Have the Bonneville sitting until I get a new Catalytic converter. It'* a high-mileage car, and it looks like the Cat has been replaced at least once. Something got me thinking. About a month before the cat went out, I added a bottle of Seafoam to a full tank of gas (18 gallons), shortly after i noticed a loss of performance which I attributed to a transmission problem (Cracked/broken modulator vacuum line). About a month before that, I had replaced a transmission in my friends Nissan Sentra, and it ran fine until he put a 1/2 can of Seafoam in his tank because the car had sat for quite a while (3 years), then after an injector swap (he had 1 good injector out of 4) it ran great until it died on him. Now i'm suspecting that car has a clogged catalytic converter too (has good spark and good fuel pressure, not throwing any strange codes). The common thread in all of this is both cars ran until the Seafoam was added to clean the fuel system. This was not a top-end clean, just added to the fuel to clean the system and injectors. Could enough carbon and junk have been dislodged by doing this to clog the cat? These are both high-mileage cars (the Bonneville is at 212,000 and the Sentra is at 180,000) and probably needed new cats anyway, but could this happen? Any thoughts, guesses etc?

If it did, it was already pretty clogged, and that was the final straw, or there was a lot of carbon buildup.

Is that the little add one bottle size or the bigger... add like 1/8 of a bottle size?

the little add 1 bottle size. Sells for about 8-10 bucks at AutoZone..

I have had nothing but good results using Seafoam. I would start with a fuel filter change. Are you sure the cat is plugged?

Well, after letting the old girl sit for a few weeks because it wouldn't start, i decided to charge up the battery and do the diagnostic tests on the ICM before I sent it out to have a new cat replaced. Tests indicated a dead crankshaft position sensor. I thought that was odd because I replaced it earlier this year. Got the harmonic balancer off, and what a suprise. Balancer keyway i sheared and the balancer hub that goes over the crankshaft snout is broken. Poor baby was out of time. Definitely explains the high fuel consumption and strange running prior to her finally going down for the count. I guess I did not have the balancer fully seated on the crankshaft when I installed the new CKP sensor. Now i'm worried that I may have screwed up the key on the crankshaft. I will get a new balancer, replace the woodruff key if I can, replace the oil seal and see what happens.

Finally got the bonneville back together. Had to take the engine down to the bare block, remove the balancer, timing cover, and timing gears to take the old woodruff key out (the old key had flat portions on the ends that laid on top of the crank so you couldn't rock it out). Got everything back together, installing the main accesory belt and the bolt on the belt tensioner strips out. Was tempted to try a JB weld repair, but went ahead and hit the Pick n pull for a replacement belt tensioner assembly. Pulled the old assembly off, and the aluminum piece of the belt tensioner that slips into the timing chain cover actually broke. Well, short session with my cotter key tool (screwdriver handle with a big hook on the end) and some new o-rings later and all is back together. Put the fuel pump fuse back in and she started on the first try. Now, i need to do an oil change and put in some top-end lube to quiet the lifter noise i picked up from having the block sit up with no oil in it for so long. So, to recap, chewed up crankshaft pulley/balancer was retarding timing making me think all sorts of other nutty things were happening (transmission slippage, plugged catlytic converter, bad O2, MAF, IAC, grounds etc.) and putting a 212,000 mile 15-year old supercharged V-6 can be a pain in the... Well, you know.