A Practical Guide to the 1955-1959 GMC PM151 Distributor Cap
If your classic GMC truck is running rough, the problem could be this simple ignition part.
- A bad distributor cap is a common cause of misfires, hard starting, and stalling on a classic GMC PM151.
- For better durability and corrosion resistance, consider a premium aftermarket cap with brass contacts.
- Always replace the distributor rotor at the same time as the cap for a complete tune-up.
- Confirm your engine type (I6 or V8) before ordering to ensure you get the correct part.
Is Your Engine Running Rough? It Could Be the Distributor Cap.
For owners of a classic 1955-1959 GMC PM151, a smooth-running engine is a point of pride. These trucks have a simple and tough ignition system, but some parts are designed to wear out. The distributor cap is one of those parts. Because it handles high voltage in a hot engine bay, it fails over time due to cracks, corrosion, or carbon buildup. Recognizing the signs of a failing cap can save you from a breakdown and keep your vintage GMC running strong.
Symptoms of a Bad Distributor Cap
A bad distributor cap can cause a variety of engine problems. You might notice one or more of these issues:
- Engine Misfires: The engine may stutter, shake, or jerk while you're driving or idling. This happens when a cylinder fails to fire because the spark doesn't reach the spark plug correctly.
- Hard Starting: The engine turns over but takes a long time to start, or won't start at all. This can be caused by moisture inside the cap, cracks, or worn contacts preventing a strong spark.
- Rough Idle: Your truck shakes or vibrates more than usual when it's stopped and the engine is running.
- Stalling: The engine may suddenly die at a stoplight or while coasting.
- Backfiring: Improper spark timing from a faulty cap can cause the engine to backfire.
Pro Tip: Check for Physical Damage
You can often diagnose a bad cap with a simple visual inspection. Remove the two clips or screws holding it down and look inside. Check for cracks in the plastic, white or green corrosion on the metal terminals, or thin black lines that look like pencil marks. These are called carbon tracks, and they show where electricity is leaking. Any of these signs mean it's time for a replacement.
Choosing the Right Aftermarket Distributor Cap
For a vehicle like the 1955-1959 GMC PM151, you don't need to hunt down an expensive original part. Aftermarket brands offer excellent quality, and in some cases, their premium products are better than the originals. The key difference often comes down to the material used for the electrical contacts inside the cap.
Brand Tiers
- Premium Aftermarket (e.g., Standard Ignition Blue Streak): These are high-quality replacement parts often recommended for classic vehicles. They frequently feature solid brass contacts instead of the standard aluminum. Brass is more conductive and resists corrosion better, leading to a longer life and more consistent performance. For a classic truck that might not be driven daily, a cap that resists corrosion is a smart investment.
- Standard Aftermarket (e.g., Walker Products, Standard Ignition): These brands produce reliable, direct-fit replacement parts that meet or exceed original specifications. They are a cost-effective choice for a regular tune-up and will perform well. They typically use aluminum terminals, which work perfectly fine but may corrode faster than brass over time.
Common Ordering Mistake
The 1955-1959 GMC PM151 came with different engines, including various Inline-6 and V8 options. These engines use different distributors and, therefore, different distributor caps. Always confirm your truck's engine before ordering a new cap to ensure you get the right part.
| Part Type | Estimated Cost |
|---|---|
| New Aftermarket | $24.44 - $34.65 |
| New OEM (AC Delco) | $40 - $60 |
| Shop Labor | $50 - $100 |
Frequently Asked Questions
Are there any recalls on the distributor cap for my truck?
No. There are no recalls or Technical Service Bulletins (TSBs) from the NHTSA for the distributor cap or ignition system on the 1955-1959 GMC PM151. Any major GM ignition recalls you may have heard about apply to much newer vehicles and are completely unrelated to your truck's mechanical ignition system.
Should I replace the distributor rotor at the same time?
Yes, absolutely. The rotor is the part that spins inside the cap. It's a wear item just like the cap and is subjected to the same heat and high voltage. Since you already have the cap off, replacing the rotor takes only a few extra seconds and ensures your entire ignition tune-up is complete. Most mechanics strongly recommend replacing them as a pair.
How often should I replace the distributor cap?
Distributor caps are considered a maintenance item. For a classic truck, it's a good idea to inspect the cap annually and plan on replacing it and the rotor every 15,000-20,000 miles or every few years as part of a major tune-up. If your truck sits for long periods, moisture can build up, so an annual check is important.
Can I replace the distributor cap myself?
Yes, this is a great DIY job for a beginner. The only tools you typically need are a screwdriver or a small socket set. The most important step is to label your spark plug wires before you remove them from the old cap, or to transfer them one at a time to the new cap to ensure the engine's firing order stays correct. An incorrect firing order will prevent the engine from running.
Technical Specifications
OEM Part Numbers: AC Delco C349
Fitment depends on the engine (Inline-6 or V8). Common aftermarket part numbers include Standard Ignition DR428 and DR429. Always verify the part matches your specific distributor.
We Have This Part in Stock
The information in this article is provided for general reference and educational purposes only. Vehicle specifications, procedures, and part compatibility can vary by production date, trim level, and region. Always consult your vehicle's factory service manual and verify part numbers before purchasing or performing repairs. Safety-critical components such as airbags, seat belts, and braking systems should be installed by a qualified professional.