Malfunctions in the car (Audi 100 C4)

Difficult start

The crankshaft turns, but the engine does not start for a long time. Eventually, the engine starts, or it may be that the engine stalls shortly after starting.

Perform a check:

• Fuel for the presence of water impurities.
• Fuel system pressure.
• Throttle position sensor sticking; the sensor signal voltage measured by the voltmeter must be below 1.25 V.
• Fuel pump relay. Connect a test lamp between circuit 120 of the pump relay and ground. The lamp should light for 2 seconds after the ignition is turned on.

Check the fuel pump shutoff valve in the fuel tank. The valve allows fuel to flow from the fuel line back into the tank after the engine is turned off. The following procedures are required for checking:

• Turn off the ignition.
• Disconnect the fuel line at the filter.
• Remove the gas cap.
• Connect the pump used to test the radiator to the fuel line and increase the pressure to 103 kPa. If the pressure does not drop within 60 seconds, the shut-off valve is OK.

Check the ignition system for:

• Compliance of the ignition system output voltage with the norm using a control device.
• No wear on the distributor shaft.
• No short circuits or damage to wire insulation.
• Idle air supply control system.
• Throttle shaft and rods for sticking. When sticking, the signal voltage from the throttle position sensor may be high (which indicates that the throttle valve is open), in connection with which, the control unit will not be able to control the engine operation at idle. Measure the voltage of the sensor signal. The voltage with the throttle valve closed should be below 1.2.
• Absolute pressure sensor in the manifold with the engine off and the ignition on. Compare the measured value of the sensor signal voltage with the voltage measured on a serviceable vehicle. The voltage deviation should be no more than 400 mV. Another test option: start the engine and disconnect the absolute pressure sensor connector in the manifold at idle. If the engine operation improves, replace the sensor and retest.
• Air conditioner compressor or relay.
• The refrigerant pressure in the air conditioning system. If the pressure is too high, check for excess refrigerant and a faulty pressure switch.
• Cooling fan.
• Compression in the cylinders.
• Battery cables. Cables must be free of dirt and securely fastened. Current leaks can cause the idle air control valve to change position and cause poor engine performance. The valve will not operate if the system voltage is below 9 volts or above 17.8 volts.

Check the CO content in the exhaust gas using a suitable device. If the CO content is too low (below 0.3%), the fuel-air mixture is too lean and the signal voltage of the CO potentiometer will be low (below 0.06 V). As a result, the ECM reduces the amount of fuel supplied to the engine, which leads to difficulties with vehicle control.

Increased content of harmful substances in exhaust gases

Measurements show high concentrations of harmful substances; a rotten egg smell may appear. The appearance of an unpleasant smell is not necessarily a sign of improper combustion. Conduct a thorough inspection. Perform diagnostics of the circuits.

Check:

• No leaks into vacuum lines.
• Cooling system and fan.
• Remove carbon deposits using engine cleaner.

Instructions for use are given on the packaging.

If high NOx concentrations are detected during measurements, check for reasons why the fuel mixture is too lean or the combustion temperature is higher than normal.

If high concentrations of hydrocarbons and CO are detected during measurements or the exhaust has an unpleasant sound, check the reasons why the working mixture is too rich:

• Fuel pressure deviation from normal.
• Clogged or stuck positive crankcase ventilation valve or clogged system hose.
• Incorrect installation of the fuel tank cap.
• Defective spark plugs, spark plug wires or ignition system components.
• Incorrect ignition timing.

Self-ignition in cylinders

The engine continues to run after the ignition is turned off, but very unevenly. If the engine runs smoothly, check the ignition switch and ignition adjustment.

Check the injector for leaks.

Apply 12V to the test terminal of the fuel pump to turn it on; increase the pressure in the fuel system.

Inspect the injector and throttle body for fuel leaks.

Backfires

Fuel ignition in the intake manifold or in the exhaust system with a loud noise.



Check:

• Ignition coil output voltage.
• Ignition order, paying attention to the distributor cap, spark plug wires, and their correct placement.
• Setting the ignition timing.
• Serviceability of spark plugs, spark plug wires and boots.

