2001 Impala - misfires under load / uphill chugging - finally FIXED!

[rfdierk]

This is my first post, so hello to everyone. I've been living with occasional misfires for quite some time. The problem almost always occurs under load - either going up a slight uphill grade, or under a gradual acceleration while cruising around 35 mph. My plugs and wires have less than 20k, and coils all checked out fine. I also replaced all of the typical maintenance items (air & fuel filter, PCV, etc), and had the fuel injectors professionally cleaned twice. It seemed like the problem went away for a week after the injector cleaning, so I had reached the conclusion that I had a bad injector. Well, last week I bit the bullet and replaced all 6 injectors, figuring that the brute force approach would certainly fix the problem. After my initial test drive, the car seemed to have more pep, and I didn't notice any uphill misfires or "chugging". The bad news came the next day, when I noticed the problem was still there - just exactly the same as it always had been. I decided to regroup and take another look at the ignition system. I reinspected the wires and even used a spray bottle to spritz the wires while running. No arcing. I checked the coil packs again, and they were fine too. Last check was the spark plugs. I pulled all 6 and found a nice tan color with no signs of fouling - picture perfect. Just for the heck of it, I checked the gaps (GM calls for a gap of .060). Well, 4 plugs had gaps of .065, and 2 had gaps of .070. I regapped all to .055 and reinstalled them. I didn't expect any change in performance. Boy, was I wrong. The next time I drove the car I noticed it accelerated smoother, with more power - and best of all - NO MISFIRES - NOT A ONE. It's been 2 weeks now and the car is driving perfect. I can't believe that such a small tweak made such a big difference. I hope this helps other folks out there suffering from the same or similar "chugging" problems.

[manicmechanix]

Good info. Well I'm a little surprised that a gap of .070 would cause it to misfire with the DIS. This is why I always check the plug gap before installing new plugs. You can also check the resistance of the plugs. they should be around 1K Ohms when new but can be up to 10K when used.

You said the plugs were under 20K miles right? The gap shouldn't have opened up. What kind of plugs are they? I think AC are good plugs but I noticed AC Delco regular platinums, not the iridiums, have a tendency to lose their platinum tab on the electrode and this will open the gap up by .010-.015 or more.

[rfdierk]

The plugs are genuine AC Delco platinum tip. All were intact, with no missing tips. BTW - I did a little research, and was surprised to find that there's a bit of a debate on GM's .060 spec. Some mechanics think it's too big - begging for drivability problems. In fact, there was a TSB issued by Saturn from the late 90's recommending that the plugs be changed to a different PN# with a .040 gap.