Low speed miss in OD/lock-up 3.8 V6

My 1989 Bonneville with 3.8 V6 is the one car in my life I just can't part with. It has developed a strange misfire (?). It will only happen when the transmission is in OD and lock-up, and you carefully lug it down to about 1600 RPM. There are isolated jolts that feel like ignition misfire, except that they are very strong. It is not a string of misfires; it is one isolated "thump". Unless you press on the accelerator, the misfire does not cause any change in transmission state--if you continue to carefully lug at low RPM, it will "thump" again in a few seconds. It can not be made to misfire under any conditions other than this. Hard accleration--no misfire. Engine speeds above about 1800--no misfire. Does not seem weather related.

The misfires are so jolting, that I even suspected some trans problem where lockup went out-in quickly. However, my best clue so far is that when I replaced plug wires, the problem became noticeably worse.

ALSO, I need to replace the harmonic balancer, but I can't get the bolt loose on the crank end. The balancer appears smooth on its face, so I don't think a puller is required, just get the bolt out and the balancer will slide out.

The Thump is likely being caused by a TCC solenoid sticking and not unlocking the convertor. If you continue driving it like that, you are risking bending a rod or doing severe engine damage. Try unplugging the TCC harness (located in the front of the transaxle, it's a square, 4 prong connector). IF the thump goes away, it's definitely the TCC solenoid. You may be able to un-stick it with some Trans-x or Seafoam tranny additive, but more likely, you'll need to replace the TCC.

The harmonic balancer is held on by a big bolt that is torqued to 240 ft/lbs. You'll need a BIG breaker bar, and an assistant to hold a screwdriver in the flywheel to keep the engine from spinning while you break the bolt loose. After getting the bolt out, you should be able to slide the balancer off. If it doesn't slide off easily, it may be rusted to crank. Some PB Blaster or other quality penetrant should do the trick.

Richtazz--thanks for info. I can't really check whether it's the solenoid valve by disconnecting it. If I understand how it works, that will disable converter lockup. Without converter lockup, I can't get the engine loaded at a low enough RPM to exhibit the problem. I'll try it anyway, though. I wonder about the change in plug wires making it worse, though. I'm still hoping for some ignition problem, since I believe that solenoid valve is buried inside the transmission.

BTW, that knocking sound that came from the harmonic balancer--has anyone else ever heard that?

Thanks again.

If you disable the TCC and the thump persists, then it's not a sticking tcc, if it goes away, it is. The entire problem could be caused from the bad balancer. If the outer ring has completely broken away from the center section, then the car's timing is all screwed up. The harmonic balancer has a shutter attached that passes through the cranshaft position sensor. When the balancer separates, it throws the signal out of phase from the crank sensor essentially messing the timing up and confuses the computer. IT can also cause the computer to think the engine is knocking, and it retards timing, which saps power.

Richtazz: Thanks again. I'll try all this when I get the car back together. I actually tore into it because of the knocking, thinking it was a water pump impeller separated from its shaft. When I pulled the pump off, it was obvious nothing was wrong with it. That's when I discovered the HB problem. There IS a "cog wheel" type disk behind the HB pulley, which appears to be a crankshaft position sensor, but it appears to be an integral part of the hub that is fastened solidly to the crankshaft, and does not appear to have any play.

I guess I've got too many things going on at once with the car. I'll just have to get the HB problem fixed and get it back togther to sort them out. I wouldn't normally put this much time into a car that old, but I've had it for eleven years, put 253,000 miles on it, and it still doesn't use a drop of oil, or leak any. It is clean, comfortable, quiet and reasonably powerful. It has been the most dependable car I've ever owned.