1970 PONTIAC BONNEVILLE

Typical onset: Original units often failing at 80,000-150,000 mi, but age is the bigger factor

Symptoms: Slipping between gears, especially 2nd to 3rd shift, Delayed engagement when shifting to Drive or Reverse, Metal shavings in pan, burnt ATF smell, Loss of all forward gears (pump failure or sprag clutch)

Fix: Full rebuild with updated clutches, bands, and seals. The TH400 is strong when fresh but servo bores wear oval and clutch packs burn up from age and heat cycling. Expect 12-16 hours labor for R&R and rebuild. Add cooler line replacement while it's out.

Estimated cost: $2,200-3,800

Typical onset: 100,000+ mi, or sooner if oil changes were skipped

Symptoms: Deep knocking or rumbling at idle, worsens under load, Oil pressure drops below 10 psi hot at idle, Metallic debris on magnetic drain plug, Catastrophic failure if ignored—spun bearing or cracked crank

Fix: Requires engine-out and crank inspection. If crank can be saved with a turn, you're looking at bearings, honing, and reassembly—18-24 hours. If crank is damaged, add machining or replacement. Many opt for full rebuild at this point since you're already in there.

Estimated cost: $3,500-6,500

Typical onset: 80,000-120,000 mi depending on maintenance history

Symptoms: Blue smoke on startup or deceleration, Excessive crankcase pressure, oil pushed out breather, Oil consumption 1+ quart per 500 miles, Poor compression readings across multiple cylinders

Fix: Rings alone won't fix it if bores are tapered or scored—requires bore/hone and oversized pistons. That's a full short-block job: 20-28 hours for disassembly, machine work, and reassembly. Most owners go full rebuild since heads should come off anyway.

Estimated cost: $4,000-7,000

Symptoms: ATF dripping from radiator area or frame-mounted cooler, Transmission running hot, burnt fluid smell, Pink fluid in coolant (if internal radiator cooler fails), Crusty, weeping steel cooler lines along frame rails

Fix: Factory cooler is often inside the radiator; if it leaks internally, you contaminate both systems and risk transmission death. External frame-mounted coolers rust through. Replace lines and add auxiliary cooler for longevity. 3-5 hours depending on access and rust.

Estimated cost: $400-900

Typical onset: Varies, often appears after clutch riding or heavy towing

Symptoms: Excessive crankshaft endplay—pushes flywheel into bellhousing, Grinding noise when clutch is depressed (manual) or torque converter rattle, Hard starts, flywheel dragging on block, Oil leaks from rear main as crank walks forward

Fix: Requires crank removal to replace thrust washers. If crank thrust surface is scored, it needs machining or replacement. 18-22 hours labor. This isn't common but when it happens, it's catastrophic if ignored.

Estimated cost: $2,800-5,000

Symptoms: Clunking when shifting into gear, Excessive driveline vibration at highway speed, Transmission tail housing visibly sagging, Harsh engagement, especially Reverse

Fix: Rubber mounts deteriorate over decades. Replacement is straightforward: support trans with jack, unbolt old mount, install new. 1-2 hours. Cheap fix, big improvement in feel.

Estimated cost: $150-300