Bet you cant figure it out - SOLVED

[shauno]

Depending on the injectors' (plural) mileage ...
Every injector does something that is not easy to explain, but here goes ...

Most people get that, for 'x' amount of energy supplied to an injector (at sufficient force), a reliably consistent amount of fuel comes out,
and if the amount of energy is reduced or increased, the amount of fuel reduces or increases PROPORTIONATELY ...
HOWEVER!
There is a lower level under which that proportional relationship changes for 'the worse' -
it is proportional, but it is a different less favorable proportion.
People who know more about injectors than we do call this the injector's 'knee'.

It is possible that the injector is old enough, that its behavior 'under the knee' has aged worse than its behavior above it.
It is also possible that the wiring to that injector falters at very low currents / voltages / whatever.
Both are also possible - all the stuff involved is over 15 years old ...

I have a full set of new injector that were installed with the same issue .
Best i can determine is something is robbing power or signal from the drivers in the ecm but not 3nough to damage the ecm it self .

 

[shauno]

Sorry if I missed it, but was a new crank sensor tried?

Saw this video on checking injector drivers.. Looks like you need a scope to check...

Yeah unfortunately I dont have a scope and its not an internal short in the computer it self it has to be something external that is pulling the ground away from the injector signal wire .

 

[shauno]

As an update today I pulled the entire fuse block and replaced it with a known good block same issue as before . I did remove all the fuses from the original box and disassemble the board to clean everything . Then reinstalled all the fuses back and installed the clean box back into the truck , also no change .

The only lead I have is the loss of signal on the injector trigger . It will blink a noid light for a few seconds then go out for a minute then come back and continue to do that

 

[j91z28d1]

Ive checked the pigtail connection to the injector . Either way my noid light fits firm in the plug and ive wiggled it to double check it wasnt loose .

To address the voltage drop across the circuit ive put a jumper wire to the injector with direct power to rule out injector 1 pulling amperage from injector 3 .

Ive verified the entire wire from the injector to the ecm with a frequency tracer (sends traceable frequency down the wire to check for internal breaks) . Ive also pushed on and reset the pins on the injector wire at thr ecm

To the guy who drop the schematic i have pro demand to pull schematics i was looking for an internal schematic for the ecm to see if there was a cross over or share circuit inside the board . But thank you good looking out on the wiring diagrams

"Ive verified the entire wire from the injector to the ecm with a frequency tracer (sends traceable frequency down the wire to check for internal breaks) . Ive also pushed on and reset the pins on the injector wire at thr ecm"

I don't feel like this is a good enough check to rule out a bad wire. depin the injector wire, ground it and have the full amp load of the injector on the wire and test the voltage drop from end to end is what I'd do before ruling our bad wiring.

cause if this isn't bad wiring.. there's not much else you can do besides cut your losses, part it out and buy something else?

 

[mikez71]

Best i can determine is something is robbing power or signal from the drivers in the ecm but not 3nough to damage the ecm it self .

I'm thinking bad ECM if you're wiring checks out. NVM I see you tested with a good known ECM.
Because there's nothing left in the schematics.. that might somehow explain why different tunes act funny.

 

[Fless]

Agree with @j91z28d1 on testing the ground switched wire. Also seems like the output circuits of the PCM should be tested.

 

[Corywg]

So I have searched and read for days but I hope we can figure this out and help others who have had a ghost like this .

Let's start with the vehicle
2009 yukon xl 5.3 4l80e
The issue is a miss at idle that slows off idle
Yes the afm has been deleted with gm parts
My background is cars . I was a tech for nearly 20 years doing diag and for this to stump me is borderline embarrassing. With that let's get into the issue

This vehicle was purchased not running after an afm lifter failure . I pulled the engine that was only a year old reman and completely rebuild it from the ground up new everything other than coils . Everything is clean . All the parts used were gm or better . Got everything in and the truck ran great for nearly a month .

Now it has a missfire on cylinder 3 . So I go down the rabbit hole . I check the plugs wires coil injector all are working properly . Do a compression test both stadic and dynamic , both are normal and close to all the adjacent cylinders . Do a leak down test , no abnormalities there either . At this point im lost . So I start grasping at straws . Pull the plenum and replace the whole intake . No changed .

Now we get into the programming . Hp tuners was used to deactivate the afm . So I pull up the original tune that was read from the computer, build a new tune and wipe the old tune . Everything is the same still missing on 3 .

