Engine Performance Problems & Possible Causes (GROUP 25, NO. 93-04)

Publication date: 1993-04-30
Reference number: GROUP 25, NO. 93-04

ENGINE PERFORMANCE PROBLEMS & POSSIBLE CAUSES

TECHNICAL SERVICE BULLETIN

Reference Number(s): GROUP 25, NO. 93-04, Date of Issue: April 30, 1993

ENGINE PERFORMANCE

Model(s): 1989-91 Audi 100, Engine Code NF

Group: 25

Bulletin No.: 93-04

Date: April 30, 1993

SERVICE INFORMATION

This Technical Bulletin lists recommendations and background information regarding engine performance. Possible driveability related performance behaviors are listed by likelihood in the following tables in four categories.

Frequent

Less Frequent

Seldom

Not A Cause

POOR DRIVING PERFORMANCE

Use VAG 1551 Scan Tool to troubleshoot all engine performance complaints. For explanation on how to connect and use VAG 1551 Scan Tool, see Repair Manual, Repair Group D2.

TEST CONDITIONS

Engine basic setting OK
Perform automatic test sequence with VAG 1551, address word 00.
Erase DTC memory.
Test drive car for at least 5 min. During test drive push accelerator pedal down once all the way and drive car briefly at more than 4000 RPM.
CAUTION: Taking measurements while driving diverts attention from the road which can lead to injury or worse. It is difficult to read and record measurements accurately while driving. Always use a second person for this task.
Read DTC memory again and repair according to Repair Manual, Repair Groups D2, 25 and 28.
If it is necessary to replace parts, print-out DTC memory and attach to file copy of the repair order.
Use the following information to correct performance problems.

TABLE 1 - DRIVEABILITY PROBLEMS

Driveability Problem	Frequent	Less Frequent	Seldom	Not a Cause
Hard to start all the time	--	16	3, 5, 6, 7, 10, 11, 12, 13, 14, 15, 17, 18, 20	--
Hard to start when cold	--	5, 7, 16, 20	3, 6, ,13, 15, 18	--
Hard to start when hot	--	7, 16	3, 5, 12	--
Poor throttle response/ acceleration	--	16	1, 2, 3, 5, 8, 12, 14, 19, 22, 23	--
Engine misfires/stalls continuously	11	13, 14	1, 4, 6, 18	--
Engine misfires/stalls when accelerating	--	11	1, 2, 4, 16, 19	--
Engine misfires/stalls during wet weather	--	--	6, 11	--
Engine misfires/stalls within idle range	--	--	1, 2, 6, 7, 16	--
Engine misfires/stalls during load change	--	16, 21	19	--
Engine has poor performance	--	16	2, 8, 15, 17, 19, 23,	--
Idle speed sometimes irregular	--	--	6, 7, 8,10, 11, 13, 16 20	--
Idle speed irregular (hunting all the time	--	9, 10, 21	2, 16, 19	--
Idle speed too high	2, 9, 10, 21		6, 7, 8, 16, 19	--
Idle speed too low	--	--	2, 9	--
Engine stalls intermittently	--	9, 10, 19	2, 3, 5, 8, 15, 16	--
Engine stalls or does not re-start	--	6, 11, 13, 16, 17	1, 2, 3,4, 5, 9,10, 12, 18, 20, 21	--
Fuel consumption to high	--	--	2, 3, 4, 5, 8, 15, 16, 19	2

Possible Causes (For Explanation See Table 2)

