2001 3.8L NA Bonneville. Motor blew, was swapped with an ~80k motor. Worked fine for a couple months. Now it'* misfiring somewhat randomly when driven, only under load, and it shudders pretty hard. It'* not consistent, but only happens in gear when pressing the accelerator. Letting off or going into neutral stops it. No codes, which is frustrating. Long story short, I drove while my mechanic watched his scan tool. The misfires would happen 1-2 cylinders at-a-time and moved around seemingly randomly. Sometimes it would be two cylinders on a single coil, sometimes not.

He has a Park Avenue Ultra and swapped pretty much everything, then blew the motor. So, he swapped his plugs, wires, ICM, coils. No change. He read about a weird alternator issue that can cause problems/destroy the ICM and/or coils, so we swapped the alternator. Still no difference. He tested fuel pressure, cleaned and checked the MAF. We're at a loss. Any ideas?

Is it possible this misfire is only happening under load in 3rd & 4th gears? The reason I ask, is it 'could' be an issue with the Torque Converter Clutch (TCC). A TCC issue can often be misinterpreted as a misfire in 3rd & 4th gear under load. If you think this is possibility there are a couple options to disable the TCC to perform a test to prove if this is the case.

If the engine is misfiring it should have codes. He was in mode 6 and r3ading misfire data?

It seems like it might be just 3rd and 4th. He said today he was leaning toward the Torque converter. He drove it and lightly touched the brake pedal enough to depress the switch and it stops. I think we’re onto something.

as for misfire, I’m not sure what mode he was in.

How many miles on the transaxle?

If you're going, say, 60MPH in overdrive with cruise control on and this starts happening, click to 3rd, does it stop or change frequency?

Does it happen when everything is still cold?

What is the fuel pressure at idle in Park?

I can't be sure about mileage. The owner before me ragged on the car pretty good, I'm convinced. I've owned it a few years now and last year replaced the motor after it threw a rod. The transmission was starting to act up, so I had him partially rebuild it while it was out. He pulled it to find out it was a replacement transmission already. So, at 205k miles it had been through two transmissions and a motor: sure seems like the previous owner was hard on it. (Why would I buy such a car? 1. Zero rust. I'm in Michigan and this one hadn't been driven in winter until I bought it. 2. It'* a 3.8L GM. Parts are everywhere, they're easy to work on, generally last for hundreds of thousands of miles, etc.

All that to say the transaxle has an unknown number of miles, was partially rebuilt recently, and the torque converter makes a lot of sense. I'm pretty sure he put in a new one when he rebuilt it, but we'll talk today to see. It may be under warranty. More to come. Thanks again for the help. I'm glad he was on the same page. The misfire threw us both for a loop. I am curious how a TCC could cause a misfire, though. My only thought it it'* locking out and putting drag on the motor, slowing the crank compared to what the ECM is expecting. I still don't understand why I wouldn't have at least some O2 codes or something.

If you're going, say, 60MPH in overdrive with cruise control on and this starts happening, click to 3rd, does it stop or change frequency?[/quote]
I haven't been able to drive it enough to test. Sounds like some of the testing he did. I got it back last night for a meeting and traded him for my Roadmaster Wagon because I need a quick oil change and wont' have time to do it before it makes a long trip later this week. When I talk with him, I'll recommend this test if he hasn't 100% IDed the TCC.

Does it happen when everything is still cold?[/quote]
I don't think so, but haven't tested that specifically. I live right in town, so I have to go straight to a faster road to test.

What is the fuel pressure at idle in Park?[/QUOTE]
I don't have numbers, but he told me all the fuel pressure tests came out normal and very close between cylinders.

Sorry, I can't edit my posts to fix the quotes. I assume they're relatively clear, though.

The TCC wouldn't be causing a misfire. It would be causing a symptom that feels like a misfire while not actually being a misfire. On a 4T6x, it'* more likely that the TCC solenoid is leaky than the TCC itself having an issue.

Look at the TCC on the scan tool. Is it being commanded on and off? Does the problem occur around 45 MPH?