Engine Timing Set Replacement Guide for the 1978-1987 GMC Jimmy
Keep your classic Jimmy's engine in sync with the right timing set replacement.
- The original nylon-toothed camshaft gear is a common failure point; upgrading to an all-metal aftermarket set is essential for reliability.
- A rattling noise from the front of the engine is the most common symptom of a worn timing chain and should be addressed immediately.
- Brands like Cloyes, Melling, and Edelbrock offer reliable aftermarket solutions, from standard replacements to heavy-duty double roller sets.
- Due to the significant labor involved, always replace the water pump, timing cover gasket, and front crank seal at the same time as the timing set.
Is Your Jimmy's Timing Chain on its Last Legs?
For the 1978-1987 GMC Jimmy, particularly those with V8 engines, the original timing sets had a known weak point. The camshaft gear was often made with nylon-coated teeth to keep the engine quiet. After decades of heat and mileage, this nylon becomes brittle, cracks, and can break off. This causes the timing chain to become dangerously loose, a condition often called 'slop'. A sloppy chain can jump a tooth on the gear, throwing off your engine's timing, and if it breaks completely, your engine will stop running and may suffer serious damage.
Key Failure Warning Signs
- Engine Rattle: The most common sign is a rattling or scraping noise coming from the front of the engine, behind the water pump. This is the sound of the loose chain hitting the inside of the timing cover.
- Poor Performance: A stretched chain can delay valve timing, leading to a noticeable loss of power, bad gas mileage, and a sluggish feel when you accelerate.
- Rough Running & Misfires: If the chain's slack allows it to jump a gear tooth, the engine's timing will be off, causing it to run rough, misfire, or backfire.
- Metal or Plastic in Oil: Finding small, shiny metal shavings or bits of black plastic in your used engine oil is a major red flag that the timing gear is coming apart.
Choosing the Right Aftermarket Timing Set
For a classic vehicle like the '78-'87 Jimmy, aftermarket timing sets are not just a replacement; they are a significant upgrade. Virtually all aftermarket sets replace the failure-prone nylon gear with a durable all-metal (cast iron or steel) camshaft gear. This is the single most important improvement you can make for long-term reliability.
Brand Quality Tiers
When shopping, you'll see several trusted brands. They generally fall into a few categories:
- Good (Standard Replacement): Brands like Cloyes and Melling offer excellent standard replacement sets. These are perfect for a daily driver or stock restoration and provide a huge durability boost over the original nylon gear design. They are affordable and reliable for most uses.
- Better (Heavy-Duty & Double Roller): For engines that work hard (towing, off-road) or have performance upgrades, a heavy-duty or 'double roller' chain set is a wise choice. Brands like Melling Select Performance and Edelbrock offer these. A double roller chain is wider and stronger than a standard chain, offering extra protection against stretching under high load.
- Best (Billet Gear & True Roller): For high-performance rebuilds, premium sets from Cloyes (their performance lines) and Edelbrock feature billet steel gears and true roller chains. These are the strongest and most precise sets available, often with multiple keyways on the crank gear to allow for advanced or retarded cam timing.
Cost to Replace Your Timing Set
The parts themselves are not expensive, but the labor to get to them is what drives up the cost at a repair shop. The job requires draining the coolant and removing the water pump and harmonic balancer just to get the timing cover off.
| Item | Estimated Cost |
|---|---|
| New Aftermarket Timing Set | $25 - $100 |
| Shop Labor | $450 - $750 |
| Total Estimated Cost | $475 - $850 |
Costs are estimates and vary by location and the specific parts chosen. It is highly recommended to also replace the timing cover gasket, front crankshaft seal, and water pump while performing this job.
Frequently Asked Questions (FAQ)
Are there any recalls for the timing chain on a 1978-1987 GMC Jimmy?
No, there are no official NHTSA recalls or Technical Service Bulletins (TSBs) for the timing chain on this specific generation of GMC Jimmy. The issue with the original nylon gear is a well-known wear-and-tear item addressed through standard repairs.
Should I get a standard or a double roller timing set?
For a stock engine used for regular driving, a quality standard replacement set with all-metal gears is perfectly adequate and a great upgrade. If you have a modified engine, tow heavy loads, or simply want the maximum durability, a double roller set is a worthwhile investment for added strength.
Can I replace the timing set myself?
This is a moderately difficult job for a DIY mechanic. It requires a good set of tools, including a torque wrench and a harmonic balancer puller. You will need to drain the cooling system and remove several components from the front of the engine. If you are comfortable with that level of work, it is achievable at home.
What other parts should I replace at the same time?
Since you'll have the front of the engine taken apart, it is highly recommended to replace the timing cover gasket set, the front crankshaft seal, and the water pump. These parts are inexpensive and replacing them at the same time saves you from having to do the same labor all over again if one of them fails soon after.
Technical Specifications
The most common engines for this vehicle are the Chevrolet Small Block V8s (305, 350ci) and the 4.1L Inline 6. The timing set consists of a camshaft sprocket, a crankshaft sprocket, and a timing chain. Aftermarket upgrades include double roller chains and all-steel gears.
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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.