Now the weird . I install the base factory tune and the truck is missfiring on cylinder 1746 but I have 3 back . So I think maybe its the tune . From this point I use techline connect (gm software) to do a full reprogram with updates and then turn off the afm again through hptuners . Still missing on 3 .
At this point im banging my head against the wall so I leave my noid light hooked to cylinder 3 while its running and the light stops blinking . So I found the problem , right? . Nope

I have narrowed it to the signal wire , I have voltage to the injector but the trigger signal will drop out . The wire to the injector has been checked all the way to the ecm with my signal tracer . No internal brakes . The odd part is if I disconnect the injector on cylinder 1 then the signal for 3 comes back . Per the wiring diagram 1&3 share a main fuse . I have supplied power to the injector thinking injector circuit 1 was drawing power away from 3 but no change . I am at a total loss we replaced the ecm with another known good ecm still no change.

This has me completely baffled . Im open to ideas or help and before you say ground I have checked all the grounds the gen4 computers have one main ground in the harness and I have supplied ground to the circuit directly at the ecm .

Im stumped , my gm tech friends are stumped . Im open to ideas up to and including a bonfire

Just for elimination purposes have tried swapping around plugs to see if miss follows possible bad plug from factory. Microcrack in the ceramic or something like that could be heat soak related on that cylinder. Possible corroded wire

 

[Tazio]

Did you swap out or exchange #3 plug? I've had ceramic insulators crack and drop down over the air gap and cause a miss but under load the plug would fire at running speed! If you removed the plug and held threaded end up it would look normal.

 

[RET423]

The stock tune bringing 3 back up until AFM is turned off is odd & would seem to eliminate a harness issue as would the signal trace, buts it's a harness issue because it has to be

Make up an external wire for number 3 with a new pin & run it externally from the ECM connector to a new #3 injector connector & see if it still misses with the original harness removed from the equation

 

[Geotrash]

I'm late to the party but thankfully I have all of the answers.

Excellent troubleshooting process so far. My next steps would be to replace the spark plug and plug wire with new ones to rule out a cracked insulator or bad lead. If still no change, then I'd do as @RET423 suggests and make up an external wire for the #3 injector.

 

[fredcook]

I'm going to put my networking hat on. Networking as in data networks.

A few times over the decades, I've come across a data loss situation where all wiring tested perfectly. No breaks, no signal weakness, etc., in the wiring. Just intermittent data loss. Long story short, the issues were a result of bleed. Bleed from a neighbor. The technical term was EMI. Basically, a neighboring cable would cause electromagnetic interference if data cable shielding wasn't sufficient. A common Ethernet issue with low quality twisted pairs. The result appears as though the data cable is being unplugged. When I've seen this as an intermittent scenario, it was because neighboring power was turning on and off.

Back to the Yukon... You've checked wiring for #3 thoroughly. Have you checked neighboring wiring? Anything worn or open running parallel or crossing over #3's wiring that might cause EMI? When you checked #3's voltage/ground, did you use a multimeter or oscilloscope (if you did I may have missed it in the thread)? Multimeter will give a nice average, scope can capture the signal of a noisy neighbor. Bottom line is, I'd be on the hunt for something unrelated that is causing interference, draw, or interruption to the signal. If this can happen with shielded data network cable, it can certainly happen with the unshielded wiring in our vehicles.

Reminds me of the time I was at a redlight and lightning struck about 50 yards away. Every modern vehicle in the intersection stalled or sputtered, moving or not. There were a couple of 1960/70 somethings that kept going like nothing happened. No damage, but enough EMI to momentarily draw away or interrupt the low voltage signals in our harnesses.

 

[mikez71]

Cool story about the lightening!

 

[Fless]

The lightening happens every morning, right as the darkness fades. Lightning is a whole 'nother thing.

 

[mikez71]

I'm thinking you might as well run a separate trigger wire to ECM as others suggested.
I know you checked that wire, but with the engine running?

Perhaps a break in the wire, intermittent with engine vibration.
Goes away under load because engine is pulled to one side.
Went away with AFM enabled because the engine was shaking so bad?

 

[shauno]

Sorry guys quick update on this mess. So out of frustration I pulled the head . This is the lifter on 3 that was causing all the issue . Never made a noise or a tick . The retainer came apart and was making the valve appear to be opening and closing properly with the cover off but it was obviously not . As for the injector pulse coming and going and for future reference for others . If your truck sees a missfire if will periodically shut the injector down to keep it from washing the cylinder. Found this helpful advice deep in a tsb .

This was a brand new gm lifter that locked up . Passed all kinds of compression tested and only found a slight abnormality when we checked it with an oscilloscope ( reason for pulling the head) ..

In all my years this was the weirdest one yet to diag .

Thank you to all who help and had good input on the issue .

 

[j91z28d1]

can't get good parts anywhere these days. I guess the cam is toast?

out of curiosity, what's the plunger retaining clip look like on the other side.

been seeing the only good oem lifters have a goldish color clip.

 

[mikez71]

I don't know how that thing looks like that.. and how that did not make noise.. and how you had ok compression with that lifter plus a retainer coming apart..
Thanks for sharing!