Check your ECU as well as the spark plugs and spark plug wires. If something happened to them (corrosion, etc) then they may not be carrying the current. It seems odd though for this to happen to an entire engine. Have you tested all the plugs?

I have a 1998 Ford Explorer that turns over, but won't start, it has no spark, 4.0 L V6 engine

Have a 1998 Ford Explorer with "won't crank" problems since October of 2007. We replaced the fuel filter and then the fuel pump. It has been towed 3 times since. Earlier in 2008 had the crank sensor replaced. It quit two weeks ago, sputtered and engine quit on major interstate. They replaced the Coil pack. Also, repair shop noted it was out of gas. They had a theft over the week and assumed gas was siphoned. Today it quit along a major road. Towed to shop and waiting to hear.

Engine cranks normally, but it does not fire: You turn the key to start and hear the starter motor crank the engine, but when you release the key, the cranking stops and there is silence. This means that the battery and starting motor are working properly, but the engine is not firing. If you continue cranking the engine over in this way, the battery will eventually run down and will need to be recharged, but the battery and starter are not the cause of your problem. There are a number of causes for this type of no-start condition, the most common being that you are simply out of gas. Assuming that you have fuel in the tank, you will need to go through a series of tests to determine what is causing the problem. The testing procedure requires that you use specialized equipment in order to determine the problem area. There are three main tests in order to get you pointed in the right direction. You will need to test for Spark, Fuel and Compression, in that order. As soon as you see a problem in one of those areas, that is where you will need to concentrate your efforts. Spark: An easy way to test for spark is with an inexpensive spark tester. This is a device that is readily available at most auto supply stores. You use it by simply holding it next to a spark plug wire. If you see the neon lamp flashing while someone cranks the engine, then you have spark and should move on to checking for fuel. If there is no spark, or a very weak spark, you will have to do a series of methodical tests that vary depending on the type of vehicle. You will need a repair manual for your car in order to get the correct diagnostic procedures. A good source for on-line repair information at a reasonable price is Alldata-DIY. Fuel: First step here is to listen for the fuel pump running inside the gas tank. When you turn the key to run, you should easily hear the pump come on, run for a few seconds to build fuel pressure, then turn off. If you do not hear it, it could mean that the fuel pump or circuitry is bad. (Fuel pump failure is a common problem on modern cars.) Fuel injected cars are very sensitive to proper fuel pressure. If the pressure is off, even by a few pounds, it will cause noticeable performance problems, or a no-start condition. To check for proper fuel pressure, you will need a fuel pressure gauge that is suitable for your type of system. A fuel injected engine (found on just about every vehicle less than 20 years old) produces very high fuel pressures and requires a fuel pressure gauge that reads up to 100 pounds per square inch. This type of gauge has a threaded connector that must match the pressure tap on your fuel rail. Since you are working with a highly combustible fluid which can be quite dangerous if you do not know what you are doing, you should leave this step to a pro. Compression: If you know that you have spark and fuel, the next step is to check for compression. For this, you will need a mechanic's grade compression tester that will screw into a spark plug hole. You will need to remove the spark plugs and use the compression tester to test the compression on each cylinder. If the compression is very low on all cylinders, that is a sure sign that the timing belt (or timing chain depending on the engine) has failed and will have to be replaced.

