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8. TUNING AND DIAGNOSIS OF MPFI ENGINES - II

In the previous lesson we studied the tuning and diagnosis of MPFI engines. We now know that a MPFI engine is controlled by a computer which not only checks itself but also diagnoses faults in the engine system and indicates to the driver or mechanic what to service and when. Hence it is easier to diagnose faults in the MPFI engine. In this lesson we will study some major engine troubles, their likely causes, and what all to check or inspect when a fault is reported.

8.0

Objectives

After going through this lesson, you will be able to: i. Diagnose the cause of engine trouble. ii. Decide upon which component to check or inspect to isolate the cause. iii. Become acquainted with precautions to be taken so that the ECM is not damaged while working on MPFI engines. iv. Understand some dos and don’ts for MPFI engines.

8.1

Introduction

Engine troubles are basically the same, whether it is a carbureted engine or an MPFI engine. Sometimes the cause may also be similar though in an MPFI engine the basic control system is much more advanced, being computerized, hence the method of fault diagnoses and rectification will differ from that of a carbureted engine. In this lesson we will look at some troubles generally reported and that need attention. Some of this knowledge is not specific to MPFI engines only and may well be useful for a carbureted engine also.


8.2

Engine Troubles and their Causes

8.2.1 Hard starting (Engine cranks OK) Possible cause

Check or inspect

Ignition system out of order

• Faulty spark plug. • Leaky high-tension cord • Loose connection or disconnection of high tension cords or lead wires • Faulty ignition coil • Cracked rotor or cap of distributor (F8D). • Faulty igniter (F8D)

Fuel system out of order

• Dirty or clogged fuel hose or pipe • Malfunctioning fuel pump • Air inhaling from intake manifold gasket or throttle body gasket

Engine and emission control system out of order

• • • •

Faulty CMP sensing rotor Faulty idle air control system Faulty ECT sensor or MAP sensor Faulty ECM

Low compression

• • • • • • •

Poor spark plug tightening or faulty gasket. Compression leak from valve seat Sticky valve stem Weak or damaged valve springs Compression leak at cylinder head gasket Sticking or damaged piston ring Worn piston, ring or cylinder

Others

• Malfunctioning PCV valve

8.2.2 Engine has no power Possible cause Engine overheating.

Check or inspect • • • • • • • • • •

Inoperative thermostat Poor water pump performance Clogged or leaky radiator Improper engine oil grade Clogged oil filter or oil strainer Poor oil pump performance Dragging brakes Slipping clutch Blown cylinder head gasket Faulty radiator fan control system


Ignition system out of order

• • • •

Fuel system out of order

• Fuel pressure out of specification o Dirty fuel filter o Dirty or clogged fuel hose or pipe o Malfunctioning fuel pressure regulator o Malfunctioning fuel pump

Engine and emission control system out of order

• Faulty injector • Faulty TP sensor, ECT sensor or MAP sensor • Faulty ECM

Low compression

• • • • • • •

Others

• Dragging brakes • Slipping clutch

Defective spark plug Worn distributor cap terminals or rotor (F8D). Faulty ignition coil Leaks, loose connection or disconnection of hightension cord

Poor spark plug tightening or faulty gasket. Compression leak from valve seat Sticky valve stem Weak or damaged valve springs Compression leak at cylinder head gasket Sticking or damaged piston ring Worn piston, ring or cylinder

8.2.3 Improper engine idling or engine fails to idle Possible cause

Check or inspect

Ignition system out of order

• • • •

Fuel system out of order

• Fuel pressure out of specification o Malfunctioning fuel pressure regulator

Engine overheating.

• • • • • • • • • •

Engine and emission

• Faulty idle air control system

Faulty spark plug Worn distributor cap, terminals or rotor (F8D) Faulty ignition coil Leaks, loose connection or disconnection of hightension cord

Inoperative thermostat Poor water pump performance Clogged or leaky radiator Improper engine oil grade Clogged oil filter or oil strainer Poor oil pump performance Dragging brakes Slipping clutch Blown cylinder head gasket Faulty radiator fan control system


control system out of order

• • • •

Faulty evaporative emission control system Faulty injector Faulty ECT sensor, TP sensor or MAP sensor Faulty ECM

Low compression

• • • • • • •

Poor spark plug tightening or faulty gasket. Compression leak from valve seat Sticky valve stem Weak or damaged valve springs Compression leak at cylinder head gasket Sticking or damaged piston ring Worn piston, ring or cylinder

Others

• Malfunctioning PCV valve

8.2.4 Engine hesitates (Momentary lack of response as the accelerator is depressed. Can occur at all vehicle speeds. Usually most severe when first trying to make the vehicle move, as from a stop.) Possible cause Ignition system out of order

Check or inspect • Spark plug faulty or plug gap as out of adjustment • Leaky high-tension cord

Fuel system out of order • Fuel pressure out of specification o Clogged fuel filter o Faulty fuel pressure regulator • Clogged fuel filter, hose or pipe Engine overheating Engine and emission control system out of order

• Faulty injector • Faulty TP sensor, ECT sensor or MAP sensor • Faulty ECM.