Check compression (detection of sticking or loose valves).

Check the valve timing.

Check the serviceability of the valve mechanism levers.

Interruptions, dips, jerking

A short-term loss of engine sensitivity to pressing the gas pedal can occur at any speed. Usually most pronounced when trying to start moving after stopping. In severe cases, it can lead to engine stalling.

Conduct a thorough inspection.

Check:

• Fuel pressure.
• Fuel for the presence of water.
• Throttle position sensor sticking.
• Setting the ignition timing.
• Generator output voltage. The generator requires repair if the voltage is below 9 V or above 16 V.
• To break the line connecting to the ignition system ground.

Skipping Flashes

Constant jerking noise when the engine is running, usually increasing as the engine load increases. The exhaust pipe constantly makes splashing noises when idling or at low speeds.

Conduct a thorough inspection. If you suspect that misfires at idle and when driving are caused by a malfunction of the ignition system, perform the appropriate checks.

Identify the non-working cylinder:

• Start the engine. Disconnect the idle air supply system drive. Using the appropriate tool, disconnect one spark plug wire at a time.
• A drop in engine speed may occur when disconnecting the spark plug wire from each cylinder (at about 50 rpm) followed by uneven engine operation and even stopping. Turn off the engine, connect the idle air supply system drive.
• If disconnecting the spark plug wire from one or more cylinders does not cause a drop in engine speed or does not result in a significant difference in the degree of drop in engine speed, then check the suspected cylinder(s) for spark discharges using an appropriate device. If there is a discharge, unscrew the spark plug from that cylinder and check for the presence of: cracks, wear, incorrect gap, electrode burning, significant deposits.

Check the spark plug wire resistance (resistance should not exceed 30,000 ohms).



If the previous checks did not produce results, perform the following checks:

• Inspect the ignition system for moisture, dust, cracks, burns, etc. Spray the spark plug passages with water mist to detect short circuits.
• Checking the fuel system (clogged fuel filter, water in fuel, low pressure).
• Compression check.
• Checking the valve timing.
• Remove the valve rocker arm covers and check for bent pushrods. worn rocker arms, broken or deformed valve springs, worn cams. Make repairs if necessary.

Low fuel efficiency

Fuel economy measured in real world road test conditions is significantly lower than expected. In addition, fuel economy is significantly lower than what this car previously achieved under the same conditions.

Check:

• Thermostat for valve malfunction (always open) or deviation from a specified temperature range.
• Fuel pressure.

Get to know the owner's driving style.

Perform a check:

• Vacuum hoses for cracks, kinks and loose fittings.
• Tire pressure.
• Mass of transported cargo (overload loops).
• Applications when driving with excessively high or frequent accelerations.

Perform circuit diagnostics.

Check the air filter for contamination.

Check the speedometer calibration.

Check the spark plug wires for cracks, hardening, and correct connection.

Check the ignition timing.

Unscrew the spark plugs, check for dirt and cracks, wear, gap changes, electrode burnout and significant deposits. Replace if necessary.

Check compression.

Check the operation of the torque converter clutch.

Check the brakes for binding.



Recheck the fuel consumption with the fuel tank full.

Check for deposits in the exhaust system.

Uneven operation and engine stalling at idle

The engine runs unevenly at idle. In more serious cases, the car may vibrate. It is also possible that the rotation speed changes (oscillations). In this case, the engine may stop. Deviation from the normal rotation speed is possible.

Check:

• Setting the ignition timing.
• Parking, Neutral mode switch circuit.
• Injector for leaks.
• The resistance of the sensor coil and contacts is normal.
• Lack of loose connections of the ignition coil.
• No moisture under the distributor cap.
• No contamination or cracks in spark plugs, wear, gap changes, electrode burning or significant deposits.

Rpm fluctuations and/or popping noises in the muffler

Fluctuations in engine power with a constant throttle opening at idle or when driving. Subjectively felt as acceleration and deceleration of the car without changing the position of the gas pedal.

