Just recently my car has begun to stumble after it is started in the morning. When I start it, it stumbles and then goes to a normal idle. If I give it a short rev it hesitates a little. It also smells a bit fumey out the tail pipe. I've replaced the plugs, the wires are about a year old, replaced the fuel filter and the O2 sensor (Delco) is about a year old. I've cleaned the TB and the IAC. Didn't mess with the MAF for fear of breaking the wire. I've also checked the coil packs and they appear to be in range of what Techinfo shows. It has thrown no codes.

Once the car gets to operating temp it seems to run fine. Although I do notice a slight rough idle every once in a while.

Any help would be great.

Thanks

Only right as you start it or as it'* warming up?

The stumble is right as I start it. If I let it go for a second the car goes to normal idle.

The hesitation is all the way up until it warms. When I say hesitation I guess I mean the throttle response. Hopefully I'm explaining that right.

Sounds good to me.

I was thinking that maybe your check valve in the fuel pump may not be working. If that is the case, when you turn the key..the system is attempting to pressurize enough and may not have quite enough to start the car properly. Causing a stumble when you first try to start it.

Hesitation could easily be vacuum hose related, it will go away as the engine heats up.