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

Hi everyone this is probably been the one that has taken the most time on my calendar.

Here are details on this 1997 Pontiac GrandAm 3.1 engine

This one stalls under just about any load.
Sometimes it stalls when put in gear.
This engine will stall and backfire thru intake at WOT.
it will intermitently shut off at IDLE in park.
Any Quick snap of the throttle and it will seem like its going to stall and backfire until it almost always shuts off. when it does start again it struggles and you can smell the raw Fuel out of the exhaust


Tests performed so far. All Waveforms were viewed with Maxiscope 4 channel scope
All power and grounds verified at PCM.
MAF frequency is good
24x crankshaft waverform is good and clean
7x crank waveform is also good, Harness replaced at this sensor because it was very corroded and worn at exhaust manifold
Camshaft sensor waveform is good also on labscope
5volt reference is steady on all sensors.
Fuel Pressure is in specification, No Fuel at regular vacuum line checked that too,

Initially thought it was ignition module so we swapped a new one in with no change

Shop Owner was able to get a used PCM to plug and play, still same symptom

No shorts to voltage on injector Drivers and solid Battery voltage on feed to injectors

the attached wavefrom Orange Channel is the Amp Clamp on the voltage feed to the injectors and the green tracer is channel to Injector 2 Driver at intake manifold connector to injectors.


Reason I went here is everytime it would stall at WOT or any load the ampereage spikes high on a few cylinders so please any input would be great.

Also did a backpressure test and it was in the green on the gauge so not clogged. Fuel Filter was also removed and i can easily blow air accross it.

HELP. This one is taking too long and first one in my life i cant figure out.

Last thing I need to mention is it will idle perfectly in park but it will intermittently shut off almost like someone just turned the key off.

I have watched with Voltmeter when it does shut off on its own and the voltage is not lost at ignition control module which is common I know but not the case here

TPS Waveform on Labscope has clean slope with no breaks

No DTCS ever on this one

Why does your injector voltage waveform not match up with your current ramps?
At least one of these ramps should be exactly lined up with that.
Second, there is something else on this injector circuit, that is running. The ramps should be dropping to 0 amps when they are off and it is not.
Third, when is this capture being taken? Idle, snap throttle?

If you are connected to the fuse that feeds all the injectors, the fuel pump is on that circuit too....

I would love to see the current ramps of the ignition coils too (on the main feed going into the ICM). At idle and during a snap-throttle when it backfires

This is taken at snap throttle. and you are right the same pink wire powers the fuel pump circuit

Here are the snap shots of Coil camps taken from primary ignition feed into ICM

THey are in order as you requested. from idle to 1200 rpms. Side note( RPM will rise smoothly all the way to the limiter if throttle is moved smoothly

Here you go.

Unfortunately, these coil primary captures do not help me. I need to see the repetition. I want to know if you are dropping coil during events. These zoomed in shots don't answer that for me.

That’s the thing if you see the last screenshot that shows two of them. The rest have disappeared completely when it’s stalling. It only captured two ramps

You just need to increase your time base and sample rate, so we can see more of those events on the screen

Ill get that this morning Thanks man

Good morning so here are the snap shots with longer time base at idle and snap throttle.

I also zoomed in the ramps that did appear on the screen incase you needed to see. I took two snap shots at different instances and you can see its like we are dropping two cylinders.

This is good learning experience for us here

Hi,
Between the two groups of ramps, i can see 4 - 5 missing fires.

rpd1125, well at least we know what is going on now! Nice captures. Let's stay focused on the ignition system, forget about the injector control picks you have for now.
There are three coils here, so it is 1-2-3, 1-2-3, 1-2-3 etc (not necessarily in that order)
What we are seeing when it is misfiring is not only are we dropping one coil but there is also a dwell time issue on one of the two ramps that are still showing.
This is ABSOLUTELY why your engine is backfiring and running bad.
Now, we need to know if this is from a garbage input or you have a module issue.
Grab the 7x crank sensor signal with your scope next and let's look at it during this event.

Hey Mr Danner,

so to go along with your suggestion, when the vehicle first came in the harness to the 7x sensor was melted to the exhaust manifold so I put a new pigtail and a new 7x crank sensor already and I have seen the waveform on it and its absolutely clean.

So after What I saw yesterday on the coil ramps, Shop owner gave me green light to put on all new 3 ignition coils and what a difference. Now it revs up under load and was able to do the Crank Variation Relearn.

It still wants to stumble sometimes under absolutely hard snap throttle. We have a new Ignition Control module coming and we will get Coil Ramps before and after we put on the new IGN module.

Will keep you posted.

Okay so here we go. here are the coil ramps prior to the new Ignition control module. I zoomed in on what looked weird to me. Im still learning how to read these waveforms and exactly what im looking at ….you wont hurt my feelings please any input would be great.

Here are screenshots with New IGN control module and all 3 new Coils installed.

Channel A is off the feed to IGN control module on Pink wire
Channel B is on the 7X Crank signal wire. as you can see at idle it seems fine and the car runs much much better but still stalls when you go heavy and fast on the throttle WOT.

I was able to capture it when it started to stall and you can see a huge air gap on both crank and Coil ramps? What do you think is happening?
Side note on this, I thought maybe we were loosing voltage to IGN Control module and ran battery voltage to the primary feed wire directly from the battery with a fuse and it had no change to it. Again its running way better and it now allows me to do a crank variation relearn but every now and then when you go heavy on the throttle it will try to stall (NO MORE BACKFIRES)

Here is screenshot when it was stalling again
What is going on right?

Channel A on ign control module primary feed wire
Channel B on 7x sensor