Check:

• Setting the ignition timing.
• Fuel filter on the fuel line for contamination and deposits.
• Fuel pressure.
• Generator output voltage. Repair is required if the voltage is below 9 V or above 16 V.
• Operation of the torque converter clutch (if any).

Check the CO content in the exhaust gases using a suitable device. If the CO content is too low (below 0.3%), the fuel-air mixture is too lean, resulting in a low CO potentiometer signal voltage (below 0.06 V). As a result, the ECM reduces the amount of fuel supplied to the engine, which leads to difficulties with vehicle control.



Unscrew the spark plugs. Check for dirt and cracks, wear, gap changes, electrode burnout and significant deposits. Also check the condition of the entire ignition system.

Loss of power, engine sluggishness, poor response to accelerator pedal input

The power developed by the engine is lower than expected. Complete or almost complete insensitivity of the car to partial pressing of the gas pedal.

Compare the behavior of the vehicle being checked with the behavior of a similar vehicle. Make sure that the complaint is justified.

Remove the air cleaner and check the degree of contamination of the air filter. Replace if necessary.

Check:

• Setting the ignition timing.
• The presence of contamination in the fuel filter, contamination in the fuel and deviations in fuel pressure from the norm.
• The condition of the lines connecting the control unit to the engine ground.

Check the exhaust system for obstructions:

• With the engine running and at normal temperature, connect the vacuum gauge sensor to any suitable vacuum port in the intake manifold.
• Measure the vacuum at 1000 rpm.
• Slowly increase the speed to 2500 rpm. Measure the vacuum when the speed is stable at 2500 rpm.
• If the vacuum level at 2500 rpm decreases by more than 21 kPa compared to the value measured at 1000 rpm, the exhaust system should be checked for obstructions.
• Disconnect the exhaust pipe from the engine and perform the procedures specified in the previous paragraphs. If the vacuum still drops by more than 21 kPa, check the valve installation.

Check the generator output voltage. The generator requires repair if the voltage is below 9 V or above 16 V.



Check engine valve installation and compression.

Check the camshaft for wear.

Check the operation of the torque converter (if any).

Check the ignition voltage with a suitable device.

Engine detonation

A knocking noise, moderate to severe in intensity, usually increasing with acceleration. A sharp metallic sound that changes as the throttle is opened.

Check for overheating.

• Low coolant level.
• Coolant pump belt loose.
• Restricted air or coolant flow through the radiator.
• Faulty or improperly adjusted thermostat.
• Electric fan circuit not working.
• Changing the sensitivity of the coolant temperature sensor.

Check:

• Fuel quality, its octane number.
• Correct selection of spark plugs according to thermal conditions.
• Setting the ignition timing.
• Fuel system pressure (is there a fall).
• Fuel system.
• Correct adjustment of the gear shift system.
• Operation of the torque converter clutch (if any).
• Correct selection of engine parts such as camshaft, heads, pistons, etc.
• Oil flow into the combustion chamber (is there any excess).

Remove carbon deposits using engine cleaner. Instructions for use are provided on the packaging.

Increased oil consumption

Increased oil consumption may be caused by one of the following reasons.

Leaks in the lubrication system. Tighten threaded connections and/or replace gaskets and seals, if necessary.

Incorrect determination of the oil level using the dipstick. When determining the oil level, place the car on a level surface and wait for sufficient time for the oil to drain into the pan.

Idle speed is too low. If the engine idle speed is not controlled by the electronic unit, adjust the idle speed according to the requirements and.

The installed pressure sensor type does not match. The pressure sensor is faulty.

Incorrect type of pressure gauge installed. Defective pressure gauge. Replace the pressure gauge.

Incorrect oil viscosity or diluted oil. Fill with oil for the appropriate season, or replace oil that has been diluted due to moisture or unburned fuel mixture.

Worn or dirty oil pump. Replace oil pump.

Dirty oil filter. Replace oil filter.

The mesh filter on the oil pump inlet pipe has become loose or clogged. Clean and/or wash the mesh filter.

The side hole in the oil pump inlet pipe is clogged. Replace the pipe.

