1997 Pontiac GranAm SE 3.1 Stalling and backfiring under load & WOT

Those spikes in your 7x ckp signal are from the ignition coil firing events and can be normal interference events as the 7x ckp sensor signal is going directly to the module. BUT I DO NOT like the one event that is actually distorting the double pulse on the 7x crank signal, this could be why you are dropping coil firings. It seems to be messing up the count. Didn't you say you had to fix the 7x harness? Did you twist the wires together (twisted pair). Also, did you route it correctly? Away from plug wires and stuff?

Yep repaired and twisted. Connections were soldered too. And completely routed away from ignition wires. They are not even close to the ignition wires at al

Here are some of the captures of the secondary ignition waveforms in the firing order to this engine respectively

Looks like it has an open wire on two cylinders. Let me know what you guys think?

This one is Fixed boys. It also had Open Plug wire on cylinder 5. This being a waste spark system making it misfire on its companion cylinder under heavy load.

Thanks Paul for your input

rpd1125 wrote:

So you are saying this image was being caused by an open plug wire? I'm just trying to clarify for everyone else.

Hey Paul good morning. It did have a Bad Coil arching to Ground but even after we replaced that IGNITION COIL it still had a bad miss where it would almost stall under heavy heavy load.

So got the Mini USCOPE and got some Secondary Ignition Waveforms and found the open wire on cylinder 2 causing misfire on cylinder 5 also being on the same coil.

My Supervisor thought i was nuts and disagreed with me and so did everyone in the shop. So i told him I would be willing to pay for the wire set if I was wrong so he got it and boom Amigo, this car is done and full power restored. You can see the secondary waveforms i posted on 03 Apr 2019 12:08. They are posted in firing order. Take a look at cylinder 2 and 5. Those were taken at about 1500 rpms.

Thanks for your input Paul this one is completely done and customer was blown away. This car has been sidelined for months because no one could figure it out. Everyone else's diagnosis was that it needed another PCM or Ignition switch. Hope this helps someone

Hi rpd1125.

Just for my learning process.

If spark were arching to ground, don't you think amp ramp should still be there?

Maybe coil was opening with heat and current didn't have a path to go?

Again, i would love to have a deep background at interpretation of this waveforms

Awesome that you fixed this one! But I just want to clarify for those following about those coil primary current ramps that were missing.
The only way that on open plug wire was causing that was from that open plug wire actually distorting the 7x crank sensor signal (for those that don't know, the 7x ckp signal goes directly to the ICM, so this interference is possible). We are absolutely seeing that event taking place the exact time one of those coils was firing (trailing edge of the current ramp). Other than that, there is no way that current ramp pattern would be missing primary events from the open plug wire. Make sense?
So again, was that latest current ramp pattern with the missing ramps taken AFTER replacing the ICM and coils and before doing the plug wires? Thanks so much!

Hey Paul

Yeah absolutely those were taken before the IGN Wires were replaced. After that we had no more of that happening and the customer was excited to take the car. Coil ramps looked perfect after the Ignition Wire were replaced.

Thanks again for all of your input I learned a lot from this one

Ruben