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Multec 2 injector swap


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I'll spare the long story of why I'm doing this, but I attempted swapping the awful stock LG5 injectors in one of my TGPs with Multec 2 injectors. It is a far, far superior injector to the old fatty multecs. I used injectors from a 4.2L Trailblazer. According to GM, LG5 injectors and 4.2L injectors share the same flow rate. I even sent both the LG5 and 4.2L injectors to witchhunter for testing/rebuild. The report I got back from witchhunter with the injectors confirmed this. All injectors flowed within a couple cc of 250cc/min @ 43.5psi.


Problem is, after the swap the engine runs extremely rich. Anything but the slightest touch of the throttle at idle will stall the engine, a Tech 1 shows extremely rich adaptives, and the car belches black smoke on heavier acceleration. I confirmed this is not an anomaly or that I screwed something else up during the swap. I picked up another cheap set of 4.2L injectors and a very cheap set of adapter harnesses and quickly threw them in another TGP and achieved essentially the same result.


This confirms the Multec 2 injectors are running much, much richer than the Multec 1 injectors. I cannot for the life of me figure out how this can be when GM's spec and Witchhunter's test confirmed they flow the same. Can anyone shed some light here?

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What's the resistance of both injectors? If it's an impedance problem you should be able to run a resistor bank like an old Honda B-Series with low impedance injectors.

It's supposed to be by flow rate lbs but I can't think of anything else that would cause it. A quick fix could also be to dial the pressure down.


This, I don't think is the issue. They are both high-impedance injectors. I remember even double checking resistance awhile back and it was the same. Around 12ohms IIRC.


Well, except for all the failed stock Multec 1 injectors reading below 10ohms. 7 out of the 12 injectors on my 2 running TGPs have had failed windings. Four on one, three on the other. This is part of the reason for upgrading to Multec 2s. Failed windings is essentially non-existant on Multec 2's. I have a feeling many, if not most of us on this forum who's have Multec 1s, have at least one injector with shorted windings. I had no idea it was such a huge issue on Multec 1s until I started hunting around for why my TGPs, which ran fine and did not miss, did not idle smooth as butter. Heat kills Multec 1s; I've noticed injectors stuck completely underneath an upper manifold plenum have a much higher fail rate than injectors which are open and do not get as much heatsoak. The poster-child is my TGP that had 4 failed injectors. The four that failed were the four underneath the upper plenum. The two out in the open were fine.



I suspect flow might be similar, but injector offsets are very different with the Multec2....


Here’s a sort of easy explanation of what I’m thinking about.




This seems plausible. My question though, is would this not have shown up on the bench flow test from witchhunter? Just as the effective flow rate is obviously varying greatly between the two on my flow bench (my "bench" being the engine), it would seem the injectors would have to behave identically on any bench including an actual flow bench. It seems to me that differing offsets due to pintle speed would have to show up in a real-world flow test.


In any case, I'll poke around the awful $8F hack file and see if there's injector offset tuning available.

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Something to consider: Witchhunter appears to list injector flow rates based on the factory fuel pressure of vehicles they were installed in. That said, if the pressures are close, that's not the issue.



It would be interesting to see one of each injector side by side on a testing rig. Do you have access to a fuel injection cleaning canister at work Jesse and a tool to pulse the injectors? If so, you could run injector cleaning solvent through them to "flow test" them. The specific gravity of that fluid might not be the same as what witchhunter and others use, but for an injector vs. injector comparison it would show for sure if one is flowing more than the other in a given amount of time. I assume this might just confirm what Witchhunter has told you, but it would be interesting if the results did end up different!


This might be of some help to you as well: http://www.tgpforums.com/index.php?topic=2906.0

Edited by mfewtrail
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  • 1 month later...

Gonna try and redo the injector offset tables. I may have to open a new thread to get views and some help here. What I need is someone with an HPTuners to open a stock 4.2L file and get the injector offsets and probably the offsets by voltage. There are plenty of 4.2L .bin files for free download, but I don't have an HPTuners. I have a DHP and Moates.net chip burner running TunerPro RT. Neither of which obviously can open 4.2L .bin files. I really need someone with an HPT.

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