1984 K10 LS Swap - Lots of Little Problems

Bruce Wingate · Apr 4, 2026

I've been quiet for a while - vacationed someplace warm.

My last tech support call with Holley said to power the fuel pump via a relay and run a ground from the pump directly to the battery. I don't see the point, but I spent today taking care of the relay. My wiring is out of control with too many things hanging off the battery - will be putting in bus bars probably on Monday. Next call with tech support will be for RMA's to send the ECU and harness back for checking.

@Ricko1966 - I can actually just swap the plugs for injectors #2 and #4. I'm nervous about playing around withthe firing order and either messing it up or forgetting to switch it back. I'm at a loss as to what this would show me - I already know that it is either the ECU driver or the wiring harness. Would doing this swap add any additional information? Especially since I've already swapped over the injectors, coils, plugs and plug wires.

Ricko1966 · Apr 4, 2026

Okay which is it? The driver? Or the harness? Now if you swap #2 and #4 and the problem moves to #4 you have verified that it is a signal problem. If it stays on #2 then you still haven't found the problem.IDC what you do it may be a driver or a harness. I'd have already done a volt drop test on the harness to eliminate it as a possibility. I'd just be making 100% sure I'd found the problem,before I condemned anything.

Bruce Wingate · Apr 12, 2026

Finally got a return authorization from Holley for the harness and the computer. Of course they have to go to separate locations, but I should have an answer in a couple of weeks. The tech said that if the unit has low hours that it may be warrantteed but could not make any promises.

Next up, while I wait and in no particular order:
- Install engine oil dipstick;
- Wire up fan relays and investigate if it is worth having the ECU control the fans when the AC is running or to just use a trinary switch (I'm leaning towards trinary switch because that is how it was wired pre-engine swap
- Dual fuel tank setup including new tanks, in-tank fuel pumps and new fuel selector switch
- Clean up wiring with a relay and fuse panel (this ties in with the fan and fuel tank work)

Bruce Wingate · May 12, 2026

Been a while, after Holley tested and OK'd the ECU, they replaced the harness. While I was putting everything back together, I managed to close my finger in the truck door. Nothing broken, but a cut that took a could of stitches to close.

So, everything is back together with no changes. I know I said I swapped coils, but I couldn't remember actually doing that, so I put a new coil on Cylinder #2, which also did not change things.

I have 2 plans of action going forward: First, retesting compression on the even bank.

Second, I'm going to follow @Ricko1966 advice and swap cylinders #2 and #4. I really feel uncomfortable doing this, because I just feel there is too much I can screw up. The plan for this is to swap the coil and injector connectors between the two cylinders and then edit the tune to swap #2 and #4 in the firing order. If the problem stays with cylinder #2, am I then looking at a physical problem with the engine?

DoubleDingo · May 12, 2026

Ummm, I would not recommend changing the firing order in the tune, just the injectors and coil packs to see if the symptoms change. Unless @Ricko1966 stated to change the firing order, which I doubt he did, because the cam isn't being reground to make it happen, nor the crank changed to make the firing order happen like that.

Ricko1966 · May 12, 2026

Bruce Wingate said:
Been a while, after Holley tested and OK'd the ECU, they replaced the harness. While I was putting everything back together, I managed to close my finger in the truck door. Nothing broken, but a cut that took a could of stitches to close.

So, everything is back together with no changes. I know I said I swapped coils, but I couldn't remember actually doing that, so I put a new coil on Cylinder #2, which also did not change things.

I have 2 plans of action going forward: First, retesting compression on the even bank.

Second, I'm going to follow @Ricko1966 advice and swap cylinders #2 and #4. I really feel uncomfortable doing this, because I just feel there is too much I can screw up. The plan for this is to swap the coil and injector connectors between the two cylinders and then edit the tune to swap #2 and #4 in the firing order. If the problem stays with cylinder #2, am I then looking at a physical problem with the engine?

I wanted you to do that to verify whether or not it was in the,harness/ecu. Now that those have been replaced. I'm going to go back and read through all this again. I don't remember it all.

Bruce Wingate · May 12, 2026

Ricko1966 said:
I wanted you to do that to verify whether or not it was in the,harness/ecu. Now that those have been replaced. I'm going to go back and read through all this again. I don't remember it all.

Thanks @Ricko1966

Reader's Digest Version: 1984 K10 with LS swap. Once I got it back, it ran poorly. I narrowed that down to a dead/cold cylinder #2. Checking the plugs early on (before swapping with another cylinder), the #2 plug looked brand new. Compression looked OK (about 180 psi in all cylinders). I swapped plugs, plug wires, injectors and coils between cylinders #2 and #4 and the problem remained with #2. I tested the injectors which all fired OK and the coils using both an spark tester and the inductive lead from a timing light and they seemed fine. Spark plugs were replaced.

When the #2 injector is disconnected, the engine runs pretty nicely on 7 cylinders. When the injector is plugged in, the engine will run very rich, smoke badly with white smoke and dump raw fuel out the tail pipe.

Ricko1966 · May 12, 2026

Put a vacuum gauge on it see what kind if readings you are getting,see if the engine runs the same with #2 injector plugged in but #2 coil unplugged,see if it runs decent on 7 that way. Wondering if #2 has a problem with the intake valve and when #2 hits it confuses the map sensor.

DoubleDingo · May 12, 2026

I didn't read the whole thread, but @Ricko1966 wondering if the #2 intake valve may have a problem, made me think, is this an AFM type engine? Could #2 be sludged up?

Bruce Wingate · May 13, 2026

DoubleDingo said:
I didn't read the whole thread, but @Ricko1966 wondering if the #2 intake valve may have a problem, made me think, is this an AFM type engine? Could #2 be sludged up?

This is a 6.0 LQ4 out of a 2005 Hummer. I don't think they had AFM

Bruce Wingate · May 16, 2026

Ricko1966 said:
Put a vacuum gauge on it see what kind if readings you are getting,see if the engine runs the same with #2 injector plugged in but #2 coil unplugged,see if it runs decent on 7 that way. Wondering if #2 has a problem with the intake valve and when #2 hits it confuses the map sensor.

Hooked up a vacuum gauge and on seven cylinders it ran at a solid 21 PSI with no wavering. When I blip the throttle it drops down and then reasonably quickly gets back up to 21 PSI. When I plug in the number two injector pressure goes down to maybe 16 psi 14 psi.

I tried unplugging the coil and plugging in the injector I got a similar result as before when I had the coil and the injector plugged in it starts running very rich starts to stumble and I just stop it at that point.

I'm going to double check compression.

Bruce Wingate · May 17, 2026

Compression is fine in cylinder #2 at 190 psi; I let it sit for maybe 5 minutes and the compression did not change. Compression in cylinder #4 was also 190 psi.
I check spark with a spark plug against the block and saw a regular spark. I don't have anything to compare the spark to, so I don't know if it was strong or weak, but it was easily visible. I put new plugs in the even bank of cylinders.

I previously checked vacuum, and it was steady at 21 psi.

Holley replaced the entire engine harness, so that is brand new. They also tested the ECU and found no problems

I did a fresh tune using the wizard. There was no change.

I'm going to poke around in the tune and see if there is a way to trim down fuel in just cylinder #2; I am also probably going to pull the fuel rail and injectors and test them using the ECU.

I'm really lost at this point. The enging has fuel, spark and compression and should run OK.

1984 K10 LS Swap - Lots of Little Problems

Bruce Wingate

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