Questions and Answers

Chevy Blazer Getting Knocked Up Q. I have a 1999 Chevy Blazer, 40,000 miles and 4.3 liter engine. I bought it used about one year ago. Not long after I started hearing spark knock, went to higher-grade gas, and didn't hear knock until recently. Eminence schedule doesn't say to replace plugs until 100,000 miles. I don't think at 40,000 I should be having this problem. What do you suggest?

I am a diesel mechanic at a truck dealer, but have little experience with gas engines. I do understand what spark knock is, just don't know what's going on with my Blazer. Don't have any engine lights, coolant temperature normal.

Please help…

A. Spark knock can be caused by timing too far advanced, low octane fuel, or high combustion chamber temperatures. Timing is not really adjustable on this engine. The distributor will move but you need a scan tool to look at cam retard degrees. Should be as close to zero as you can get it. A non working EGR valve can also cause this but this should set a code because the PCM does many self tests to check for malfunctions.

There is also a knock sensor on the block that "senses" any abnormal noises and then if knock is detected it will retard timing. My guess is that there is a problem in this circuit, but all above mentioned items should be checked.