Oil viscosity is not within specification. Use SAE standard oil for the corresponding operating temperature.

Long periods of driving at high speeds or with a trailer will usually result in increased oil consumption.

Malfunction in the forced crankcase ventilation system.

Wear of the oil-reflecting caps and / or valve guide bushings, or lack of oil-reflecting valves. Bore the guide bushings and install larger diameter valves and / or new oil-reflecting caps.

Poorly seated, worn or broken piston rings. Allow new piston rings to bed in for some time. Replace broken or worn piston rings if necessary.

Incorrect piston installation or incorrect size.

Low oil pressure

Low oil pressure can be caused by the following reasons:

• Excessive clearance in the bearing. Replace the bearing if necessary.
• Cracks, pores, plugs in the cylinder block channels. Repair or replace the block.
• Lubrication system plugs are missing or installed incorrectly. Install plugs or replace if necessary.

Noise in the valve timing mechanism

Noise in the valve train may be caused by any of the following reasons:

• Low oil pressure. Eliminate the cause.
• Loose valve levers. Check and fix if necessary.
• Worn valve lifter and/or tappet.
• Broken valve springs, valves sticking.
• Worn, dirty or faulty valve lifters. Clean, check and replace if necessary.
• Worn or defective timing shaft. Replace shaft.
• Worn valve guides.

Knocking in a cold engine, lasting two to three minutes and/or increasing with increasing load

The flywheel is touching the casing. Adjust the casing installation.

The balancer or drive pulley has come loose or broken.

Increased clearance between pistons and cylinders. Rebore the cylinder and hone to size. Replace the piston. Piston knocking on a cold engine usually disappears after grinding. Piston knocking on a cold engine that disappears after 1.5 minutes is acceptable. Bent connecting rod.

A loud knocking noise from a warm engine that occurs under load

Balancer or pulley hub failure. Replace if necessary.

Loosening automatic transmission bolts.

Accessory drive belts too tight. Replace or adjust tension.

The exhaust pipe touches the road surface. Secure the exhaust pipe.

Crack in the flywheel.

Increased clearance in main bearings. Replace if necessary.

Increased clearance in connecting rod bearings. Replace if necessary.

A light knock from a warm engine

Detonation. Check fuel quality.

Check the ignition setting as well.

Loosening automatic transmission bolts.

Exhaust manifold leak. Tighten bolts and/or replace gasket.

Increased clearances of connecting rod bearings. Replace bearings if necessary.

Knocking noises when starting the engine, lasting only a few minutes

Oil of the wrong viscosity. Fill with oil of the correct viscosity for the operating conditions. When the engine is not running, some valves will be in the open position. The spring pressure on the hydraulic valve clearance adjustment mechanism will cause the hydrostatic pressure to bleed off. Repair is only required if the problem persists.

Increased axial clearance of the crankshaft - replace the thrust bearing.

Excessive clearance in the front main bearing. Replace worn parts.

Knocking noises on a warm engine at idle

Poor tension or wear of drive belts. Tighten and/or replace as necessary.

Air conditioning compressor or generator bearing. Replace if necessary.

Gas distribution mechanism. Replace parts if necessary.

The viscosity of the engine oil does not meet the requirements. Fill in oil with a viscosity corresponding to the season of operation of the vehicle.

Too much clearance in the piston pin. Replace the piston pin and piston.

Connecting rod adjustment. Check clearances and replace the connecting rod if necessary.

Insufficient clearance between cylinder and piston. Hone the cylinder and fit a new piston.

The crankshaft counterweight has come loose. Tighten and/or replace worn parts.

The piston pin is offset from the axis to the other side. Install the piston correctly.

The engine gets very hot

Engine cooling system leak, transmission oil cooling system leak. Check for leaks and, if any, eliminate them. Check the expansion tank, hose, and radiator cap.

Drive belt is slipping or damaged. Replace belt or tensioner if necessary.

Thermostat stuck in closed position. Check and replace if necessary.

Operation of the electric fan.

Leak through the cylinder head gasket. Check and fix if necessary.