TABLE 2 - EXPLANATION OF POSSIBLE CAUSES

Possible Causes	Recommendations And Background Information
1 Oxygen (O2S) sensor Heater faulty or short Circuited.	Deposits on ceramic part of sensor cause incorrect control (lean misfire). Caution, Do not use sealants, liquid gaskets or sprays that contain silicone on engines with oxygen sensors. Silicone entering the intake air system is not burned during combustion and can damage the oxygen sensor Always read ingredients of the material you are using. It must not contain silicone. Short circuit causes mixture enrichment while driving. O2S sensor incorrectly measures oxygen level as too low. - Test drive with O2S sensor disconnected which forces O2S control to refer to fixed map values and sets pressure regulator current at 0 mA at idle. Voltage can be measured at the oxygen sensor connection using test leads (parallel check). Voltage must fluctuate constantly between 100 mV (lean) and 800 mV (rich). Similar measurement is possible using adapter leads from VW 1594 at the Engine Control Module (ECM) at the O2S sensor signal input.
2 Mass Air Flow (MAF) Sensor (G19) faulty.	Sensor plate sticks (bent). Foreign objects prevent free movement. - Check visually. Break in wiring/short circuit at potentiometer Negative effects on driving performance if output voltage of potentiometer is below tolerance limit. -Check rest position and clearance of sensor plate, also check potentiometer for MAF sensor, see Repair Manual, Repair Group 25
3 Differential Fuel Pressure Regulator -(N73) or control, faulty.	- Check differential fuel pressure regulator see Repair Manual, Repair Group 01. - Check engine idle speed, CO content and engine timing, see Repair Manual, Repair Group 25.
4 Fuel distributor Faulty, control Plunger sticks.	-Check sensor plate lever and control plunge for free movement, see Repair Manual Repair Group 25. -Check engine idle speed, CO content and engine timing, see Repair Manual, Repair Group 25.
5 Engine Coolant Temperature (ECT) Sensor (G62), faulty.	- Check ECT sensor, see Repair Manual, Repair Group 25. DTC for O2S sensor or basic engine setting.- Check O2S sensor control, see Repair Manual Repair Group 25. Corrosion caused by coolant leak leading to high resistance or open circuit in connector terminals. ECT sensor simulates colder conditions than actual and enriches, resulting in mixture enrichment. ECT sensor simulates warmer conditions than actual. Cold start valve is not triggered causing starting problems.
6 Spark plugs fouled Because of frequent Cold starts high fueLConsumption or engine Oil over-filled.	Spark plugs damaged during installation. Spark plug connector loose or damaged. - check plug connectors for proper seating, corrosion, carbon arching and heat cracks.
7 Fuel injector faulty.	Injector is leaking, has poor or incorrect spray pattern. - Check injectors, see Repair Manual, Repair Group 25. Check air-shrouded injectors for contamination and carbon deposits.
8 Barometric Pressure (BARO) sensor (F96) Faulty.	Simulates altitudes that are higher than actual. Faulty regulator or associated wiring causes control to lean out mixture. - Checking: disconnect BARO sensor harness connector and measure current at different pressure regulator: * must be 6 mA at idle or when accelerating. Simulates altitudes that are lower than actual - Pressure regulator causes engine to be too rich and to smoke at high altitude because low air density is not corrected (F96 does not sense altitude), see Repair Manual, Repair Group 25.
9 Idle Air Control (IACvalve (N71) sticks or is not triggered.	- Check IAC valve, see Repair Manual, Repair Group 01. Starting problems with closed throttle (engine does not get enough air). - System in limp-home mode. Open limp-home slot in IAC valve contributes to increased idle speed. - Check idle adjustment with adapter harness and multimeter. Experience value for warm engine:540-610 mA at idle. If values are very low (approx. 400 mA) check engine for air leaks.
10 False air (air leak).	At gaskets for cylinder head cover, oil dipstick, intake manifold, injector O-rings hoses for Idle Air Control Valve, air supply to air-shrouded injectors, leaky or disconnected vacuum hoses. Sensor plate is not raised enough causing lean mixture. Check all hoses and connections for secure fit and damage. - With air leak, current to IAC valve reaches lower control limit of approx. 400 mA (corresponds to IAC valve mechanically closed), see Repair Manual, Repair Group 01
