2004 F115 Fires but won't Start

[Queequeg2]

Went to the marina and fired up both engines. They both started and were idling while I was getting ready to leave the dock. Then the starboard engine stalled and wouldn't start. So far I've:

1. Ran YDS. No error codes. Tested coils, HPP, Injectors.... everything looked good
2. Checked the compression in all four and they are consistent at ~120
3. Pulled the plugs and ran coil tests again to make sure.
4. Swapped the ECNs to see if that was it.

Now when I first crank the engine it starts for a few seconds. If I keep cranking it will intermittently fire. Wait 60 seconds and it will repeat.

Lost for ideas and appreciate any guidance.

[Queequeg2]

Got desperate and swapped entire intake manifold with all sensors, rail, and injectors.
Pulled the flywheel to check timing. It's okay.
Swapped the relay (didn't think it was it but was easy enough so tried)

I'm leaning toward something in wiring or fault ignition switch. Is there a way to test? Someway to bypass the switch maybe. Also, is there anything in the shift control box which might be it.

[99yam40]

timing light hooked to the individual plug wires would tell if there is spark, and tell you more about when it was sparking(timing)

[Queequeg2]

Well, I've narrowed it down to a 1st for me. I had all 4 injectors unplugged and for some reason (not sure why) I turned the key. Fired right up and ran like new until whatever fuel was in the chamber burned out. I thought it was some miracle fluke. So plugged the injectors back in hoping things resolved. Wouldn't start. Systematically unplugged one injector at a time and trying to start. When I got to the bottom injector it fired right up. Obviously running poorly on 3 cylinders. Plugged the injector back in and back to no start.

I assumed a short in the harness. So pulled it off and tested every wire. Continuity through each wire but nothing across wires. I also checked the injectors for resistance.

Has anyone ever heard of this happening? What the heck could cause it.

[99yam40]

Well, I've narrowed it down to a 1st for me. I had all 4 injectors unplugged and for some reason (not sure why) I turned the key. Fired right up and ran like new until whatever fuel was in the chamber burned out. I thought it was some miracle fluke. So plugged the injectors back in hoping things resolved. Wouldn't start. Systematically unplugged one injector at a time and trying to start. When I got to the bottom injector it fired right up. Obviously running poorly on 3 cylinders. Plugged the injector back in and back to no start.

I assumed a short in the harness. So pulled it off and tested every wire. Continuity through each wire but nothing across wires. I also checked the injectors for resistance.

Has anyone ever heard of this happening? What the heck could cause it.

are you saying by unplugging all the injectors the motor will start and run?
I see no way for the motor to run without the injectors letting fuel into the cylinders.

are you saying there is no spark on any cylinder at all unless you unplug #4 injector?

[Queequeg2]

are you saying by unplugging all the injectors the motor will start and run?
I see no way for the motor to run without the injectors letting fuel into the cylinders.

are you saying there is no spark on any cylinder at all unless you unplug #4 injector?

Correct, and crazy I know. I'm assuming while cranking the engine fuel is building up in the cylinders. It will start and run with ALL injectors unplugged until all of that fuel runs out. maybe 30 second or so.

No, there is spark but the engine will not start until I unplug #4 injector. It will pop now and then but will not run. Once I unplug #4 it will start and run (poorly) on 3 cylinders.

Tomorow I can try to take a video and post it on youtube. If I can post a link I'll do so. Assuming I can't post videos here.

[99yam40]

no fuel injected motor I have ever heard of can run for 30 seconds with all the injectors disconnected.

check to see if there is spark happening on any plug when the motor will not start.

[Queequeg2]

I'm not arguing. I couldn't understand how it did more than fire once. I'll try again later today and time how long it runs. I was told on another forum that the injectors see 12V constant and ecu grounds to activate them. I tested the entire harness. Continuity on every wire and no shorts across any. All Orange/Yellow grounds are tied as I believe they should be. I'm wondering if the injector could be shorting somehow and sending 12V through the ground.

When I tested with YDS I pulled all plugs and ran coil test. It can be hard to see spark in the daylight but I could sometimes see it and always hear it. I was testing 2 at a time so possible the sound was coming from just one. I can isolate them and try again. I did swap both coils with the port engine and they worked fine on that side.

Looks like I can insert a video here. I'll try to capture it and post later this evening.

[Queequeg2]

Okay, let's see if I can post these clips.
https://youtube.com/shorts/m_uI1HghNYg?feature=share
https://www.youtube.com/shorts/M4GO46Qeij8

I didn't try to crank it very long with injectors plugged in so it only ran a few seconds once unplugged. If I crank longer, it will run a bit longer. Also, this morning it will no longer run on top 3 injectors. Whatever was allowing that yesterday has stopped. Luckily, I had my wife with me yesterday to observe it running on 3 or I would question my own sanity. I did remove the entire wire harness and search for shorts, Found nothing. Still lost on what's wrong. I'll head back out to test each plug for spark with injectors plugged in. I'm solo today so I'll have to use YDS to trigger them.

[99yam40]

only thing I can think of is,
if there is too much fuel and the plugs are sparking, the cylinders will still not burn the fuel.

guess too much fuel or not enough air

make sure the rail pressure is correct and that the Idle air control valve is open to 100% at start\
good compression, good spark at the correct time, and the proper fuel to air ratio is needed for the motor to burn the fuel properly.

[Queequeg2]

Hats off to you 99. I hooked up the YDS to test the spark and it coughed up an error code 29 (Intake pressure sensor). Unplugged the sensor and it fired up. Can increase / decrease throttle if I go slow and it keeps running. I guess unplugging the injectors helped to have less fuel and more air. Where I'm confused is the sensor is on top of the intake manifold. When I swapped manifolds, it came along. Now I need to see if it's the actual sensor or something about the signal.

[Queequeg2]

It's not the sensor, but it was what 99 indicated, too much fuel/ not enough air. Somehow the swirl flaps were completely closed. I adjusted the idle crew to open them a crack and everything appears to be working. Since I never touched ANYTHING, and the motor always ran great, I don't know what caused it. I'll spend some time investigating further to see how it happened. The screw does not easily turn and I can't see how vibration would do it.

[99yam40]

I would think that a 4 stroke motor would have an Idle air control valve that is suppose to adjust the air flow at idle with a stepper motor con*****ed by the ECU.
if it does you jumped around it, adding air manually with the throttle valve.

if it does have one and it is Stuck/not moving, and it keeps trying to move it, there is a chance of burning out the ECU circuit for that operates that IACV

[Queequeg2]

It definitely has an IAC valve. I didn't leave the sensor unplugged and I'm not running the engine until I understand what's wrong. I don't think the IAC valve is stuck. As I've mentioned, I swapped the entire intake manifold with the port engine. The IAC valve is built into the manifold, so it swapped along with it. I'll pull it and test to see if it moves freely anyway.

Feels like one step forward two steps back.

[99yam40]

Hats off to you 99. I hooked up the YDS to test the spark and it coughed up an error code 29 (Intake pressure sensor). Unplugged the sensor and it fired up. Can increase / decrease throttle if I go slow and it keeps running. I guess unplugging the injectors helped to have less fuel and more air. Where I'm confused is the sensor is on top of the intake manifold. When I swapped manifolds, it came along. Now I need to see if it's the actual sensor or something about the signal.

check to see what the weather station says the barometric pressure is and what the YDS says.
it tells what the motor thinks the altitude is and how much O2 is available