Low compression

8.2.5 Surges (Engine power variation under steady throttle or cruise. Feels like the vehicle speeds up and down with no change in the accelerator pedal.) Ignition system out of order

Fuel system out of order

Engine and emission control system out of order

• Defective spark plug (excess carbon deposits, improper gap, and burned electrodes, etc.) • Leaky or loosely connected high-tension cord • Cracked rotor or cap in distributor (F8D) • Variable fuel pressure o Clogged fuel filter o Kinky or damaged fuel hose and line o Faulty fuel pressure regulator • Faulty MAP sensor • Faulty injector • Faulty ECM


8.2.6 Excessive detonation (The engine makes sharp metallic knocks that change with throttle opening. Sounds like pop corn popping.) Possible cause Engine overheating Ignition system out of order

Check or inspect • Faulty spark plug • Improper ignition timing • Clogged fuel filter and fuel line • Faulty ECT sensor or MAP sensor • Faulty injector • Faulty ECM Excessive combustion chamber deposits

Fuel system out of order Engine and emission control system out of order Others

Self-check Questions Give reasons for the following 1. Hard starting can be due to both air inhalation and compression leakage. 2. Engine overheating can be due to improper lubrication. 3. Improper engine idling can be due to malfunctioning PCV valve. 4. Engine hesitates due to leaky high-tension cord. 5. Engine surges due to leaky or loosely connected high-tension cord 6. Detonation could be due to excessive combustion chamber deposits

8.2.7 Poor mileage Possible cause Ignition system out of order

Engine and emission control system out of order Low compression Others

Check or inspect • Faulty spark plug (improper gap, heavy deposits, and burned electrodes, etc.) • Leaky or loosely high-tension cord • Cracked rotor or cap in distributor (F8D) • Fuel pressure out of specification • Faulty TP sensor, ECT sensor or MAP sensor. • Faulty injector • Faulty ECM • • • •

Dragging brakes Slipping clutch Thermostat out of order Improper tyre pressure

8.2.8 Excessive engine oil consumption Possible cause

Check or inspect

Oil entering combustion • Sticky piston ring chamber • Worn piston and cylinder • Worn piston ring groove and ring • Improper location of piston ring gap


• Worn or damaged valve stem seal • Worn valve stem

8.2.9 Low oil pressure Possible cause

Check or inspect • • • • • •

Improper oil viscosity Malfunctioning oil pressure switch Clogged oil strainer Functional deterioration of oil pump Worn oil pump relief valve Excessive clearance in various sliding parts

8.2.10 Engine noise Note: Before checking the mechanical noise, make sure that: • • •

Ignition timing is properly adjusted. Specified spark plug is used. Specified fuel is used. Possible cause Valve noise

Piston, ring and cylinder noise Connecting rod noise

Crankshaft noise

• • • • • •

Check or inspect Improper valve lash Worn valve stem and guide Weak or broken valve spring Warped or bent valve Loose camshaft housing bolts Worn piston, ring and cylinder bore

• • • • • • • • •

Worn crankpin bearing Worn crankpin Loose connecting rod nuts Low oil pressure Low oil pressure Worn crankshaft journal bearing Worn crankshaft journal Loose lower crankcase (bearing cap) bolts Excessive crankshaft thrust play

8.2.11 Excessive Hydrocarbon (HC) emission or excessive Carbon-monoxide (CO) emission Possible cause Ignition system out of order Fuel system out order Engine and Emission

• • • of •

Check or inspect Defective spark plug Leaky or loosely fitted high-tension cord Cracked rotor or cap in distributor (F8D) Fuel pressure out of specification

• Lead contamination of three way catalytic


control system out of order

• • •

• •

converter. Malfunctioning PCV valve. Faulty EVAP control system Closed loop system (A/F feed back compensation) fails o Faulty TP sensor o Faulty ECT sensor or MAP sensor o Faulty oxygen sensor Faulty injector Faulty ECM

Low compression

8.2.12 Excessive Nitrogen oxides (NOx) emission Possible cause

Check or inspect

Fuel system out of order • Fuel pressure out of specification Engine and emission • control system •

Lead contamination of three way catalytic converter Closed loop system (AI F feed back compensation) fails (vehicle with oxygen sensor) o Faulty TP sensor o Faulty ECT sensor or MAP sensor o Faulty oxygen sensor • Faulty injector • Faulty ECM

Self-check Questions Give reasons for the following7. Poor average can be due to improper tyre pressure. 8. Excessive engine oil consumption can be due to improper location of piston ring end gap 9. Low oil pressure could be an indication of worn engine. 10. Engine noise can be due to improper valve lash 11. Excessive emissions could be due to fuel pressure out of specification.

