Clark's Garage

Introduction

This troubleshooting guide is intended to help identify and repair engine starting and running problems and engine lubrication system problems. Symptoms covered:

Engine Will Not Rotate
Engine Will Rotate But Will Not Start (Fire)
Engine Is Difficult To Start When Cold
Engine Is Difficult To Start When Hot
Engine Idles Rough When Cold
Engine Idle Surges
Engine Shudders At Idle
Engine Hesitates or Stumbles During Acceleration
Engine Misses
Engine Backfires
Engine Oil Pressure Low
Engine Oil Pressure High

Engine Will Not Rotate

This situation is the easiest to troubleshoot because there are fewer possible causes than for an engine that will not fire.

Possible Cause	Refer To
Discharged or Faulty Battery	ELECT-15
Battery Terminals Loose or Corroded	ELECT-15
Bad Starter Motor or Starter Solenoid	ELECT-16
Bad Ignition Switch	ELECT-17
Broken or Loose Wiring In Starter Circuit	ELECT-17
Broken Teeth on Starter Pinion or Flywheel	See note below

Broken teeth check: Remove starter and inspect starter pinion teeth. Roll the engine using a 24 mm (or 15/16") socket and ratchet and inspect the flywheel for broken teeth.

Engine Will Rotate But Will Not Start (Fire)

When the engine will rotate but will not start, it can typically be attributed to one of two causes:

Lack of fuel
Lack of spark

A fuel pressure gauge, a spark tester, and a digital multimeter are essential for diagnosis. These two problems can be diagnosed independently, but there are several failures that will simultaneously prevent the fuel pump from running and cause a no-spark condition.

Testing for Lack of Fuel

A fuel pressure gauge is almost essential for this test. You may test for fuel supply by removing the end cap from the fuel rail, installing a hose, and directing the open end to a catch pan. However, even if you get fuel from the rail during testing, there is no guarantee the fuel pressure is adequate to start the car.

Connect a fuel pressure gauge to the end of the fuel rail.
Attempt to start the vehicle while watching the fuel pressure gauge.
If pressure rises, compare the reading to specifications in FUEL-01.
If there is no increase in fuel pressure and you suspect the fuel pump is not starting, refer to FUEL-16 for fuel pump troubleshooting.
If fuel pressure is low or rises very slowly during cranking, refer to FUEL-16 for further fuel-related troubleshooting.

Testing For Lack of Spark

First verify that a no-spark condition actually exists using a spark tester (available at most auto parts stores). Remove the fuel pump fuse to keep the fuel pump from running during spark testing.

Disconnect one plug wire and connect the spark tester in series with the spark plug. Crank the engine and look for spark pulsing inside the tester.

If there is no spark at the spark plug, disconnect the main lead from the ignition coil to the distributor cap and connect the spark tester in series with the coil wire. Crank the vehicle and check for spark to the distributor.

If you get good spark at the main lead to the distributor but no spark at the spark plug, the distributor cap and/or rotor is bad and should be replaced.
If there is no spark at the distributor main lead, proceed with ignition troubleshooting using IGN-04.

Engine Is Difficult To Start When Cold

Possible Cause	Checks	Procedure
DME Temperature Sensor	Check sensor resistance	ELECT-19
Fuel System Vapor Lock	Vapor lock can be caused by a bad Fuel Pressure Regulator (FPR), bad fuel pump check valve, or leaking injectors — any of which cause fuel rail pressure to drop rapidly after shutdown. Heat causes remaining fuel to flash to vapor, creating voids in the rail. Perform a fuel pressure and leakdown test (FUEL-01). If the system fails the leakdown test, determine the cause using FUEL-17.	FUEL-01
Fuel Pressure Too High	Disconnect one fuel injector connector and attempt to start. If the car starts and runs where it would not before, the FPR is most likely bad. When the FPR fails high, injectors see too-high differential pressure, draw excessive current, and the injector driver shuts down. Disconnecting one injector reduces current enough to allow starting. Note: the car will run very rough on three cylinders.	N/A
Speed / Reference Sensors	A failing or misadjusted Speed and/or Reference sensor can cause hard starting, particularly when hot but also when cold. Try wiggling the sensor wires and connectors. Remove the sensors and clean the ends — magnetic field buildup causes them to accumulate metal fines from the flywheel ring gear. Also check Speed and Reference sensor clearance.	IGN-02
DME Computer	Bad solder joints in the DME can cause various starting problems. Try tapping on the DME while attempting to start. If the car starts while tapping where it would not before, there are bad solder joints that should be repaired. Open the DME and inspect for bad solder joints before replacing the unit.	DME-05

Engine Is Difficult To Start When Hot

Possible Cause	Checks	Procedure
DME/Fuel Pump Relay	One of the first signs of a failing DME/Fuel Pump relay is difficult hot starting. The car typically starts fine cold but refuses to restart after reaching operating temperature. After cooling for 30 minutes to 1 hour, it will start again. If you experience hot start problems, try installing a jumper for the DME/Fuel Pump relay (FUEL-05, "Emergency Repairs") to see if it solves the problem — this test must be performed while the hot-start problem is occurring to be valid.	FUEL-05
Fuel System Vapor Lock	Same as cold-start vapor lock diagnosis above. Perform a fuel pressure and leakdown test (FUEL-01).	FUEL-01
Speed / Reference Sensors	This is the most common condition for speed/reference sensor problems. Try wiggling wires and connectors, clean sensor ends, and check clearance.	IGN-02
DME Computer	Same as cold-start DME diagnosis. Eliminate all other causes before replacing the DME.	N/A

