The most important tool you'll need to work on your C4-ZR1 is the year specific, GM authorized, Corvette Service Manual. Print and digital versions are readily available from on-line retailers. Of particular importance is Chapter 6E, entitled: "Driveability and Emissions - VIN J". The "J" is the VIN designation for the LT5 engine, which is found in the C4-ZR1.
The ZR1 has a built-in OBD1 code reader. Simply jump A-G on the Assembly-Line-Diagnostics-Link (ALDL) connector and the Driver-Information-Center buttons and the speedometer become an in-car reader. For additional clarity, see the ALDL connector image below. Be sure to troubleshoot driveability issues using the diagnostic codes first.
Resolving driveability issues without diagnostic codes, requires deductive reasoning that starts by isolating ignition symptoms from fuel symptoms. To achieve this, spark plug testers and fuel injection 'noid' lights are highly recommended. A 12 volt test-light with an alligator clip on one end and a probe on the other will be useful as well. These inexpensive testing devices can be found at many automotive retailers or on-line.
A GM Authorized Corvette Service Manual is the most important tool you can own.
Knowledge Carries a Lot of Torque!The LT5's Direct Ignition System (DIS) is managed by its ignition module, and the Sequential Fuel Injection System is managed by its Engine Control Module (ECM). Management of some functions, including timing advance, are shared by these two devices.
Essentially, as the engine is initially turned over, the Crankshaft Sensor provides a pulsed signal to the ignition module, which in turn waits for a sync pulse from the crankshaft. Once received, the ignition module sends consecutive signals to the coils which provide a high voltage signal to the spark plugs and, at the same time, the ignition module sends a continuous pulse train to the ECM on the Crankshaft High circuit. The ECM uses this incoming pulse to activate all the fuel injectors simultaneously until the engine starts. After which, the ECM coordinates this crankshaft pulse with a pulse from the camshaft sensor, in an effort to time the activation of each primary fuel injector sequentially, on a cylinder-by-cylinder basis, so that fuel arrives at each cylinder as it reaches its respective intake stroke.
NOTE: It is good engineering practice to replace the Crankshaft Sensor when driveability issues are noticed. It is located on the right side of the engine block close to the dipstick tube. The sensor is held in place by an 8mm bolt and both can be easily removed and replaced from under the right-front of the vehicle. A failing Crankshaft Sensor might not produce a diagnostic code while causing driveability issues. The engine will not start or run with a completely failed Crankshaft Sensor.
DIS = Direct Ignition System and ECM = Engine Control Module
Knowledge Carries a Lot of Torque!There are a number of simple ways to confirm there is a spark at each cylinder. One method uses a basic screwdriver, another uses an actual spark plug, while another other utilizes a spark plug test light.
To prevent the engine from starting while testing, remove the Gray or Yellow ECM connector. To check for a spark, the engine can be turned over from the ignition key, while watching the 'spark' gap or the test light. Spark tests should be performed one cylinder at a time. Some locations on the plenum may not provide a suitable ground for these tests.
Disconnect the Gray or Yellow ECM Connector when testing for a spark!
Knowledge Carries a Lot of Torque!A crude hi-tension lead test can be performed with a screwdriver placed atop the engine plenum with its stem inserted snuggly into the boot of the plug wire. There should be less than a 1/4 inch 'spark' gap between the screwdriver's stem and the plenum or other appropriate grounding spot.
A spark plug can be readily used to test for a spark on the LT5 engine. Depending on your circumstances, a single 'test' plug can be borrowed from a cylinder or a new plug can be brought into play. In some cases you might want to test each plug individually. Simply connect the hi-tension lead to the spark plug and let the threaded part of the plug rest on the plenum or some other suitable grounding point near the engine.
If you're using a spark plug test light, normally it is inserted in series, between a hi-tension lead and its respective spark plug but due to the depth of the plug shafts on the LT5, it is sometimes necessary to exclude the spark plug connection and provide a jumper directly to ground on the spark plug side of the test light.
The LT5 uses a waste-spark system comprised of a package of four ignition coils, which means two cylinders are attached to each coil. The plugs are paired as follows: 1/6, 8/5, 4/7, and 3/2.
This suggests that if only one hi-tension lead fails a spark test, it is likely a faulty lead or spark plug but if two leads fail from one pairing, it is likely a faulty coil or the ignition module.
If all the hi-tension leads fail, a blown fuse, faulty Crankshaft Sensor or a malfunctioning ignition module could be the cause.
To replace the hi-tension leads, ignition coils or ignition module, it is necessary to remove the plenum.
If the leads, plugs, ignition module and coil pack are deemed to be in working order, checking the fuel delivery system is the next step.
Tag: C4, Corvette, ZR1, LT5, ECM, DIS
