2001 GSR, recently the car died on me randomly, no spark at all, so my local shop diagnosed it was the ICM and replaced it.

Upon driving the car home, it had a CEL [[i]P1607 Engine/Powertrain Control Module Internal Circuit Malfunction] which I cleared.
The CEL never came back, but I immediately noticed a lack of low end power and VTEC was much less pronounced or noticeable at the 4400 RPM crossover.

It's had a buddy club spec 2 catback for almost 10 years, so the change in power / sound is extremely apparent.
VTEC used to hit nicely at the crossover, now it's almost completely inaudible and there is no real power until the butterfly runners open at 5800 rpm (which I can now hear since the exhaust is so much quieter - before the exhaust would be louder than the intake in VTEC).

I tested a p28 ECU w/ a stock GSR basemap on it to see if the ECU was the issue, but it runs the same as the stock p72 ECU.
I'm thinking my ignition timing changed when they took my dizzy apart to replace the ICM? I'm struggling to understand how just replacing the ICM would have such an effect on the engine.

Any ideas/insight would be much appreciated. Cheers

 

To replace the ICM its not necessary to remove the distributor. But they may have who knows.

What you can do is simply check the ignition timing with a timing light or have the shop do it if they admit to removing the distributor. First simply ask the shop if the distributor was removed for the ICM install or if was moved in any way? If so they will be responsible for setting the timing back to spec. Its not hard at all so they should do it with no problem.

Try removing the Neg battery terminal for a few mins to clear the ECU and see if the power is back. The ECU is most likely in Limp mode.

Was the check engine light present when you picked up the car from the shop? or did the check engine light pop on while driving and was it then that you felt the power loss?

 

So they did infact take the dizzy off, but they seem very confident that the timing was not changed as they marked it before removal.

They still offered to check the timing for me, which I'll have them do.

I did initially try to reset the ECU a couple times with no luck. The CEL was present right when I picked it up from the shop, I noticed it felt different right away and I took it easy home until I could read & clear it.

Apparently they also used a different ICM (LS/GS vs GSR one), as that's all they had at the time - but I don't think this would cause the issue? It's my understanding that the dizzy internals are interchangeable though

 

Have them double check all the progs for a tight fit on the ICM.

Also i think it may have something to do with your ECU having an immobilizer