When you posted you checked for spark, you did that experiment by shorting the starter relay. That’s not the best way to check for spark, b/c that’s not the normal method to start the car. You need to repeat this experiment with a helper turning the key to “start” while you check for spark at several spark plugs.
Think of it this way. The engine will start when you turn the key to “start” – with only a few exceptions – if the following five things are working correctly:
Good healthy spark on each cylinder
Spark occurring at the right time in the cycle
Correct amount of fuel and air entering the cylinders
Sufficient flow through the exhaust system to rid the cylinder of exhaust gasses
Compression in each cylinder meets manufacturer’s specs
Suggest you or your shop start your diagnosis by investigating those five items.
The one case I had where the above five items above tested “ok”, but the car still wouldn’t start was a VW Rabbit, years ago. The reason it wouldn’t start intially was a clogged fuel injection system. But I fixed that. I discovered later that my repeated starting attempts had flooded the cylinders with so much gasoline even with the fuel system problem corrected, it still wouldn’t start. All that was required was remove the spark plugs and let it sit overnight so the gasoline could evaporate.
At this time and actually prior… We need to check all things needed for combustion. We all know what those things are…any one left out of that recepie and you gots problems.
If you used start fluid with no change…methinks you have no spark.
Those coil packs die out all the time… just check for spark at the end of any plug wire…then test at another. If no spark…check fuses for ignition system, that roll over safety relay/circuit breaker thing in the pass kick panel.
This is one of those situations where I wish I were onsite. Sometimes loosening the main plugs to the ecu and plugging them back in…refreshes a connection or two and Voila…she runs.
Id be looking for spark…if none…dont stop till you get spark…Injector pulse would also be nice…the two of these are directly related to each other via the ECU actually.
Uh oh… THAT my friend is NOT GOOD. Many causes for this…but my guess would be a deterioration of the timing chain tensioner and chain…then a jump of time…then bent valves.
If you arent a mechanic…the vehicle is toast. Shame…those 4.0 V6 engines are actually very very durable. My Explorer went over 250K and the engine was the strongest part when I let a friend use the vehicle and they crashed it “for me” It ran just as perfect as it always had to the salvage yard. Was a sad day for me.