[HELP] Crank-No-Start After ECM Swap and Tune ? Fuel Pump Command 99% But No Prime

[navedj786]

Hi everyone,
I?m working on a 2007 Chevy Avalanche with the following setup:

Engine: LC9 Gen 4 5.3L (stock cam, stock injectors, stock headers)

ECM: E38 (used replacement, matched OS)

Transmission: 4L60E

Fuel system: Stock

Tune: Modified via HP Tuners (VCM Editor/Scanner)


Here?s what happened and what I?ve done so far:

1. I replaced the ECM and flashed a tune with VATS disabled


2. Engine cranks but won?t start


3. Using VCM Scanner, I confirmed:

Fuel Pump Commanded = 99% during crank

RPM = 0 (likely no crank signal)

No injector pulse, no fuel pressure



4. I suspected the fuel pump wasn't priming, so I:

Swapped the fuel pump relay

Checked fuses

Jumped the relay manually ? pump works fine when jumped

Verified ECM is outputting command but no voltage at the pump plug unless jumped



5. Crank relearn is pending because the engine hasn?t started


6. No DTCs appear in VCM Scanner


7. All grounds have been checked and cleaned



At this point, the ECM is commanding the fuel pump, but the relay doesn?t seem to close unless manually jumped. Is it possible the ECM output pin is damaged? Or is this a wiring continuity issue between ECM → relay → pump?

Any help or direction is appreciated!

Thanks in advance.

[gtstorey]

no crank signal kills a lot of things as a safety but I don't "think" fuel pump is one, but I could be wrong. But it's pretty much impossible to diagnose electrical problems remotely.

[Alvin]

Does the tach move while cranking?

Since you just swapped motors double check everything especially grounds. Even the pro's will make mistakes.. Don't take it for granted.

[blindsquirrel]

Verified ECM is outputting command but no voltage at the pump plug unless jumped

If that is true and your testing was accurate, how is that going to be fixed by changing settings in the ECM?

screenshot.08-04-2025 21.48.33.png

In your testing, did you verify ECM C1 #50 has +12v when pump is commanded ON, and no volts when it's not?

When you 'jumped the relay manually' did you do that across pins 30/87? That tells you something about the power side of the circuit, but nothing about the control side.

On the control side, the relay has a constant ground at #85, and is activated by +12v from the ECM on #86. The way to test the control side of the circuit is, does it have a good ground all the time at #85? G102 is unlikely to be the problem, as that also feeds seven other relays (including RUN/CRANK, RUN, STRTR, along with FUEL PUMP) and you say it cranks on the key...

If you put your own +12v to #86 does the relay click and the fuel pump run?

Then, is there +12v at #86 when ECM commands pump ON? If there is +12v at the ECM, but not at relay #86, the wire is broken somewhere in between.

[RoninDusette]

Check to make sure you are getting spark too. A lot of people when doing swaps into older chassis's forget to make sure the ECM has power WHILE cranking (you can check by setting the key to run, and jumping the starter directly AT the starter with a screwdriver or wires between the main 12v post and the 12v switched post. JUst make sure it is NOT in gear).

If you can start it like I mentioned, then you are losing power during cranking, and not getting spark. It's a very common issue I see a lot (I tune and work on swaps full-time).

You don't have to suspect if it primes. You should be able to hear it pretty clearly. If not, make sure you are running power to it (wire coming off ecm is 12v, not ground, if I am not mistaken). You could also just pull one of the fuel lines, and have someone turn the key. Aim it toward a bucket and see if it is kicking out fuel.

If you have no pulse, then pressure won't matter. Maybe also try jumping the fuel pump directly to 12v at the battery and then jump the starter with the key set to RUN like I stated above. If it runs and stays idling, then it's not vats.

As for VATS, make sure you set the VATS patch (or patches) in the OS tab to DISABLE, then go engine diag tab, and uncheck SES and set to NO ERROR REPORTED for ALL VATS-related DTC (anything with the word "immobilizer", not given fuel pump enable signal or something like that, immo environment not correct, etc). I forgot the whole lot of them, but they stick out like a sore thumb. If it sounds like immo-related, it probably is). Then write ENTIRE, key off for 30 seconds, then it should start if wired correctly.

First and foremost, take the ECU out of the equation, and try to jump the started as I stated. If that doesn't work, then check if you are even getting fuel by unhooking the line and watching for fuel via key. If not via key, then jump the fuel pump directly to 12v. If it doesn't work, then bad fuel pump or wiring. Just narrow it down, and take the bulk of it out of the equation. Process of elimination will yield the culprit quickly.