likely ignition problem has me stumped

[Q-Ship]

My '01 SSEi started progressively running worse with the main problem being a miss in 4th gear over 55 MPH or so and at about 4 PSI of boost under load, like going up hills on the highway. It got worse to the point of missing on any spirited acceleration. The coils were iffy so I replaced all of them with new AC Delco ones. Slight improvement, but still definitely there. Since it had been about 40K miles since the plugs and wires were replaced I replaced those with some BWD Select wires (lifetime warranty) and some NGK Iridium IX plugs. I checked the gap on all the plugs before installing them and they were all within spec at .053. The problem is much better but it still has a miss when in 4th going up hills on the highway at about 4 PSI of boost. Tonight I swapped the ignition module with the one from my wife's '99 3.8 Firebird and the miss is still there.

I'm at my wits' end here. I thought about closing the plug gap to about .040 and trying it out. Closing the plug gap on my Mustang SVO (2.3 turbo) helped with a miss under high boost (over 17 PSI) and that was going from .035 to .028 but I wouldn't consider 4 PSI to be "high boost." No codes except P0401 (insufficient EGR flow) but I just replaced the EGR valve about 4 months ago. It's always seemed to not like the above highway hill conditions and I'd like to solve this problem once and for all.

Any ideas? This is our main commuter car.

[nos4blood70]

What's the mileage on the car? Ever ran any fuel injector cleaner through it? Perhaps the cat is clogging up.

[Q-Ship]

Just did an oil change a few hundred miles ago and ran a bottle of FI cleaner through it then. That tank's been used up already, too. The car has almost 175K on it. Still has great oil pressure, too. From my experience with my wife's Firebird it seems like the ignition system is an Achilles' heel of the distributorless 3.8. For comparison, my old '89 Mustang LX 2.3L (not the SVO I mentioned above) is still on its original TFI ignition module and IIRC I replaced the coil once...in 20 years and it has 188K on it (got it with 54K on it when I was in high school and I abused that poor car driving through fields, rough gravel/dirt roads, and delivered pizza in it for 2 years). My mom was driving it until last year when she had to quit driving after her stroke (she's basically OK, FYI). I'll admit I'm mainly a Ford guy. My '94 Explorer also has about 188K on it. I've had it for 9 years and the only thing I ever replaced was the plugs and wires. It's DIS coil pack is the same one that was on it when I bought it at 130K. It's an offroad toy only now, though.

Although the cat could be a possibility I doubt it since the miss/stumble improved but didn't go away after the recent coil/wire/plug replacement.

[Q-Ship]

UPDATE

Took the car on a longer drive this morning and while the slight miss is still there going up hills on the highway the more severe bucking never happened. It was a noticeable improvement. I pulled a long hill at about 65 in OD around 4-5 PSI and it didn't stumble. Maybe the ignition module was going out? I think I might try regapping the plugs, time permitting today. I've got to replace the front transaxle mount (going to try the hockey pucks) and replace the rear rotors and pads.

[crash93ssei]

Is the car throwing any codes?

[Q-Ship]

Is the car throwing any codes?

Like I said, just P0401, insufficient EGR flow. That one seems odd to me since I just replaced the EGR valve about 4 months ago. Going to have to investigate that one further, too.

[michtom]

Check at the EGR inlet both tube and intake for any accumulation. This could cause stumble under light loads.

[Q-Ship]

Check at the EGR inlet both tube and intake for any accumulation. This could cause stumble under light loads.

Will do-thanks!

Also wanted to update that I'm on a work trip with the Firebird which you'll remember now has the Bonneville's ignition module. Now the Firebird has the bad stumble under load. I think I've isolated the problem there. Looks like I'll be replacing an ignition module not long after I get home.