How do you diagnose the failure?
Comes from seeing many
examples of this failure from maintenance neglect, using low-quality oil of too little viscosity in highly demanding conditions not anticipated by factory engineers. It all comes down to lack of lubrication and rod-journals contact and wear away rod-bearings, then rod end-caps themselves.
Engine-rods face vastly different amounts of force depending upon power-stroke vs. exhaust-stroke. Even with 200-300bhp, 4A/7A rods won't experience more than 1000-1500 lbs on power-stroke. However, on exhaust-stroke, there's no resistance and forces at top of stroke is immense, 10000-lbs or more! The higher the revs, the higher the tension-force stopping piston's upward motion and
then pulling it down again. This is what ultimately breaks rod-bolts. You can see pieces of rod-bolts still stuck in rods here. Would need to look at rod end-cap to determine if it was lack of lubrication or over-revving that actually broke the bolts.
Another case where transmission was down-shifted to 2nd-gear @ 150mph instead of 4th-gear. In this case, extreme tension force @ +12k RPMs snapped piston-pin.
Since these typically are purchased as economy cars, usage-patterns anticipated by Toyota typically doesn't require stringent oil-lubricating properties. Even so, at 200K-miles, my wife's DX had worn-out rod-bearings from using quick-lube outfits during her university years. They usually supply very low-end oils that get too thin under regular use and allow rod-journals to contact rod-bearings and wear them out. You can see that babbit layer has been rubbed through and some copper base is starting to show.
Now, ideally, one adjusts their oil based upon anticipated usage. Here's oil-chart for my Porsche. Note that 10w30 oil most commonly used is only recommended up to 68F. Higher temperatures increase engine-temps (less delta-T for radiator to cool) and thicker and thicker oil needs to be used to maintain lubrication at high-RPMs under racing conditions (or viscosity breakdown under high-mileage).
People often get the two oil-conditions confused and collapsed into one spec. On cold-start, with street-driven cars, sure a lot of wear can occur. So you want a low-viscosity starting# of 0, 10 or 15w to ensure fast oil-flow to engine to build pressure quickly. What many forget is that AFTER
engine is started, there's also high-viscosity requirements when operating temperatures have been reached.
At operating-temps of 220-230F for street-cars in regular operating condition, the oil loses viscosity and pressure, so thicker 30, 40 or even 50w rating is needed to ensure adequate lubrication. In racing cars running at 250-300F oil-temps, an even thicker 40, 50, 60w rating is needed to maintain proper thickness for lubrication of bearings, rings and cam-lobes (same cSt as street car). Along with higher amounts of extreme-pressure/extreme-heat additives like ZDDP. During ZDDP-drought fiasco of API SL ratings, only Mobil-1 20w50 oil retained its high ZDDP levels since this is preferred OEM fill for Porsche & BMW.
I've rebuilt my track-car's engine every 1-3 yrs on regular basis for past 20-years, sometimes blowing them up prematurely due to too much boost. This gives me (and lots of other racers) some insight on wear and tear under harsher-than-street conditions, such as full-throttle 100% WOT for hours at time. Monitoring oil-temp and oil-pressure is paramount to engine's survival and vast experience of racing over past 100-years have pointed to high-viscosity oils as saving grace in maintaining oil-pressure under high-temp racing conditions. Many oil-vendors provide oils with +10 hot rating for extra heavy operating viscosity for racing. Obviously full-synthetic is first criteria:
HPL <-- this is what I use in turbo race car
Liqui Moly <-- Sponsors MotoAmerica and international MotoGP race series
Motorex <-- This is what I use on my race bikes