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04 Grand Prix Stuttering on acceleration

Not sure if this will help but maybe your throttle body? My friends Torrent. the throttle body was going and it also had problems of shuddering and such and all were fixed by replacing it
 


A brand new throttle body is on its way. I bought a used one on ebay for 34 bucks and believe that it may have the same issues as when I swap them out, the symptoms are the same but have varying degrees of intensity. FYI - New on ebay = 110.00, new at autozone and other related sites = 375-400.
 
If I were you, i'd take it to get diagnosed at the stealership. My issue turned out to be the MAP sensor and I never would have guessed after all that I went through to try and figure it out. It was $95 to get it diagnosed and my car wasn't even throwing a code.
 
I'm going to go with the clogged CAT crowd on this one. Especially since my car experienced similar issues, and is actually at the shop now having the CAT replaced. The hair pulling I went through was similar. The car was due for new plugs and wires, after I replaced the coil packs in Feb. So that's where I started. I also cleaned the MAF, and replaced the TPS sensor, although I did not have a CEL and the car wasn't throwing any codes. Scanned on 3 different machines. Tranny fluid was a little low, but still red and did not smell burned up. No vaccuum leaks, and the Fuel pump and filter were less than a year old. I had noticed over the last few months that it always felt like I was accelerating into a head-wind from a cold stop. But nothing that made the car jump, or act skittish. Over the last few weeks it had increasingly gotten worse. Car started fine, idled fine, didn't overheat. If you can't drop the CAT yourself, partially jack it up, and rap on the CAT with a rubber mallet. If you hear any debris kicking around in there...that's a pretty good indicator. Really gotta think simple before you go mad scanning and checking things like voltages and especially the O2 sensors. They aren't the culprit. However, when they fail and don't do their job (report issues and throw codes), it makes us go through mind numbing checks and re-checks, because we've become so reliant on lights, buzzers and codes to diagnose. A bad CAT will not always throw a code if the O2 sensor(s) are not doing their job, or failing themselves.
 
I will check the CAT again, had already temp tested it and it passed with what I would expect flying colors (about 500 on the output) but I did not rap on it. I bought a brand new Throttle Body and it improved my symptoms a LOT, even thought it was fixed the first day. Alas, that is not the case as it still bogs down just not nearly as bad as it had been. I'd say the new throttle body improved the symptoms 85%.
Things of note:
- 02 sensors reporting a lean condition on bogging, both of them at 0 volts when the issue happens which tells me that something is telling fuel management to cut fuel to the engine at this time.
- Knock retard spikes to almost 15 degrees when bogging, doesn't spike at all when not.
- Valves rattle pretty bad when bogging even with new throttle body, no rattle when not.
- Short Term Fuel Trim spikes to 25 when bogging, Long Term appears unaffected.
- Better example of what happens now with the new throttle body. Accelerate lightly until it shifts into 2nd, then give it about 1/3rd throttle and it will not downshift and will start bogging. It doesn't pull itself out of the bog until RPM's increase from about 1500 to 3k or until I give it WOT.
 
My car is doing the same thing. I Have A 08 l26. I had the check engine light on for a while. But my car started to bog down and sputter. I took it to a shop and had it checked the said it was my upstream o2 sensor and possibly a clogged cat. I changed the o2 sensor and the light went off but the problem is still there so I think it's the cat

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Easiest way to test for a clogged cat is to remove the upstream O2 sensor and drive the car around. If runs better with a big hole in the exhaust, then it's probably the cat clogged.
 
Fixed! It got real bad, dying on the freeway after dropping to 500 rpm's under acceleration. Not starting for 5 mins in heavy traffic was quite the experience from the center lane...

Called a friend I knew worked as a mechanic and he told me to turn the key to on, unplug the EGR and then start it and see if it has issues. My EGR is the issue - drive it for a week unplugged then plug it back in and if it's bad replace it.

It accelerates like brand new on acceleration now.

Thanks all for the help!

Also got a nice tool to check if the cat was plugged today before calling him, an O2 Sensor socket. If this link works from O'reilly's http://www.oreillyauto.com/site/c/d...1603&keyword=sensor+socket&pt=N2546&ppt=C0195. CAT is fine and changed nothing about how it was running when the upstream sensor was taken out.
 
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