It’s pretty unlikely that a 1987 Toyota van even has a limp-home mode. These were pretty simple engines and shouldn’t really tax a good mechanic too badly. Provided you have spark, fuel and compression, an old Toyota four-cylinder engine is more or less obliged to run.

If you’re happy that the carburettor and ignition are spot on, the next step is to perform a compression test to see if the engine is internally worn out. This is a possibility and would explain the sluggish performance. You should also make sure things like the air filter and fuel filter are clean and not restricting the flow of fuel.

There’s a very good chance that your problem concerns the cable or linkage that connects the gearstick in the cabin with the actual gearbox. It’s very common for one cable or linkage to operate half the gears, and a second linkage to operate the rest. If this linkage has fallen off or become detached somehow, there’s no longer a physical link between the gearstick and the transmission selectors for those gears. At which point, it’s impossible to select the affected gears.

A quick check under the gearbox will confirm whether this is the case, and fixing it should also be a simple case of reattaching the linkage or possibly replacing a broken cable.

This hasn’t surfaced as a common problem, but that doesn’t mean it isn’t happening in some instances. The HiAce rear axle should have seals on the actual differential centre as well as each axle. If these are failing continuously, there are a number of possibilities.

The first is that there’s something bent or damaged that is placing undue force or damaging the seal, causing it to fail prematurely. The second is that the seal (and its replacements) are from a dud batch of seals with either poor manufacturing tolerances or a flaw in the material from which they’re made. The third culprit would be a blocked differential breather which is not allowing the pressure inside the housing escape, and that pressure is causing the oil to be pushed out past the seals.