GM 3800 ICM not working after spark plug and wire replacement

[bamx2]

I just changed the spark plugs and wires on my 1999 GM 3.8 . The engine ran briefly then died . Yes, firing order was correct. Now no spark on coil pack 3/6 and low spark on others. I went through a diagnostic procedure ( http://easyautodiagnostics.com/gm_ignition_3800/test_ignition_module_1.php ) and and determined that the ignition control module (ICM) was bad. I did not check the gap on the plugs (Denson PT16TT 4511 )before I installed them but later found out the gap out of the box was too small (0.35 vs .060 ). The Autolite wireset ( 96650) seemed to be fairly close to the original in length wires on but cyls 3 and 5 were a bit longer than the original . Would either of these (small plug gap or wrong wire length) cause damage to the ICM ? - Thanks

 

[phucheneh]

It is an unspoken rule among plug wire manufacturers that aftermarket wires cannot be the correct length.

Really, they NEVER fit like the factory ones. Especially on a V engine. Old V8's seem to be the worst...I think they sell the same wires in a different box for every old carbed V8, and just figure 'meh, distributor on front left, front right, front center, rear...who cares, just make sure you can reach all 8 plugs.'

Anyhoo...no, wires aren't the problem. Plug gap shouldn't be, either. You also shouldn't need to change the gap; modern platinum and iridium plugs should come pre-gapped. While you CAN gap platinum plugs like coppers (don't try it with iridiums), you shouldn't NEED to. I pretty much use Denso and NGK plugs exclusively and, while I will check the gap (just using your standard 99 cent gapper as a feeler gauge), I've never needed to alter it.

Are you positive you're not getting spark on the middle coil pack? What are you using to test? How are you quantifying the spark on the others as 'weak'? Adjustable spark checkers are pretty cheap, and allow a more concrete conclusion to be drawn.

I doubt it is your ICM. Maybe wiring related to it. Check for any damage at the connector and make sure it's tight (if it's the style with the bolt in the middle though, don't go nuts on it). You can also trace that harness back across the intake manifold and look for any obvious damage. You can remove the coils and look for corrosion on the terminals beneath them (just the two small bolts per coil) and also swap them to see if anything changes. Just make sure you keep your wiring straight (obviously, if you swap coils around, the labels won't identify the correct cyls anymore).

Past that, there is unbolting the actual ICM (three nuts on underside, IIRC) to check for ground issues- the ICM gets its ground directly from its attachment to the engine. If in doubt, clean off the bottom of the module and the bracket it attaches to.

 

[bamx2]

phucheneh - OK, I guess the ICM was about to fail and the failure was a coincidence .I think that I have later model than you are referring to . The mounting bolts for the 3 coils also hold the ICM on. I re-gapped the plugs and installed a $25 used ICM ( cleaned the ICM and heatsink) . It is running fine. - Thanks