97 Bonneville N/A 280,000 kms. Series 2 3800
Car runs great most of the time but over the last 15,000 kms car has been acting up. If cruising at 55/60 kms an hour and give either partial or wide open throttle car will either take off and run great or it may shudder, surge and sometimes even back fire through the intake. This can also happen at WOT from take off. I can not get the SES light to activate, has been scanned 3 times and no codes. At times the car will run like it was new with no issues. Usually works great if gradual throttle is applied, and it will wind up to 180 kms hr. with no issue.
Mechanic drove it for 10 miles really hard with scanner hooked up but although the surge and backfire issue did happen, nothing showed on scanner. Car not running lean.
I have tried with tranny in 2nd, 3rd or drive and always the same symptoms.
Problem can occur every time I floor it or it can only act up every other 3rd or 4th time.
Have replaced wires, plugs, fuel filter, checked out the coils just from the top and all read the same ohms. MAF has been cleaned as well as the plate in throttle body and screen. Problem is still exact same as before.
Not sure if I am chasing a spark, fuel or timing issue.
Very frustrating as I am afraid to pass anyone on highway in case car acts up.
Any suggestions?
Thanks
Martin

Check for vacuum leaks and change the fuel filter

Tap on the MAF. Does the engine stumble when you do that?

Thanks for the reply,
Fuel filter was changed, used a propane torch (unlit of course) and no vaccum leaks found.
Think I will invite daughter over as she drives a 2000 Grand Prix GT, and start swapping some of her parts LOL.

I have tapped, no change. When I unplugged Maf car stalled. Tried driving it for 20 kms but car would stall every time I let off throttle, plus same issue was still present while unplugged.
The backfire through the intake will happen sometimes at 60kms, sometimes at 100 kms hr.
Usually just pops once or twice but loud.

Pull the plugs and inspect them. You may have a cylinder not firing right and dumping raw fuel in to the exhaust.

Plugs have been pulled 3 times, changed twice but still same issue.
Plugs all dry and clean.
Thinking of changing all 3 coils.
Do the timing chains act up in the 3800? Crank sensor?
No matter how hard I push it even when missing SES light will not set. Only time it came on was when I unplugged MAF.
I have even went from 40 kms hr. right up to 170 kms hr with engine surgeing, back fireing all the way but still no code.
Its baffling because sometimes the car runs like a rocket.

It could be an exhaust leak. This will trick the O2 sensor in to thinking the engine is running lean and add more fuel. What color are the plug electrodes? Do they all look the same? Is there any chance you can get the engine scanned for live data and have that graphed? I would like to see what the O2 sensor is doing, as well as your LTFT

All the plugs look the same , clean and dry, slight tan colour.
The mechanic that drove/scanned car said that everything was within peramiters but I dont know if his Snap On scanner can give a print out. May have to take the car to somebody who can do a print.
I will check with original mechanic and see if he can re scan and pay special attention to the long term fuel trim/short term fuel trim and 02 sensor.
Still amazed the power this car has when running right.
Thanks