My Bonneville has 199,000 miles on it. In the mornings my car starts and runs well for about 15 minutes. Once the car gets warmed up the car begins to have stalling problems. The stalling occurs only when I am braking or if I take my foot off of the gas pedal. I never have a problem with the car starting back up right away. I used to coast to a complete stop and place the car in park and it would restart. Sometimes I have to apply the gas pedal right after I start it to keep it running. Lately, I have placed the car in neutral and started it while coasting. I can prevent the car from stalling if I keep pressure on the gas pedal while using the brake to come to a complete stop. Once the car comes to a complete stop I can remove my foot from the gas. However, while I am stopped my car will occasionally throttle up as if I am revving the engine.

The rough idle also happens in the afternoon when I start the car. Often times the car will act as if it is going to stall right after it starts, but then immediately revs up to 2500 - 3000 RPMS. This all happens for before the car is put into gear and last 15 - 20 seconds.

Additionally, once the car has been running for about 15 minutes I notice that at times it acts as if the car is misfiring and I lose acceleration power. This is a problem that I was having a couple of years ago (summer '09 or '10). At the time I replaced the spark plugs and wires. I also had the ignition coil replaced.

Lastly, someone suggested that I might have a mass airflow sensor problem. As a first step, I bought MAF cleaner and cleaned the MAF. This did not seem to help any. Is this a path that I should continue down?

Please help!

Check your egr valve for a sticking pintle.

Vacuum leak.

Power brakes work on vacuum. The first 15 minutes you operate your car the fuel mixture is enriched by the computer as it's cold. When the fuel enrichment switches off and you hit the brakes, the car stalls.

The vacuum hose from the power brake booster to the UIM is the probable culprit. If that's not it, look elsewhere, but I'd bet a lot it's a vacuum leak.

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462 cubic inches of REAL Pontiac power.

Bob,

Can you please elaborate on how one would look into a potential vacuum leak for those of us less experienced with cars or bonneviles? I'm experiencing similar issues with my 1999 pontiac bonneville SE.

-robodude666

Get a can of carb cleaner and lightly mist around each vacuum line. If you see or hear the idle change when you spray a particular line or area, you've found your vacuum leak. Make sure not to soak the lines or engine in the cleaner though, just a light spray will do the trick.

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Joseph

Current: 2013 Infiniti G37 Journey
1999 Infiniti G20T - Sold
1999 Buick Lesabre - Race Couch - gone but not forgotten