The loudest year in the run — a brake pedal that turns to stone, and a pump that can take the engine with it.

This is the part most buyers miss, and it is why the issue above is rated the way it is. An independent used-car dealer who says he has sold twenty to twenty-five of these explains that the vacuum pump "can freeze up, and then if that happens it can take out vital engine parts" — describing other dealers who sold cars where the pump let go and "all of a sudden the engine was gone". The federal file corroborates it from the owner side: a 2018 Terrain filing reads "brake vacuum pump failed causing engine damage", with the car sitting in a shop for six weeks waiting on a camshaft that was on national backorder. The same dealer notes the saving grace — if the car still has powertrain warranty left, GM covers the engine; if it does not, "then you are in a big problem". On a 2018 that warranty is long gone. Treat a hard pedal as an engine risk, not a brake complaint.

The same dealer reports that of the second-generation Terrains arriving at his shop, "most of these cars" have significant oil leaks, and that they come from two places: the upper oil pan and the vacuum pump. The pan is the expensive one. He describes the upper oil pan on the 2.0L as roughly a fifteen-hour job, because reaching it means dismantling a great deal of what sits above it — labour that dwarfs the cost of the gasket itself. Put the car on a lift during the pre-purchase inspection rather than relying on a glance underneath, and treat wet oil around the pan-to-block joint as a serious negotiating point.

The 2018 redesign replaced the gear lever with a row of buttons GM calls electronic precision shift — a shifter with no mechanical linkage, where the car decides what gear you asked for. It gave the launch year trouble. GM bulletin 10137535 addresses cars that will not shift out of park, get stuck in gear, refuse to shift into park or reverse, or blink the gear display, with fault codes P187D, P2731 and U0100. A separate engineering bulletin describes the car dropping back into neutral when the driver selects reverse. A Service Update covering 2018 to 2020 addresses a condition where the car can be shifted out of park when it should not allow it. Work every button several times, cold and warm, and confirm the gear the dash reports is the gear the car is actually in.

A small number of 2018 Terrains left the factory with a 1.6L turbodiesel, and GM wrote a Service Update just for them. Bulletin 18209 covers cars built with the clamp and gasket joining the turbocharger to the catalytic converter improperly aligned, which can cause a diesel exhaust leak and smoke from under the hood. Two diesel cars in this year's federal file report the vehicle decelerating to 20 or 25 mph with a check-engine light — limp mode. Beyond that specific fault, a diesel this age carries the usual modern-diesel running costs: EGR, the exhaust gas recirculation valve, and the DPF, the diesel particulate filter that traps soot, both clog on short city trips, and DEF — Diesel Exhaust Fluid, the additive that cleans the exhaust — has its own pump and sensors to go wrong. Rare enough that a diesel specialist should look at it before you commit.

The 2018 carries the heaviest recall load in the run and two of them are genuinely serious. Recall 17V516 covers a right front driveshaft that can fracture and separate, taking away drive and letting the car roll away if it is parked on a slope without the parking brake. Recall 20V668 covers a start-stop transmission accumulator endcap missing its bolts, which leaks transmission fluid and can progress to a total loss of drive or, on a hot exhaust, a fire. Recall 18V358 replaces a high-pressure fuel pump that can detach from its mounting flange and damage the fuel line — note that this is a different failure from the pump problem on the 2022 and 2023 cars, and completing this recall says nothing about those. Recall 18V340 reprograms the crash sensor that fires the airbags, which can fail to power down and be dead on the next start. Recall 18V576 bleeds the brakes to clear gas pockets in the rear calipers. All five are free.