Question: I have a 1992 Chevrolet Silverado with a misfire. I changed the spark plugs and wires, coil, and cap, but it's still missing. It does it a lot when I get up to 40 mph.
Answer: I didn't get the exact model or engine, so I'll give you the basics. You first have to determine if it's a single- or multi-cylinder misfire. If the misfire is present at idle, you can do a cylinder balance test by removing the plug wires one at a time from the good old GM HEI distributor cap with a pair of insulated pliers. Watch the rpm while killing each cylinder. If killing one has no effect, but the other five (V-6) or seven (V-8) drop evenly by a few hundred rpm, that's the cylinder with the miss. If it's hard to determine which is good and which is bad, that's most likely a multi-cylinder misfire. If it's a single miss, what did the spark plug at that cylinder look like? Was it clean, black with oil, or wet with fuel? It appears all 1992 Chevy gasoline truck engines used the dual-injector throttle body, not six or eight individual injectors (port injection). So if it is indeed a solid single-cylinder misfire, we can eliminate fuel as the cause. If one of the two throttle body injectors were bad, it would affect four or three cylinders. A compression and cylinder leak-down test would be required to confirm or eliminate an internal problem. It could be a worn camshaft lobe, burned valve, bad head gasket, etc. If several cylinders have the problem, watch the fuel spray coming out of the injectors. It should be visible with the air cleaner removed, and have a uniform, steady, umbrella-shaped pattern. Then check fuel pressure, then for fuel contamination, and engine mechanicals as a last resort.
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