2001 GS Shuttering 30sec After Start

[irbf]

2001 Regal GS Supercharged 3.8 with 120K

After a normal start, about 30 seconds later, the engine starts to shutter with RPM dropping low and surging. If I do not play with gas pedal will ultimately stall. Once it is past its fit, which lasts about a minute, maybe a little more, the engine then runs and performs fine until the next start. This happens with a cold or hot engine. I have replaced the fuel pump and filter. Still have the issue. All battery connections, grounds, etc. are good. No check engine light.

Okay you GM 3.8 SC experts, any ideas? Help appreciated.

When it starts to shutter it is significant. Reminds me of trying to start a cold piston aircraft engine on a cold day after a couple shots of primer and goosing the throttle.

[Tech II]

Have seen a vac leak in the gasket between the sc'er and the lower intake.....spray carb cleaner around the edge of the joint of the upper SC'er plenum and the lower intake and see if you notice a change in rpm while spraying....

[irbf]

Thanks. Will try this.

[irbf]

The problem turned out to be the upstream O2 sensor. As a test I unplugged the sensor then started the car. The symptom did not occur. I replaced the O2 sensor and the problem is solved.

If you attempt to replace this sensor it is in a very tight location between the firewall and the back of the engine. I purchased the Lisle 22mm O2 3/8" socket with the slot in it. Without this it is near impossible to get a wrench on it. I painted PB blaster at the threads of the old one and let soak (cannot get can in there so will need brush).

Be careful to not let the socket snag the adjacent plug wire.

Question: Should the downstream sensor be replaced any time the upstream is replaced?

[irbf]

Problem still not fixed. Have been driving the car and putting up with this. After replacement of the O2 sensor the car seemed fine for about 15 trips. The symptom then came back and has gotten gradually worse.

Once in awhile it will throw the 1133 O2 code.

I did find that the thermostat was sticking open and replaced it. This made a significant difference in fuel economy but did not solve stumble problem.

Once the engine finishes going through its fits (seems to start after ECU goes into closed loop) about 60 seconds into running it stops stumbling then runs fine.

So start, at closed loop stumble perhaps stall, then about 60 seconds clears up.

I have also noticed a steady clicking noise while in the car from the engine compartment that seems to be at a steady 5hz frequency at idle. Sounds almost like a bad plug wire and arching but have not found this at all.

I have two of these cars and so swapped a known good MAF sensor and this did not solve the issue.

This chump is stumped...

[Tech II]

Could still have a vac leak, that could cause a P1133.....

[irbf]

I have looked closely and cannot find an obvious vacuum leak.

Another clue... If the car has been setting for a day or so and cold it will start and run without issue. Even the second start after a cold start there might be a little stumble but not much. Worse is when the car has been driven around town with multiple stops and warm. It then stumbles repeatedly after a start.

If there were a vacuum leak or intake manifold leak would it not stumble when cold? In the case of the manifold as the engine warms and metal expands I would expect any small leak to seal.

I wonder if the air intake temperature sensor could be the issue?

Any help, thoughts, outbursts, etc... appreciated.

[irbf]

Replace fuel pressure regulator. Seems to have fixed problem.