8.3

Inadvertent Damage to the ECM ECM may get damaged in the following cases

• • • • • •

While connecting and disconnecting ECM coupler with ignition switch ON If tester is connected to the ECM with its coupler disconnected. Connecting an ohmmeter to the ECM with its coupler connected. By touching electrical terminals of ECM by hand (static electricity from body can damage these parts). By removing negative terminal of battery with ignition switch ON. By short-circuiting specific terminals of relays, sensors etc.


8.4 • • • • • •

8.5

If power of TP and MAP sensor circuits gets shorted to ground.

Dos and Don’ts for MPFI Vehicles Do not push start. Always use unleaded petrol, Never disconnect negative terminal of battery when engine is running. Do not start engine by pressing the accelerator pedal. Only those accessories whose couplers are provided should be fitted. Minimum battery voltage should be 11 V for any kind of inspection.

Home Assignment

1. Revise the causes for each engine trouble given in tables above and try to reason out why it is responsible for the trouble.

8.6

Summing Up

In the last 3 lessons we studied engine tuning and diagnosis. Engine tuning is an important job a mechanic is called upon to perform regularly. Hence you should become proficient in it as early as possible. Even when the engine is properly tuned, it needs to be attended to at regular intervals to ensure that its performance does not deteriorate with time. This is taken care of by Periodic Maintenance. Periodic maintenance services will be our subject of study in the next lesson.

8.7

Possible Answers to Self-check Questions

1. excess or deficient air will not allow formation of combustible mixture 2. improper lubrication will generate more friction and hence engine overheating 3. PCV valve directs proper amount of crankcase gases into the engine cylinder for re-combustion; a malfunctioning PCV valve may cause excess crankcase gas to enter the cylinder and cause improper engine idling if the proportion of fresh charge to crankcase gases is less. 4. a leaky HT cord leaks high voltage energy required for sparking and hence engine may miss 5. a leaky HT cord behaves erratically and hence the engine may fire properly sometimes and not at others 6. excessive deposits decrease the cylinder volume and cause greater pressures inside the engine cylinders causing the end charge to detonate 7. low tyre pressure puts load on the engine, hence low fuel average 8. piston ring end gaps are staggered; if they happen to line up, oil may get pumped up in the combustion chamber thus increasing oil consumption 9. in a worn engine, clearances between shafts and bearings increases hence low oil pressure will be recorded as oil will come out of the bearings 10. greater valve lash will cause the rockers to hit the valves when contacting them for opening 11. high or low fuel pressure will cause rich or lean mixture to be supplied; when


mixture is rich more CO-HC will be produced, when mixture is lean more NOx will be produced.

8.8

Terminal Questions

1. List a few of the engine troubles generally reported and give their causes. 2. List the actions which may cause damage to the ECM. 3. List the dos and don’ts for an MPFI vehicle.

8.9

References

1. Maruti MRF 308/410 Service Manual.

8.10 Glossary PCV valve

Positive crankcase ventilation valve, which permits calibrated amounts of blow-by gases from the crankcase to flow into the intake manifold for re-combustion.

Detonation

The action of the fuel charge firing or burning too violently, almost exploding. It sometimes results in a noise called "pinging." Detonation is caused by auto-ignition of the "end gas" i.e., that part of the charge not yet consumed in the normal flame-front reaction. Detonation occurs because piston motion and compression of the end gas raise its temperature and pressure to the point where the end gas auto-ignites. The pinging or knocking noise is the result of intense pressure waves in the charge which cause the cylinder walls to vibrate. Also called "fuel knock."

Emission

The passing of gases and other toxic substances into the atmosphere.

Catalytic converter

A pollution-control device found in the exhaust system of automobiles which acts like an afterburner to re-burn unburned gas in the exhaust. It looks like a small muffler and is usually made of stainless steel. It contains platinum, rhodium, or palladium which is a catalyst for the chemical reaction needed to burn off any unburned hydrocarbons and carbon monoxide by turning them into water vapor, carbon dioxide and other less toxic gases.

EVAP control system

A system for reducing evaporative emissions by means of a sealed fuel tank, a vapour-liquid separator, a three-way valve, an activated carbon filter, and a network of interconnecting hoses. Also called "evaporation control system"

Unleaded petrol

Petrol which has no tetraethyl lead mixed in it. TEL was earlier used to reduce knocking in petrol engines.


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