1990 GMC SIERRA 1500

Typical onset: 150,000-220,000 mi

Symptoms: Rod knock on cold start that quiets when warm, Low oil pressure at idle (under 10 psi hot), Metallic ticking that worsens under load, Oil consumption over 1 qt per 500 miles

Fix: The 5.7L TBI small-block has soft cam bearings and piston skirts that wear oval. Most common is rod bearing failure from oil starvation. Requires full rebuild or short block replacement: 18-24 hours labor for experienced tech, includes R&R, machine work if salvaging block, and reassembly. Many opt for reman long block swap instead.

Estimated cost: $3,500-5,800

Typical onset: 120,000-180,000 mi

Symptoms: Slipping or flaring on 3-4 upshift, No fourth gear (stuck in third), Burnt transmission fluid smell, Check engine light with shift solenoid codes

Fix: The non-electronic 700R4 (pre-'92) has weak 3-4 clutches and the governor can stick. Requires transmission rebuild with upgraded clutches and often accumulator spring upgrade: 10-14 hours for R&R and rebuild. Cooler line failure accelerates this—always replace cooler lines during rebuild.

Estimated cost: $1,800-2,800

Typical onset: 80,000-140,000 mi

Symptoms: Transmission fluid leaking near radiator, Fluid dripping along frame rail, Low transmission fluid without visible leak at pan, Pink fluid residue on radiator or frame

Fix: Steel cooler lines rust through where they pass along the frame or at crimp fittings near the radiator. The lines run the length of the truck and are exposed to road salt. Replacement requires new pre-bent lines or custom fabrication: 2-3 hours labor. Always inspect during any transmission service.

Estimated cost: $250-450

Typical onset: 100,000-160,000 mi

Symptoms: No-start with cranking but no fuel pressure, Intermittent stalling when fuel tank below 1/4, Fuel pressure drops under load or hot conditions, Whining noise from fuel tank area

Fix: The in-tank pump on TBI systems runs at lower pressure (9-13 psi) but heat-soaks in steel tanks. Common for pump to fail or sock filter to clog with sediment. Requires dropping tank: 3-4 hours labor. Many techs replace pump, strainer, and rubber hoses together as preventive.

Estimated cost: $400-650

Typical onset: 120,000-180,000 mi

Symptoms: Coolant seepage at front or rear of intake, Rough idle when cold, smooths out hot, Slight coolant loss without external leak, White residue or crusty buildup at intake corners

Fix: The TBI intake uses composite gaskets that deteriorate and allow coolant into the valley or cause vacuum leaks. Not as catastrophic as Vortec intakes but still requires intake removal: 4-6 hours labor. Replace valley gaskets, end seals, and thermostat while in there.

Estimated cost: $600-950

Symptoms: Erratic idle or misfiring that moves between cylinders, Ignition timing drifts over time, Metal shavings in oil filter during change, Backfiring through throttle body

Fix: The iron distributor gear can wear prematurely with modern low-ZDDP oils, especially if cam is worn. More common in trucks run on cheap oil or extended intervals. Requires distributor removal and gear replacement, sometimes cam inspection: 2-3 hours labor. Preventive: use high-zinc oil or additive.

Estimated cost: $300-500