Calamity averted
COINCIDENTALLY, once I had buttoned up the repairs to the 300Tdi, my very next job was a timing belt change on another 300Tdi-engined Defender, this time as part of a routine service visit. This was a factoryfit 300Tdi which sits a long way further forward in the engine bay than the one I had just been working on. Even with the fan and cowling removed, there isn’t a huge amount of room between the radiator and the front of the engine, but I soon had the front cover off, revealing another large pile of rubbery fluff and a belt which had been badly worn along its front edge.
A short history lesson: as originally designed the 300Tdi had shoulders on the tensioner pulley to stop the belt wandering off the pulleys, along with a plain, unshouldered toothed crank pulley like the 200Tdi. Within a fairly short time the factory started getting reports of cambelt failures on 300Tdis caused by the belt rubbing against the tensioner shoulders and wearing away until it became so narrow that it snapped under load.
There were several issues with the design, the main one being that the timing case was not quite as strong as it needed to be. Unless great care was taken when fitting the injection pump, the case could bend a very tiny amount, just enough to move the injection pump pulley off-square so that the belt was pushed up against one of the tensioner shoulders (usually the one at the front of the engine). Land Rover produced a couple of modification kits, in which the main feature was that the shoulders were now on the toothed bottom pulley, with the tensioner being plain and unshouldered.
This design performed a little better but still did not entirely solve the problem. In the end Land Rover redesigned the rear support bracket for the injection pump, fitting a couple of sliding sleeves which stopped the pump pulling the timing case out of shape when the mounting bolts were tightened. The later tensioner and pulley set-up fits the earlier casing, the only differences being the removal of a large washer behind the idler pulley and the use of a short cap head screw rather than a long bolt and clamp to hold the tensioner in place.
Most 300Tdis still in service will have had the later set-up fitted by now: if I come across one with the early design I always change it to the later system, not least because INA (the original manufacturer) no longer supplies tensioners of the early type. The shouldered crank pulleys originally had thin steel discs spot-welded to the pulley centre. These can fail in service, and I usually bin them in favour of the later one-piece pulley (part no. LHH100660).
Sometimes I find that when I fit the new belt it doesn’t run true on the pulleys: after turning the engine by hand half a dozen revolutions I should be able to slip a thin feeler blade between the edge of the belt and the guide shoulder. If the belt is being pushed forwards against the shoulder this can usually be dealt with by fiddling with the injection pump bracket to fractionally change the alignment of the pump pulley relative to the belt. On early 300Tdis Land Rover recommended replacing the entire timing case, but I’ve never found this necessary.
So what happens if you just fit a late tensioner and idler to an early engine without changing the bottom pulley? You get a chewed belt and a timing case full of black fluff, which is exactly what I found myself looking at with this vehicle. The belt must have been running reasonably true on the pulleys. There was a groove in the front cover where the belt had been rubbing against it, but looking at the belt I suspect it had done a fair few miles like that. It would have failed in the end: the belt had been worn to about three-quarters of its original width and was contaminated with oil from a leaking crank seal. These seals last about as long as the belt and should be changed at the same time. The seal on this vehicle was rock hard and most likely the original from 1995.
On 300Tdi engines there is an argument to be made for inspecting the timing belt every 36,000 miles rather than letting it run unchecked to the full change interval (60,000 miles on Defender, 72,000 on Discovery and Range Rover). If you know that the belt has the later tensioner set-up, solid bottom pulley and that the belt is running true, there’s no need to worry, but it only takes an hour or so to remove the outer crank pulley and front cover, inspect the internals and put everything back together. That might save you from having to deal with the fallout from a broken belt.