint-free cloth, rather than spraying large quantities of cleaning fluid directly into the throttle, which introduces an excess of fluid into the engine.

FAILING 12-VOLT BATTERY

The condition of the 12-volt battery, as well as the high voltage propulsion battery pack, is extremely important. Often, simple starting problems with Priuses have been traced to failure of the 12-volt battery, although many people assume that the car’s main high-voltage battery pack is at fault.

Having said that, Japanese batteries tend to last well. In our experience, five years is a typical lifespan for a battery before faults arise, although our 2005 car here appeared to still have the originalfit battery. A good battery tester will determine the battery state, but it’s worth using a basic voltmeter to check for 12-volts – remember the system won’t operate without this level of voltage.

If replacing the battery, it’s always

worth fitting a high-quality replacement, and main dealer prices have become a lot more competitive in recent times. As usual with any battery that is connected negative earth to the vehicle, tackle the negative connection first, then the positive.