Q&A

Mailbag: Troubleshooting an Engine That Won’t Shut Off

1987 Chevy Camaro IROC-Z black

(Image/Hemmings)

You’ve got questions, we’ve got answers. We work with the Summit Racing tech department to help you tackle your auto-related conundrums. In this week’s Mailbag, we’re helping a reader solve a diesling problem where he can’t get his engine to shut off. 

Q: I have a 1987 Camaro with a 305 4-barrel. I am having problems getting the car to shut off. I have tried pulling the coil wire, and even removing the battery cable, but the engine keeps running until I choke off the carburetor. What is causing this?

A: This problem is known as dieseling. It can be caused by one of—or a combination of—the following conditions:

  • Ignition timing is advanced too far. Not sure how to adjust your timing? Check out this video put together by our friends at MSD Ignition, and read Jeff Smith’s Ask Away! column about ignition advance.
  • Idle speed set too high. You didn’t mention what specific type of carburetor you had, but check its service manual on idle speed adjustment. It’s often a simple matter of adjusting a set screw.
  • A vacuum leak causing fast engine idle and letting excess air into the engine. This can be as basic as a loose or cracked hose, or it may require fixing a gasket leak in the carburetor or manifold. Smith addressed this topic as well in his Ask Away! column How to Identify Gasket Leaks and Fix Them.
  • Excessive carbon buildup in the combustion chambers, which causes a “glow plug” effect. A fuel system additive or motor treatment may do the trick, but in extreme cases, you’ll have to remove your cylinder heads and physically remove the carbon buildup by hand.
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