Engine Idles Rough When Cold

Possible Cause	Checks	Procedure
DME Temperature Sensor	When cold, the DME temperature sensor tells the computer to supply a richer mixture (cold engines atomize fuel poorly). When the sensor fails it typically sends a higher-than-normal temperature signal, causing the computer to lean the mixture — the engine runs rough or may not run at all. Check the cold resistance of the DME temperature sensor.	ELECT-19
Vacuum Leak	Inspect all hoses connecting to the intake manifold downstream of the AFM.	N/A

Engine Idle Surges

Possible Cause	Checks	Procedure
Idle Stabilizer / Auxiliary Air Valve	Clamp the line from the Idle Stabilizer/Auxiliary Air Valve to the intake manifold and see if idle stabilizes. If it does, remove the valve, clean it with carburetor and choke cleaner, reinstall, and check operation. If surging persists, replace the valve.	N/A
Vacuum Leak	Inspect all hoses connecting to the intake manifold downstream of the AFM.	N/A
Throttle Body Butterfly Sticking	Remove throttle body and clean the area around the butterfly valve.	N/A

Engine Shudders At Idle

Possible Cause	Checks	Procedure
Engine Idle Set Too Low	Check idle speed and reset if necessary.	FUEL-08
Early US/Canada 944 DME Programming (pre-1985.5 US/Canadian models only)	These cars had a momentary fuel cutoff when the throttle was released from any off-idle position. The idle would drop too low before fuel cut back in, resulting in a shudder. Several fixes exist: replace the DME with a 1985.5 unit; replace with an ROW (Rest of World) DME; or adjust the throttle stop (loosen the lock nut) until you hear the Throttle Position Switch click, then tighten the lock nut and adjust the idle screw to approximately 1000 RPM. A software fix is also available for those comfortable with DME reprogramming.	N/A

Engine Hesitates or Stumbles During Acceleration

Possible Cause	Checks	Procedure
Dead spot in Air Flow Sensor / Air Flow Meter (AFM) potentiometer	Test AFM.	ELECT-19
Vacuum Leak	With a large vacuum leak, the car may idle correctly but hesitate, stumble, or cut off when the throttle opens. Connect approximately 10 psi compressed air to the intake manifold with the engine off and listen for air leaks.	N/A
Spark Plugs Fouled	Remove spark plugs and check condition.	ENG-18

Engine Misses

Possible Cause	Checks	Procedure
Bad spark plug wires	Run the engine in a dark area and look for arcing from the spark plug wires to the engine.	N/A
Cracked distributor cap	Remove cap and check condition.	N/A
Faulty spark plug	Remove spark plugs individually and inspect for broken or excessively worn electrode or cracked insulator. Check condition by color.	ENG-18
Vacuum leak	Pressurize the intake manifold with compressed air and listen for leaks.	N/A
Clogged fuel filter	Replace filter.	FUEL-03
DME Computer	A faulty DME can cause a variety of starting and running problems. Problems are usually the result of bad solder joints, which can often be repaired. Without a known-good substitute DME to swap in for testing, there is no direct way to confirm the DME is bad. Eliminate all other possible causes first. Open the DME and inspect for bad solder joints before replacing.	DME-05

Engine Backfires

Possible Cause	Checks	Procedure
Faulty ignition system	A cylinder that is not firing or fires intermittently can expel raw fuel into a hot exhaust system where it ignites. Check for faulty spark plug wires and/or faulty spark plugs.	N/A
Vacuum leak	Pressurize the intake manifold with compressed air and listen for leaks.	N/A
DME Computer	Same as the DME diagnosis described under Engine Misses. Eliminate all other causes first.	DME-05

Engine Oil Pressure Low

Possible Cause	Checks	Procedure
Oil Level Low	Check oil level and top off if necessary. On 1987 and newer 944s, check for a low oil level warning light.	LUBE-03
Crankshaft pulley bolt loose	Check torque on crankshaft pulley bolt (155 ft-lbs).	LUBE-03
Bad oil pressure sending unit	Check sending unit.	LUBE-03, ELECT-19
Oil pump lost prime	Not uncommon on cars that have been sitting without being run. To prime: remove the oil filter and pour oil down the center hole of the filter housing while turning the crankshaft counter-clockwise. Install a new oil filter when done, then crank the engine to check pressure.	LUBE-03
Bad or stuck-open oil pressure relief valve	There is no direct way to check operation. Remove the relief valve, clean it, and reinstall it ensuring it does not bind.	N/A
Worn engine bearings	If oil pressure reads low only when the engine is fully warmed up, the cause may be worn main and/or rod bearings. Refer to LUBE-03 for normal pressure specifications.	LUBE-03
Broken oil pump pickup tube	Not a common occurrence. There is no way to check without removing the oil pan. This is a last resort if all other attempts to restore oil pressure have failed.	N/A
Bad oil pump	The oil pump on these engines is generally very reliable. Failure typically results from loss of prime, which can destroy the pump and the main and rod bearings. To inspect: remove the pump, disassemble it, and inspect the pump gears.	N/A

Engine Oil Pressure High

Possible Cause	Checks	Procedure
Bad oil pressure sending unit	A faulty sending unit is the most common cause of a high oil pressure indication. Refer to LUBE-03 and ELECT-19 for symptoms and troubleshooting.	LUBE-03, ELECT-19
Stuck closed oil pressure relief valve	Not a common problem. Remove the relief valve, clean it, and reinstall ensuring it does not bind.	N/A
Blocked oil passage	Most commonly caused by the check valve for the oil supply to the cylinder head sticking closed. This will be accompanied by loud tapping from the hydraulic lifters. A blocked oil passage into the camshaft housing itself typically produces an oil leak at the cam assembly-to-cylinder head gasket.	N/A

Troubleshooting - Engine