ECM-1 keeps blowing on key-on when injector harness connected 250 miles after MFI upgrade

[CSX55]

I recently did heads-up refresh of my 1999 Tahoe 5.7 4WD that included replacing the intake manifold gaskets (no leaks, but at 130k wanted to get ahead of the problem) as well as upgrading the CFI to MFI. I've put 250 problem-free miles on it since then.

Last night, pulling onto an empty freeway, I decided to, ahem, "blow the carbon out" and stress test my work with a WOT run. I got startled by the sudden intervention of the speed limiter at 98 MPH, but cruised home another 15 miles with no issues. When I parked it, the Tahoe was at about 1/4 full as indicated on the fuel gauge.

This morning, upon trying to start it, the fuel gauge went immediately to the 3 o'clock position and stayed there, and I was unable to connect to the ECM via my OBDII adapter and Torque. Turns out the ECM-1 fuse was blown. I disconnected everything downstream of the ECM-1 fuse (EVRV, EGR solenoid, injector harness) and the fuse is ok and the fuel gauge returns to its normal position only when the injector harness is unplugged.

I measured the resistance across each injector, and it ranges from 12.6 (lowest, Injector# 1) and 13.1 ohms. With the harness disconnected and a new fuse, I measured 12V across each of the pin pairs on the harness side.

Electrically, I'm not seeing what's causing the short every time I key-on with the injectors connected considering the consistency in voltage/resistance I measured on each side. So I'm curious if something may have happened as a result of the fuel-cutoff related to the speed limiter, or if perhaps I have a problem with my ECM? Is there something I need to reset? Any advice would be greatly appreciated.

 

[CSX55]

Well, a bit of an update...I checked each of the intake-side pins on the fuel injector spider assembly, and it may not show up well in the attached photo, but the highlighted pin is discolored and more of a dull golden brown color compared to the other 15 pins, which are all shiny. Also note the evidence of the plastic insulation around that pin having bubbled up through; both to me are indicative of high heat exposure.

Although the pins across that injector measure the same resistance as the others, both of those pins also have continuity with ground. None of the other 14 injector pins do. I'm not sure how or why, I can only guess that the injector failed, creating high heat which may have melted the insulation on the wires and it is now in contact with the lower intake manifold.

Unless someone has any reason to suggest otherwise, it looks like the only fix is to take apart the upper manifold and replace the spider assembly. As least that doesn't require as much work as the intake gaskets...

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[joep88cheyenne]

Check at the injector and see if there is a cause for the high heat. It should not have any difference on resistance then the other give or take 10%.

You make have burnt one injector/controller up causing the condition. In the process you may find a bate spot that caused it.

 

[shamrock246]

You can also replace just one injector if you want but for the money i would just get a new assy. Any kind of warranty left on it?

 

[CSX55]

Check at the injector and see if there is a cause for the high heat. It should not have any difference on resistance then the other give or take 10%.

You make have burnt one injector/controller up causing the condition. In the process you may find a bate spot that caused it.

That's the plan - it's either #5 or #7 cylinder, so it'll be easy to narrow down the problem one. I also plan to plug in the new spider assembly uninstalled just to make sure the installed spider is 100% the issue.

 

[CSX55]

Well, pulled the upper intake to discover that the #5 fuel injector wire appeared to be pinched between the bracket and the spider assembly, so with time and vibrations, it eventually rubbed through the insulation and caused the shorted spider. Could have definitely gotten away with replacing just the single injector, but for the same effort, I just replaced the entire spider and made sure to route the wires and place the heatshink such that there is no longer any chance of this happening again.

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