1997 GMC Suburban 1500 P0302 Misfire: Diagnosis and Fix with CPI Injection Module Issues

The ignition coil is likely the culprit, especially with over 198,000 miles. Even a relatively inexpensive replacement (around $40–$50) can make a big difference. Also, use a multimeter to check voltage on the high side of the coil for proper spark. I replaced mine after 100,000 miles when it started causing low idle misfires—this was a key fix. In future, consider doing a compression test on cylinder #2 to rule out mechanical issues. Also, verify fuel pressure and check if injectors are clogged.

Thanks for the advice! I’ll replace the coil tomorrow—it seems like a cost-effective first step. Since I already removed the valve cover during inspection, I’m in a good position to do further checks. If the issue persists after replacing the coil, I’ll perform a compression test as suggested. I also plan to check fuel pressure—maybe multiple components are operating at minimum specs, contributing to the misfire. I’ll update everyone on progress. Cheers!

Hi there—don’t forget to check fuel pressure and inspect the injectors in cylinder #2, as clogged injectors can cause misfires. Also, confirm whether your engine uses a TBI (Throttle Body Injection) or multi-port fuel injection system, as this affects diagnosis.

Hello—my truck has a TBI system, so that rules out a single-cylinder misfire issue (though it's still a good tip). However, I'm wondering if there’s an advantage to having a quick oil change shop perform a full TBI cleaning service for about $80 instead of using fuel tank additives. The latter is cheaper but less effective. I haven’t replaced the coil yet—so I’ll do that tomorrow (it's not a daily driver). Thanks for the input, V3!

Hi! Cleaning the TBI system can be helpful, but it’s better to clean it using professional equipment in a garage rather than relying on fuel tank additives. That said, since you're experiencing only one cylinder misfire, it's less likely that injectors are faulty—though still worth checking. I recommend rechecking the ignition system: verify spark and wiring for cylinder #2. Please keep us posted!

OP: Could you clarify? Isn't this a Vortec engine? If so, it doesn’t have TBI—it has port fuel injection with a CPI (Central Sequential Injection) system. These engines are known for CPI module failures that cause random, shifting misfires across cylinders.

Darkrooms: Thanks! I was mistakenly thinking it had TBI—this is actually my mistake. The engine does have a CPI system (Central Sequential Injection), as noted in the Chilton’s manual. I’ll now inspect the CPI module and check for faulty or clogged injectors. This insight was very helpful—thank you for pointing that out, V3!

Yes, CPI is the same as Central Sequential Injection (CSI). When CPI modules fail, they often cause random misfires that shift between cylinders—one injector fires briefly and then fails. You can test this using a scan tool: activate each injector individually—when faulty, it will fire once and then stop. If you’ve already removed the intake manifold, simply replace the CPI module. A Borg Warner replacement exists—it relocates injectors from the central module to the ends of injection tubes, improving performance—but it’s expensive. I've shared this before—if you search for 'spider' injection module, you’ll find more details.

Hi all—thanks for the help! I replaced the CPI module with a new Delphi unit (OEM replacement for original). It took some time to locate the correct model at AutoZone. While I looked for a Borg Warner spider-style module, I couldn’t find one. The Delphi module includes updated injectors and control units. After installation, the truck runs much smoother—better idle, improved acceleration. I’m still monitoring fuel economy. Huge thanks to everyone who helped clarify the issue! Cheers, VeeThree