The history about my 1997 Ford Explorer starting problem. I bought this explorer from a woman who wanted to get rid of it because it had a random problem of not wanting to start. She was nice enough that she didn't want to “dump it on anyone”, but I bought it thinking I can take care of this. Her mechanic had never been able to find the source of the problem. Well, neither could mine. I went four years with the problem. Twice the car had to be towed to the mechanic and each time it started right up. The last time it wouldn't start in my driveway, I decided “I have to troubleshoot this myself”. And I did. Here's is how to troubleshoot for the problem my explorer had. You will need a battery charger because you may run the battery down as you troubleshoot. My explorer would crank but not fire. This means no gas or no spark is getting to the engine. To check gas let the car sit for an hour. Then smell the output of the exhaust pipe (this is to get a reference sniff). Then, crank over the engine for 20 second intervals about four times and go to the output of the exhaust pipe and smell to see if fresh gasoline is passing out the exhaust. If the answer is no it means the fuel injectors are not working or the fuel pump is not working. You can check the fuel pump by connecting a fuel pressure gauge to the shrader valve on the gas line located near the front drivers side top of the engine. When cranking the engine, the fuel pressure should be about 40 psi. If not at this level, you most likely have a fuel pump problem. If okay, next you must check for spark. You can buy a cheap neon bulb sensor at the auto parts store that you can hold near a spark plug wire, or you can pull a wire and hold it using pliers with insulated handles. Hold the tip of the plug near the engine block and have someone crank the engine over. No spark is actually a good indication as it confirms that the engine control module is not firing the fuel injectors and it is not firing the spark plugs. This is because the module is not getting enough information to determine when to fire the injectors or the spark plugs. This points to a bad sensor somewhere. I do not have a manual for the 97 explorer but on many cars the computer looks at many sensor signals. There is a sensor to measure air flow, a sensor to measure the position of the throttle (gas pedal), a sensor to measure the position of the cam shaft (which opens the valves), a sensor to measure the position of the crankshaft (it must know where the engine is on its compression stroke), a sensor to measure pressure from the fuel pump, and who knows what else. On my 1997 I had my mechanic change the cam sensor and it did not solve the start up problem. So my first thought was: “Which of these sensors would start working if the car is towed?” That was puzzling, so I decided to proceed to the other sensors one at a time. I concluded the crankshaft sensor was important because if the engine control computer didn't see that signal it would not fire the injectors or the spark plugs. The crankshaft sensor is located on the bottom passenger side at the front of the engine. At the lower front of the engine you will see a crankshaft pully with grooves in it. The sensor module is a hall effect sensor that includes a magnet. As the crankshaft pully turns, the grooves and peaks in the pully alter the magnetic field from the magnet and the hall effect sensor detects this and sends a series of pulses to the engine computer to tell where the pully is in its rotation cycle. If the computer does not get a good signal from this sensor it will not fire the injectors or the spark plugs. I (being an electrical engineer) understood that if the sensor is to far away from the grooves on the pully, it will not produce a good signal. And because the car would start up if it was towed I figured if I pushed on the sensor with a wrench toward the pully, maybe it might change how it behaved. I did this and then got out from under the car and it started right up. I then knew the crankshaft sensor was the problem. Here is the sequence to change the sensor. You need to release the serpentine belt. On my explorer there is a picture of how to do this on the top of the radiator area. But first you must remove the air intake tubing to get to the tensioner pully. Put the right socket wrench in the square hole on the belt tensioner and rotate it so as to release tension on the belt and remove it from the top of the alternator pully. Under the car move this belt out of the way so you can reach up to the crankshaft sensor area. Oh, there is a plastic guard across the bottom area of the engine that has a lot of dirt that will fall in you face so push on this guard with your hand and make all the dirt fall out before you put your face down there. To remove the sensor, you will need patience, determination, and a 10 mm socket with a small socket wrench with 2 ½ inch extension, a 10 mm box end wrench (straight, not angled). First thing is to remove the wiring connection. You do this by pushing in on the square metal ring that is around the connector. Push it in and while holding it in, pull the connector loose. Then use the 10 mm wrench to loosen the two bolts and the socket to finish taking them out. Note that one of them is difficult and I had to use my fingers as you can not get the socket on the bolt as the connector is in the way. They changed the angle of the molded connector on the new sensor. The new sensor also has a much more powerful magnet than the old one. Replace the sensor, replace the serpentine belt, replace the air intake tubing.

FIRST CHECK THE CRANKSHAFT POSITION SENSOR, TAP ON IT WITH A HAMMER HANDLE OR SCREWDRIVER AND JIGGLE THE WIRES ATTACHED TO THE SENSOR ITSELF, THEN TRY AND START THE CAR, IF IT STARTS, REPLACE IT SOON. I SEE THIS WAS ANSWERED AND I IWENT THROUGH THE EXACT HEADACHE AND AMOUNT OF TIME WITH THIS PROBLEM. I AM SURPRISED MECHANICS DON'T KNOW ABOUT THIS PROBLEM OR HOW TO FIX IT. lOCATED UNDER THE ENGINE PASSENGER SIDE NEXT TO THE SERPENTINE PULLEY WITH THE GROVES.