Had that same problem on my 01 Pontiac Grand Prix GT. 3800 engine. 122,000 miles. Running on level ground, it was fine. Start to pull a hill or jump on the accelerator to pass someone and it would bog down then surge forward then fall back again. Finally determined it was the catalytic convertor starting to go. Replaced it and it runs like brand new again.

 

Greeny I take it you had the same confusion that I have now with no engin light coming on? So im guessing if it is the cat, that the O2 sensor is not reading out of its normal operation range so that is the reason for no check engine light at this point. Well like I had said earlier, a tune up is not going to hurt the car at all and if it turns out to resolve the issue it will have been for less money that the cost of replacing a cat. I will keep you guys updated as I go along.

 

Man I had Firestone take it out and drive it and while driving it about 10 miles, it suddenly started running right and they brought it back and said, whatever it was, it is now running right. They checked the fuel pressure, the fuel filter and finally said it just started running right by itself. Well a week later and $65 to them and it was running even worse for me. My brother and I switched out coil packs to no avail until I finally gave up and had it towed to the pontiac dealer. Well $750 dollars later and a new catalytic converter, the car runs great. Course if you find the right kind of mechanic it could cost you about $140 for the cat and maybe a $100 to install it. My mechanic was down in Florida for 2 weeks and I needed the car badly or else I would have waited.

PS: The whole time the only time any warning lights came on was when it was under load and I tried to keep the pedal floored and the engine service light would come on.

 

Hmm...mine is not that bad of drop in performance, and I never get a light that comes on. Im starting to lean toward the fuel filter because it does it even on flat roads now. I guess Ill replace that and the tune up and see where that leaves me.

 

Well to say the least the fuel filter was pretty gross, the fuel that spilled out of the filter was bown and after I started blowing chunks came out, so the filter definitly needed to be changed. I also put new plugs in even though there had already been a new set put in once before. But this did not solve the problem, the car does idle smoother, although i didnt really notice a problem with it before hand. Im starting to suspect the tranny, it seems to only surge when its in OD and around 2K, its almost like it cant decide if needs to down shift or not. It shouldnt need to, it surges when I press the gas ever so lightly while crusing or when going up hills and I dont yam it to force a downshift. The tranny fluid looks clean, but my next step is to replace the filter and fluid. But I dont really see that solving my issue. I am stumped at this point...

 

Sounds exactly like mine was doing and the fuel filter was clean and I thought the tranny was going in it. Until it got so bad that it would not even go down the street without running like a piece of junk did I put it in and they diagnosed it as th Catalytic convertor. Now my son has the car and he says it really runs smooth.

 

I guess the next thing to do then is to get the cat checked out...does you guys have any idea what the post cat O2 sensor should be reading on a good cat? this way I can get it scanned and see what the O2 sensor is reading at this point to see if I can make any kind of diagnosis myself before I take it in somewhere. I am strapped for cash at this point being a student and all, so I want to spend as little on unnecessary parts replacements as possible.

 

Well I have been doin some shopping for cats, the high flow from random looks pretty sweet, but the price is a lil much for me right now. I did find this direct replacement from Magnaflow (Car Sound). Im gonna call them to make sure but the pic from a few different sites shows NO U BEND! and for $133, I dont see a different option. I was thinking bout waiting a few weeks till I get back to school and try and fab my own pipe with a high flow cat and no U bend. But for the price and the time saved I think this might be the way to go...check it out


http://store.summitracing.com/partde...5&autoview=sku

 

A couple of other things that come to mind:


Check the alt load. If the alternator is failing it will cause havoc with the electrical system and yes even your trans. I had a similar experience with my wifes truck. Took me a couple of days jacking with it before the alternator finally failed. After I replaced it the trans and other drivability problems went away...

 

There is an extended warranty on the cat, I think it was to 100K now. You should have gotten a letter about it. Mine was hesitating on acceleration at 52k, I took it in with that letter and they replaced the cat and it ran fine. The letter just says that it isn't a recall but if you experience certain problems, bring it in to see if the cat is causing them... I forgot all the symptoms they listed, but it was something like poor fuel economy, hesitation, check engine light on, rattling noise from the exhaust..