PCM needs to 'initialize' each time you start the truck. (at least, that's the way I see it.... no real 'facts' to back up this claim, but, it sure does make sense given what I know about the sensors....) Crank sensor tells the PCM "a" cylinder is approaching TDC, cam sensor indicates which "side" of the firing order. (side "1" 1,8,4,3 side "2" 6,5,7,2) as it is an On/Off sort of deal, and switches "state" once each crankshaft revolution. (takes two full revolutions to fire all eight cylinders.) So, the PCM knows a cylinder is coming up from the crank sensor, as soon as the cam sensor switches state, it knows WHICH cylinder is coming up. At that point, it will start firing the injectors in the correct order, at the correct time. (assuming everything is set within spec). Since the cam sensor only switches state between cylinder 2 and 1, and then again between 3 and 6... and cylinder four is kind in the middle...... cam sensor is more than likely NOT your problem. (especially considering the other seven work right??) So, that leaves the crank sensor. Which, given past experience in another thread, can cause some REALLY odd behavior. That's why I am so curious about spark on number four. If you are getting a nice fat spark, the crank sensor is doing ITS job. If all the other injectors are firing correctly, the cam sensor is doing IT's job. That leaves the PCM......

 

Correct. Or the injector itself, they can go bad.

 

Brian, that's a good explanation of the spark side, but it's really how the PCM controls the firing of the injectors that I'm the most unclear on. Since it seems that the PCM just knows that "a" cylinder needs a spark, and then the distributor delivers it to the right one (via the coil, of course), how does it keep track of which injector needs to get fired?

 

The computer knows the firing order.

 

This. When the cam sensor switches state, the computer knows exactly which cylinder is next. It gets reminded once per crank revolution. It just keeps track from there.

 

I was actually looking for something a little more technical; specifically, how is that information stored in the PCM, and whether or not it makes much sense that the computer circuitry would get corrupted so that it forgets where #4 is, but remembers all the rest. Trying to rule in or out a bad PCM for the OP.

And I still say it's not the injector, since it fires when physically grounded. I still have 5 cents riding on the CKPS.

 

The computer operates from programming. Firing order is part of that programming. When Crank sensor sends sends a signal, AND cam sensor switches state, depending on which way it goes (on>off, off>on) the PCM KNOWS which cylinder to fire the injector for next. Programming tells it on the NEXT crank sensor signal, to fire the next injector in the firing order. After the initial 'state switch', the cam sensor is basically redundant, and only serves as a method of ensuring sync should the crank sensor miss a beat, and confuse the PCM.

Injectors are all individually controlled thru the ground path. All have a common power feed path. (thru the ASD relay) Only one injector is fired at a time. Not like the old batch fire system. When the PCM wants to fire an injector, it just provides ground path for whatever pulse width it deems necessary given current sensor inputs.

So, you CAN have an injector not fire, even though the spark plug does. Now, I will grant, it is indeed possible, since the injector in question is sorta in the middle of on bank, that if the PRECEDING crank sensor signal didn't show up, (in this case, cylinder 8, which would then also have a miss.) The PCM may then think that since this is the next crank sensor event it IS getting, and cylinder 1 is the last cylinder it fired, it may be firing the number 8 injector, instead. (give a multi-cylinder miss.) But, then comes cylinder 3.... which is the break point in the firing order, where the cam sensor switches state, and the PCM again gets clued in as to the correct injector to fire.

Looking at it that way..... checking for spark on cylinder 8 wouldn't be such a bad idea.

 

I didnt have much time tonight.I did replace the crank sensor,truck starts much better no more pop through throttle body.I still have a code P0204.All cylinders have good spark at the plugs.Injector pulse with a noide light on seven cylinders.No pulse on number four.I backed the grounding pin out of the injector and grounded real fast,engine skip changes.This tells me the injector is ok.OHMED and did a voltage drop on the wire from the injector to the PCM both ok.I think Im looking at a bad PCM.Can i use the one out of my 2000 1500 with a 5.9 auto.Do I have to program the VIN in it before I fire it up.

 

OK, that's good news that it runs better, but bad news that #4 still isn't firing. Not even sure why it's running better, unless you have the old "two problem/double-whammy" that I had when I had a bad coil AND a bad CKPS at the same time.

it looks as if HeyYou/Brian might be right about the bad PCM, but since I absolutely HATE it when someone else is right about something, here's one last theory:

I'm not even sure how the injectors physically get grounded [well, through the PCM I suppose], but is it possible that when YOU ground it it fires fine, but it's not grounded properly when the PCM tries to ground it? I guess what I'm driving at is the wiring between the injector and the PCM. Have you tried checking for continuity on both wires to the injector, for as far as you can, between it and the PCM? Or - try putting a test light between a good ground and the hot side of the #4 injector, wiggle the wiring harness around, and see if you can get the test light to flash.

If all that fails, then I'd have to say it's the PCM, although I have to say, that based on my (admittedly limited) experience, they don't seem to go bad all that often.

Keep on fighting the good fight, you will solve this.

 

OK, this is war. Re-reading this thread, you said:

"I have power to the injector, power out of injector. I did a voltage drop from injector to PCM, thats ok. The PCM is not grounding the injector. If I supply a ground to the injector it fires"

This leads me to think that either the ground wire between the PCM and injector is bad, the connections to it are bad (did you check the wiring harness connector that goes right onto the PCM? Take it off and spray the crap out of it with WD-40?; carefully examine the connector that goes onto the injector?); or ... the PCM really is bad.

Also, I haven't read that you actually tried switching injectors; I think you have to do that at this point. Couldn't tell you exactly why, you just have to.

Lastly: "I have a code P0204 injector number four".

http://www.obd-codes.com/p0204

P0204 - Cylinder 4 Injector Circuit Malfunction


P0204 means that the PCM detected a fault in the injector or the wiring to the injector. It monitors the injector and when the injector is activated, the PCM expects to see the voltage pulled "low" or close to zero. When the injector is switched off, the PCM expects to see a voltage that is close to battery voltage or "high". If it doesn't see the expected voltage the PCM will set this code. The PCM also monitors resitance in the circuit. If the resistance is excessively low or high it will set this code.

The symptoms for this code will likely be a misfire and a rough running engine. [Duh]. Poor acceleration. The MIL will also be illuminated.

The causes of a P0204 engine light code could be any of the following:

Bad injector. This is usually the cause of this code but doesn't rule out the possibility of one of the other causes:

======> Open in the wiring to the injector <======== (Emphasis mine)
======> Short in the wiring to the injector <======== (Ditto)
Bad PCM [Note this is listed last].

Possible Solutions

First, using a DVOM check the resistance of the injector. If it isn't within specs, then replace the injector.
Check for voltage at the fuel injector connector. It should have 10 volts or more.
Visually check the connector for damage or broken wires.
Visually check the injector for damage.
If you have access to an injector tester, activate the injector and see if it operates. If the injector operates, then you likely have either an open in the wiring, or a blocked injector. If you do not have access to a tester, swap the injector with another one and see if the code changes. If the code changes, then change the injector.
At the PCM, remove the driver wire from the PCM connector and Ground the wire. (Make sure you have the correct wire. If you're not sure, do not attempt) The injector should activate
Replace the injector
[And, of course, check the wiring; why they listed this as a cause, and not a solution, I have no idea, except for this theory of mine that states that most people are stupid].

Good luck.