Trouble Shooter

Motor, Nov 2004 by Seyfert, Karl

Eliminating spark knock can be a devilish proposition. Make sure you've thoroughly interrogated all of the usual suspects before resorting to wholesale parts replacement.

Knock, Knock

I am working on a 1998 GMC K 1500 pickup. It's got 64,000 miles on the odometer and a 5.7L Vortec V8 engine under the hood. The owner is complaining about a spark knock that does not seem to be affected by engine temperature. Rot or cold, the engine knocks under load, until you back off on the gas. The following parts have been replaced: fuel pump, fuel pressure regulator, distributor cap, rotor, ignition wires, spark plugs, knock sensor, thermostat (colder than stock), two oxygen sensors and an air filter. The timing has been adjusted with a scan tool. I've also tried a power programmer and higher octane fuel, but the problem is still there. I'm going crazy trying to figure this one out.

Matt Rousar

Winfield, IL

You didn't explain why any of the mentioned parts were replaced or the diagnostic method you used, so we'll have to assume the truck "needed" all of them. The engine is still knocking, so let's start at the beginning and maybe I can help you fix the truck.

Unlike an engine miss, spark knock is a combustion event that takes place at the wrong time. In your customer's case, the fuel is igniting before it's supposed to, causing the knocking noise when the vehicle is under load. Spark knock may also be called ping, spark rattle or preignition. It doesn't matter what you call it, as long as we're sure we're talking about the same thing. Spark knock can have several causes, or combinations of causes. Among the most common (in no particular order) are improper ignition timing, combustion chamber deposits, improper EGR system operation, ignition system crossfire, a defective knock sensor or low fuel octane.

You mentioned that you had adjusted the ignition timing, so let's begin there. Although the 5.7L Vortec engine in your customers truck still has a distributor, it can't be used to adjust timing in the conventional sense. Rather, turning the distributor serves only to adjust what GM calls the cam retard offset. Actual ignition timing is controlled by the PCM, based on the information it receives from various input sensors.

To adjust the cam retard offset, connect a scan tool to the DLC. Bring the engine to operating temperature, then hold it steady at 1000 rpm. Monitor cam retard offset on the scan tool. It should be 0° ±2°. To compensate for a negative reading, rotate the distributor in a counterclockwise direction, To compensate for a positive reading, rotate the distributor in a clockwise direction. Recheck the reading after tightening the hold-down bolt. It may change. Also note that attempting to retard the cam retard offset reading outside the recommended range will not cure spark knock, and may cause the PCM to store a diagnostic trouble code.

Take a look at the routing of the spark plug wires. If they run too close together between the distributor and the spark plugs, or are tied to one another with zip ties, the spark knock may be caused by spark induction. If they're too close together, the current traveling through one spark plug wire can induce a current in the adjacent wire, causing that cylinder to fire at the wrong time. Depending on the position of the adjacent cylinder in its combustion sequence, you may end up with one or more cylinders that are suffering from preignition. Cylinders 5 and 7 are possible candidates for this scenario, as one follows the other in the firing order.

You mentioned that the engine has spark knock regardless of whether it's hot or cold. This may point to combustion chamber deposits, which artificially raise the compression ratio. The deposits can also cause localized hot spots inside the combustion chamber, causing the fuel to ignite spontaneously and prematurely. All but the most stubborn deposits can be removed with the array of top engine cleaners formulated for the purpose.

Combustion chamber deposits can also result in a higher octane requirement, which is why using a higher octane gasoline may temporarily prevent pinging. Combustion chamber deposits are at least partially related to the aromatic hydrocarbons in gasoline. Aromatic content varies widely from brand to brand and may be over 50%, which is why the simple act of switching to another brand of fuel may correct a spark knock condition. Ironically, fuel surveys have indicated that the average aromatic content is often slightly higher in premium grade than in regular grade gasoline. So switching to premium may actually make the problem worse. Combustion chamber deposit formation is also affected by the types and concentrations of deposit control additives (detergents) in the gasoline.

Another possible cause for spark knock is a plugged or inoperative EGR system. The EGR adds a measured amount of inert exhaust gas to the combustion chamber, lowering its temperature and preventing knock. This is a very important function, which explains why OBD II systems now monitor the EGR system to make sure it's operating within design specifications. However, an EGR system may function just well enough to pass an EGR monitor, but not well enough to prevent spark knock Check the passages to make sure they're free and clear.