I'm not familiar with your specific model but I can share some generalizations. First of all, you found the cause of the insufficient EGR flow and fixed that, but that has nothing to do with a cranks / failure to start. The EGR valve should be closed at idle and low speeds, so during cranking that just leaves you with fuel and fresh air in the cylinders.
Normally there is only a small rubber vacuum hose going to the fuel pressure regulator. There must never be any gas leaking out of that port. GM has a lot of trouble with that on their older trucks, but leaking regulators are rare on all other brands.
The next thing is to understand how all fuel pumps work. If a fuel line gets ruptured in a crash, that would lead to raw fuel being pumped onto the ground, and a major fire hazard. To prevent that, with a ruptured line there will be no fuel pressure. With no pressure, the injectors can't spray fuel into the engine. With no fuel, the engine stalls. One of two methods are used then to stop the fuel pump. On some engines, especially on older cars, a tap on the oil pressure sending unit sends current to the fuel pump or the pump's relay. Engine stalls, oil pressure is lost, switch turns off, and the fuel pump stops running.
On most cars the fuel pump is run by a relay, either a dedicated fuel pump relay or an engine relay. Chrysler calls that the "automatic shutdown" (ASD) relay. Some import manufacturers call that the "injection" relay. They all work basically the same way. That relay will turn on for one or two seconds when you turn on the ignition switch to insure fuel pressure is up for starting in case it bled down over days or weeks. Next, the Engine Computer turns that relay on again when it sees engine rotation, (cranking or running). As long as the engine is spinning, the fuel pump will be running. The problem is it's not uncommon for fuel pressure to bleed down to nothing, and that one or two-second burp isn't enough to get it up to normal. The pump will run, but much slower than normal, while you're cranking the engine, but the pressure will still be high enough within a few seconds.
The point of all this is you need to connect a fuel pressure gauge, and the readings only have validity when you're cranking the engine. If you just turn on the ignition switch and expect to have fuel pressure, you might be disappointed when there's actually nothing wrong. This is where most people get stuck. The clue you would like to observe, if possible, is to hear the hum of the pump for that one second when you turn on the ignition switch. If you hear that, move on. Fuel pumps often fail intermittently at first, but they fail to start up. If you hear it running for that first second or two, it's working and you don't have a fuel delivery issue.
What IS pretty common though, especially on domestic models, is a failure of the crankshaft position sensor or the camshaft position sensor. A few engines only have one of those but most have both. It's the pulses from those sensors that tell the Engine Computer the engine is rotating and it's time to turn the fuel pump back on. On almost any brand of car, the most common failure of either of those sensors is an intermittent failure when they get warm, then they work again after they cool down for an hour. The typical story is the engine wouldn't restart after filling with gas or after running briefly into the store. That gives the engine time to go through "hot soak", meaning there's no air blowing though the car to keep those sensors cool. The internal engine heat migrates around to those sensor and causes one to fail.
Normally the clue to a failed sensor is you have no or low fuel pressure since the pump isn't being turned on during cranking, AND you have no spark. The Engine Computer needs those signals to know when to fire the spark plugs and the injectors, so with a missing signal, it doesn't know when to fire either of them. Most do-it-yourselfers get hung up on the first thing they find missing, either the loss of spark or the loss of fuel pressure. In most cases, the ignition coil(s) and ignition system are probably responsible for one or two percent of no-starts. A failed fuel pump probably causes five percent of no-starts, although that's what most people replace first when they're guessing; and by far the majority of no-starts are caused by a failure of what gets both of those systems turned on, which are those two sensors.
The Engine Computer has the ability to detect a missing signal from one of those sensors and set a diagnostic fault code to indicate which circuit to diagnose. The problem is there is always a pretty good list of requirements that must be met to set a code, and those may not be met in the few seconds it takes for a stalling engine to come to a stop. Often the code won't set during cranking either. In those cases when you have no spark and no fuel pressure, you need to connect a scanner to view live data to see what's missing. I use Chrysler's DRB3 scanner. A lot of independent shops bought them because it can do emissions-related stuff on all brands of cars starting with '96 models. On that one, each sensor is listed as "No" or "Present" during cranking. Without a scanner, all you can do is guess based on the symptoms, and hope, if you replace a sensor, you don't add more problems. Specifically, some of them have to be adjusted to very precise air gaps.
The last thing to be aware of is when you do get a fault code or when you find a signal listed as missing, that doesn't mean that sensor has failed. Way too many people read the fault code description, then say, "the code told me to replace... ". In fact, fault codes never say to replace parts or that they're bad. They only indicate the circuit or system that needs further diagnosis, or the unacceptable operating condition. When a part like a sensor is referenced in a fault code, it is actually the cause of that code only about half of the time. What you're paying the mechanic for is the time to rule out wiring and connector problems, and mechanical problems related to that sensor. Reading and recording any fault codes is just the first, the fastest, and the easiest step in a long list of diagnostic steps.
Monday, September 28th, 2020 AT 4:33 PM
(Merged)
