Vibe GT hesitates when throttle pushed during cold launch

[vibe owner]

After 49,000 trouble-free miles, my first minor driveability issue. It behaves like a bad throttle pump on a carburetor, but no carb here! I'd like to figure it out before it becomes major, but all service manual procedures begin with the diagnostic system, and I don't have one. Here is the deal:- happens only in the first minute, not when warmed up- most noticeable when launching in first gear; you have to push the throttle down very slowly to avoid a hesitation- seems more likely to happen when leaving a sunny parking lot in the afternoon, not the garage in the morningThis last point makes me suspect something related to tank pressure and the EVAP system -- could it be something as simple as a sticky valve?

[rainvibe]

Well, mine does the same thing, but not enough of a probelm for me to do anything yet. Mine is in the morning though.

[slowvibe]

i have an 05 gt. not long after i got it (about 6 weeks) i noticed that when i push rpms to 6500 that i was getting a stumble in power/performance at about 4500 rpm and sometimes as early as 3000 rpms which is a rpm level that i consistently reach regardless of whether i'm "getting on it". last week it got really bad, i couldn't even baby it around town without noticing the problem and then the check engine light came on the next morning so i took it to the dealership and of course they claimed they couldn't get the code to come up while testing it, but they said there was a code stored in the ECM that they cleared out and then added a bottle of fuel injection cleaner to the gas tank. it just so happen that the very next day a cold front moved in and the temp. got down to 50 degrees that night, so the next morning i took it out for a spin (having already read this discussion) and the problem was almost as bad as when the check engine light was on, after the engine warmed up it was still noticable in the upper rpm ranges. some would say if it only happens at full throttle then don't "get on it", but if that was the case i would have bought a base model not a fully loaded gt. the code in the ECM was P0304. does anyone know ECM codes?

[vibe owner]

Sounds like slowvibe has a different and much more serious issue. Code P0304 is a "type B" code -- it only causes a malfunction light if it happens in two consecutive ignition-on periods. It means "engine misfire detected". Engine knock is a different code, so I assume this means a bad spark? I doubt if it would detect a lack of fuel, but that is another possibility. Is there any visible or audible indication that unburned fuel is going through the engine? Could be very bad for the catalytic converter.As for my original hesitstion issue, I plan to give written descriptions to the Toyota and Pontiac dealers and see if either offers a plausible explanation.

[vibe owner]

Reading further, here is the full description of code P0304:DTCs P0301-P0304 are cylinder-specific misfire codes. Each cylinder is monitored individually for a misfire condition and can set a cylinder-specific diagnostic trouble code (DTC). The powertrain control module (PCM) uses the crankshaft position (CKP) sensor and camshaft position (CMP) sensor to determine misfire. The CKP and CMP monitor their respective components and evaluate changes in the crankshaft rotational speed for each cylinder. Irregular changes in the crankshaft rotational speed indicate a possible misfire. [wouldn't this only work at slow revs?] The malfunction indicator lamp (MIL) illuminates when the misfire rate equals or exceeds a predetermined count. A misfire rate that is high enough can cause the catalytic converter to overheat under certain driving conditions. The MIL will flash on and off when the conditions for catalytic converter overheating are present.It goes on to say the computer remembers the rpms at the time of failure. Sounds like the light only blinks on at first, but stays on if it happens the next cycle. The diagnostic procedure starts with an engine scope (to capture more events), and goes on for 3 pages. Could also be only a faulty CKP. A vacuum leak or bad mass air flow sensor could command the engine to go lean. Could be water in the gas -- easy to rule out with an appropriate additive. Cold temperatures after damp weather will cause condensation in an unfilled tank, even if the gas started dry.

[slowvibe]

oh and i only have about 3500 miles on my gt and it's already been to the body shop cause some a-hole backed in to the right rear corner while it was parked.

[slowvibe]

hey (vibe owner) thanks for the tech info i got pulled away from my computer earlier and just read your thread.

[rainvibe]

Hey VO, I am going to try cleaning the air flow sensor, see if that clears it up.

[ToolGuy]

Have you ever changed out your spark plugs? If not I would do so, I do not care if they are long life, issues can come up sooner with long life plugs. I always change mine out at half the recommended...Second, run a bottle or two of Chevron Techron Fuel System Cleaner in your car. Do not use anything else here...Have you ever done this too?You may also want to clean or have cleaned the throttle body. My feeling is if it were EVAP you would get a code. OBDII is very sensitive and if it malf even a little, you get a code.

[vibe owner]

Thanks for the comments. Still original plugs. I have my doubts -- it isn't running rough, just hesitation on acceleration. I won't get a chance to work on it until late this month.

[vibe owner]

I tried something new -- wrote a letter describing the problem, and left a copy at service department of both the Pontiac and Toyota dealers. The Toyota dealer called 2 hours later to say my injectors need cleaning. The Pontiac dealer never contacted me. I suppose I'll take it to the Toyota dealer...

[ToolGuy]

Quote, originally posted by vibe owner »The Toyota dealer called 2 hours later to say my injectors need cleaning. Looks like your are on your way to having it fixed! Kind of what I thought the issue was too...

[vibe owner]

I took it in to Toyota last week. They cleaned the injectors and changed the oil. It is a big improvement, with most of the hesitation gone. I am not sure it is exactly like new -- there is still a brief lag and I've stalled it by lugging down. But if it isn't 100% better, it is at least 80%.