Hello all havent posted in quite a while. Having some misfire issues and need to get emmision test! 2003 ss 3.8 , 255k miles.getting 28mpg. Have code p303(cylinder 3) sometimes blinking cel light sometimes constant cel light. I've done pretty much all the basics....changed coil, changed#3 plug again, old plugs an wires done approx 30k ago.(a-cdelco) Cleaned air filter , changed cat converter , cleaned EGR and Idle control valve , replaced 1 lil pc of vaccum hose. Wondering what anyone else did to solve this problem running out of ideas.THANKS!!!!!

 

Note: Doesnt seem be missing power- purrs at highway speeds ....shakes and runs rough at idle mainly.

 

Sorry to read about your problem with your Monte
I'm not a tech, but a google searcher & I hope the below link may help...Please check back to see what our expert tech member's post to help...Good Luck
Part 1 -How to Diagnose Misfire Codes P0300-P0306 (GM 3.8L)
Click above link

 

With mileage that high I would suspect either the ignition control module or coil pack. Over time the heat sync lube between the coil and ICM dries up causing ICM to over heat and damage circuitry. Be sure to install new lube if you replace either. A misfire is often ignition related, so that narrows things down quite a bit.

 

I also suggest the ignition control module (piece that the coils bolt too)

Side note a missfire can also be from not fueling properly it doesn't always have to be spark related.

 

x2, try moving the fuel injector to a different one and see if the misfire moves with it.

 

Agreed, but I believe if the problem was in the injector electrical circuit the code would be P0200-P0299 instead of P0303. Could be a clogged injector though.

 

Thanks for the info yall !!...i was kinda thinking ICM but if it was that , wouldnt i be lacking power and have multiple or random misfire codes?? Hard to find anywhere that test them anymore, and not a cheap part . Around 150 bucks.... Doesnt the ICM also power the crank & cam sensors

 

My local Advance Auto parts and also Auto Zone can bench test the icm. Did you check with them?

 

None of my local part stores do it ...seems they dnt have the connectors/ or something of the sort, def something i wanna check before jus throwing money. Need me a good GM service manual - the haynes dnt tell you squat really