11 Ignition distributor Dirty, faulty.	- Replace if cracked, broken or if damaged in other way. - Remove light carbon deposits with clean rag small traces of carbon are no reason for replacing part.
12 Diaphragm pressure Regulator faulty.	- Check system, differential and residual pressure, see Repair Manual, Repair Group 25. - Check that fuel supply quantity is sufficient, If necessary check fuel delivery quantity, see Repair Manual, Repair Group 20.
13 Ignition distributor Hall sensor (G40) Faulty.	Hall sensor is sensitive at high temperature. Interfering signals from defective ignition cables, spark plugs and distributor affect the Hall signal. - Check Hall sensor, see Repair Manual, Repair Group 28. - Clean parts, if damaged, replace.
14 Fuel pump check valve Faulty.	- Check system pressure, differential pressure and residual pressure, see Repair Manual, repair Group 25. - Check fuel delivery quantity, see Repair Manual, Repair Group 20.
15 Timing adjustment not Correct or knock Sensor supplies Incorrect signal.	- Listen for abnormal engine noise (pinging or noise from loose components). - Check timing, see Repair Manual, Repair Group 28. - Check knock sensor mounting (tightening torque). - Check knock sensor cable for continuity, see Repair Manual, Repair Group 28.
16 CIS-E III Engine Control Module (ECM).	- Before testing control module be sure to check connectors for corrosion and damaged terminals (terminals spread, pushed back). - No start enrichment. Check differential pressure regulator current. With cold engine, current fluctuates down from approx. 90-100 mA to 0 mA as engine warms up. - Check engine idle speed and CO, see Repair Manual, Repair Group 25. - Deceleration fuel shut-off is always active Pressure regulator current values are far into the negative range, see Repair Manual Repair Group 25. - Oxygen sensor control, see Repair Manual, Repair Group 25. Measure pressure regulator current during test drive. Display value must always fluctuate and go to the positive by approx 3 mA when Wide Open Throttle (WOT switch is activated.
17 Ignition control Module faulty.	Water in control module housing. - Check installation location for exposure to moisture or water. - Check that control module is protected with plastic protective cove.
18 Ignition coil with Power output stage 1 (N70) faulty.	- Perform electrical testing, see Repair Manual, Repair Group 28.
19 Closed Throttle Position (CTP) switch (F60), Wide Open Throttle (WOT) switch (F81) faulty or not Correctly adjusted.	- Check CTP switch and WOT switch, see Repair Manual, Repair Group 25. - Check for corrosion on switch, if corroded replace switch.
20 Cold start valve Always open or does Not work.	Engine adjustment too rich, possibly caused by wom wire. Engine does not start at low outside temperatures, possibly caused by defective cold start valve or loose connector. - Check cold start valve, see Repair Manual, Repair Group 25.
21 Throttle Body.	Throttle cable is difficult to move or sticking, especially in second stage. - Increased engine idle speed, perform functional test, visual inspection, check throttle body for sticking. Dashpot on throttle body not correctly adjusted or is faulty. - Check adjustment and function of dashpot, see Repair Manual, Repair Group 20.
22 Intake valves have Carbon deposits causeBy insufficient fuel Additives.	Lean condition caused by carbon deposits absorbing fuel. - Treat carbon deposits with fuel additive Part No. G 001 700 03. - Use quality fuel with sufficient additives.
23 A.) Catalytic converter core melted. B.) Sulfur odor after shutting engine off.	a.Measure exhaust pressure at CO tap tube (using slack tube tester). If pressure is greater than 0.3 bar (absolute pressure 1.3 bar) replace catalytic converter. b.Explanation: Sulfur odor develops from sulfur dioxide that is stored as sulfate in the catalytic converter during slightly lean operation (deceleration). With rich engine operation (Idle or fast acceleration), sulfate is reduced to hydrogen sulfide which causes the odor. Odor formation depends on the kind of fuel used. The odor decreases as the car accumulates mileage. Replacing the catalytic converter would make the odor worse.