Hello all!
Long story short, I have a 5.7L motor that is 1500 miles old. Truck now doesn't run, only cranks. When the motor was put in, every sensor on the motor was new, as well as distributor, spark plugs, wires, etc. Truck ran fine, then i noticed it would kind of lope, and want to stall when you put your foot on the throttle. Now the truck just wont start. I've replaced the injector spider thinking it was that but it isn't. The injectors aren't firing, but will fire manually when a snap-on scanner is used. So the ECM isnt sending the pulse to make them fire. The crankshaft position sensor is sending the signal to the ECM, as well as the Crankshaft Position sensor has 12v going to it with good ground, as well as injectors have 12v going to it when the ignition is turned on like its supposed too, just no signal being sent from the ecm to injectors to make them fire. Im at a loss right now because there really isn't a specific answer as to what it could be. I'm 700$ into this repair right now and no results, which is why I havent bought anymore parts until I can get some form of concrete answer. Spark is good, compression is good, fuel pressure is perfect. Just something is causing injectors not to fire, and I have no clue what it could be at this point!

Sorry this was a very long post, I'm just super frustrated and have 0 clue what to do at this time. Any help is appreciated!

Thank you all!

Nathan Gilronan

 

You’re not getting any codes? Even without a check engine light, codes can be stored.

The ECM relies on the camshaft position sensor (inside distributor) to pulse injectors during startup. Verify 12v on the pink wire (pin C of the connector on the distributor) with the key on. Pin B (brown/white) is the signal circuit back to the ECM. Check the output signal using a meter set to AC volts (an analogue meter works best here). I know with the engine running you should see at least 0.7 v AC but during cranking I would guess about half that.

If the signal is good at the distributor, check it at the ECM - in both cases make sure to back probe the connectors.

If the signal is present at the ECM, inspect the connectors at the distributor and ECM for loose, damaged or corroded terminals. Perform a wiggle test on the harness, using a meter to watch continuity on the circuit. Any reading 0.3 ohms or higher is reason to suspect the harness (touch meter leads together first as a reference - if it reads 0.1 or 0.2 ohms then subtract that from the circuit resistance).

 

No codes at all, present or stored codes. I'll have to check that wiring out there then. I didn't check it at all. I just knew that I put a new sensor in when I put the motor in, ill check that then. Thank you!

 

Did you check the ecm fuse? I've had issue twice in the past with ecu not reading pulse signals, it was the ecm. It would just die, or not start on occassion, it got more frquent with the isse. If you have the test light, make sure it is the injector issue.

 

Yup fuse is good. First thing I checked actually and have been checking just to make sure it is still good. It is just an injector issue at this point, but cannot figure out for the life of me what is causing it. Just replaced the camshaft sensor, sending signal just fine. But I find it hard to believe its the ECM only because if a snap-on scanner is capable of making the injectors fire manually, the ECM is allowing the signal to be sent. That's whats making me think its something else, but I can't figure out what.

 

If the ecm is original, it is over 20 years old. Not saying that is the issue, but since you've thrown $ at it, and still nothing.

 

Yeah that's what i'm thinking. I just bought one, gotta pick it up and get it flashed. That's literally the only thing I can think it is. Something between the camshaft sensor wire going into the ecm and the wires coming out of the ecm going to the injectors isnt happening. Especially if the injectors can be fired manually, essentially bypassing any sensor having to deal with making them fire. So that's the next thing im gonna do and see what happens.

 

You’re correct in thinking that being able to command the injectors with a scan tool rules out the ECM.

Are you absolutely sure the injectors aren’t pulsing during cranking? If you use starter fluid or carb cleaner in the intake, will it fire? If not then it’s a spark/ignition timing issue.

Edit: Good luck in throwing parts at it. I hope you solve the problem by pure chance.

 

100% injectors are not pulsing. No smell of fuel from them, also verified with test light as well as my fluke DMM. When I spray starting fluid it'll fire up but die as soon as the starting fluid is used up. So clearly its not getting fuel in my mind. Like I said above, the scan tool can make the ECM fire the injectors manually, but the truck isnt doing it. Which in theory doesn't technically rule out ECM due to the scantool bypassing any type of sensor making the truck fire. All sensors having to deal with that are working fine, there is signal going to the ECM, but nothing coming out, but again the scan tool can make the ECM fire the injectors. So something is telling me that there is something wrong in the ECM between the signal coming into it and going out (Obviously lol). The really only way I know how to verify that is to buy a ECM, flash it to my truck, throw it in and see if it starts. If it does then I know that was the issue. ECM is 20+ years old now, and from what I've read there are some things that are known to go in it? Havent dove in depth on it, just skimming through, so idk how true that is. But, we shall see. I'm open to any more info that I may be missing! I'll keep this thread updated as time goes on.

 

*UPDATE*

So its been a few days, got the new ECM installed and flashed, still nothing. Dealer said that the truck had spark but wasn't getting fuel... Turns out, it was getting fuel still up to the injectors but injectors still wont pulse. Again, I have replaced crankshaft sensor, camshaft sensor, ECM, fuel pump is only a couple months old and is working. I'm at a loss now as to what it could be. The only sensors that havent been replaced are the Throttle Position sensor, Idle Air Control Sensor, or Ignition Control Module. I wouldn't, think it would be the Ignition Control Module only because the truck turn on and cranks, just doesn't start because of no fuel being fired into the injectors. Any one else have any other ideas as to what it could be? I'm at a total loss now, and not sure what else to check. (P.S. Yes all fuses as well as relays are good)