If you just picked up your first full-size truck or are fairly new to wrenching on them, this list is for you. These are the mistakes that come up over and over — most of them are easy to avoid once you know about them.
1. Skipping the Fluid Inspection on a Used Truck
When you buy a used truck, change the fluids — don’t just check them. Color and level don’t tell you enough. Coolant can look fine and be degraded. Transmission fluid can smell burnt without appearing dark. Differential fluid rarely gets changed and the condition isn’t obvious at a glance.
What to do: Budget $150–200 for a fluid service on a newly purchased used truck. Change engine oil, coolant, trans fluid, front and rear diff fluid, and transfer case fluid. This alone can save you from expensive repairs down the road.
2. Using the Wrong Transmission Fluid
This one kills transmissions. GM trucks from the late 1990s through the 2000s require Dexron VI in the 4L60E, 4L80E, and 4L65E. Older Dexron III is not equivalent. Running the wrong spec fluid causes shudder, clutch wear, and shift solenoid issues.
Also: Transfer cases (especially NP246 in GMT800s) require Autotrac II or GM equivalent — not generic ATF. Running the wrong fluid in the t-case causes clutch shudder and premature wear.
What to do: Always check the owner’s manual or a reliable forum thread for your exact trans and t-case fluid spec before you fill.
3. Ignoring Coolant Maintenance
GM trucks from the OBS era through GMT800 used DexCool (orange extended-life coolant). DexCool is fine when properly maintained — but it goes bad and turns acidic over time, especially if the system ever got air in it or the coolant level dropped. Acidic coolant eats intake gaskets, water pumps, and heater cores.
What to do: Check the coolant condition (test strips are cheap). If you have a GMT400 with a 5.7L, the intake manifold gaskets are a known failure — inspect them regardless of coolant condition.
4. Over-Torquing Lug Nuts by Hand (or Impact)
New truck owners often blast lug nuts on with an impact gun and move on. Over-torqued lugs warp brake rotors and can cause the lug stud to stretch or snap — especially on aluminum wheels.
What to do: Use a torque wrench. Full-size Chevy/GMC trucks are typically 140 ft-lbs on 14mm studs (most 1/2-tons) and 165 ft-lbs on 9/16" studs (3/4-ton and up). Torque in a star pattern.
5. Buying the Cheapest Parts Available
Rotors, brake pads, and tie rod ends from the bottom shelf at an auto parts store are cheap because they’re built cheap. On a 5,000–7,000 lb truck, your braking and steering components matter. A $15 tie rod end that fails at highway speed is not a savings.
What to do: For steering and suspension, use OEM or known brands (Moog, Dorman for most items, TRW for steering). For brakes, Raybestos, Wagner, or similar are solid mid-tier options that hold up well.
6. Not Checking for Frame Rust Before Buying
On OBS and early GMT800 trucks especially, frame rust can be a serious issue — not just surface rust, but through-rust at the crossmembers and body mounts. A truck that drives fine can have compromised structural integrity.
What to do: Before buying any fullsize truck from the salt belt (Midwest, Northeast, Canada), get under it with a flashlight. Poke the crossmembers with a screwdriver. Frame rot is a deal-breaker on anything you plan to use or tow with seriously.
7. Not Checking Tire Age
Tires that look good can be dangerously old. Rubber degrades from the inside out — a 10-year-old tire with good tread depth can still blow at highway speed.
What to do: Find the DOT code on the tire sidewall. The last 4 digits are the week and year (e.g., “2318” = week 23 of 2018). Most manufacturers recommend replacement at 6 years, maximum 10. If you bought a used truck with unknown-age tires, check the codes.
8. Skipping the Alignment After Suspension Work
Even a leveling kit changes your front alignment — at minimum your toe will be off. Driving on an out-of-spec alignment eats tires fast and wears out ball joints and tie rods prematurely.
What to do: Any time you touch suspension components, get an alignment. It’s $80–120 and saves tires and parts.
What’s the mistake you wish someone had told you about earlier? Drop it below.