Thread: Car lopes and sometimes dies when gas cap is opened.

99 Grand Prix SE 3.8 249,000 on the clock.

Car will lope and hunt to find idle when gas cap is opened.

I checked out the Evap solenoid and it is closing when the car is off. Have yet to replace it as i seen no need to. Anyone know what else to check under the hood to see what would be keeping the solenoid open?

What do you need to keep the gas cap off for when the cars on anyway lol

What happens when you remove the gas cap, does pressure go up or down?
Isn't it tiring running next to the car to open the cap with it driving down the road?
The cap technically shouldn't be getting removed with the car running because gas fumes plus spark equals undesirable result.

Sounds stupid I know. But I guess theres times the gas cap did not seal 100% and has caused some Drive-ability and safety issues. Causing the engine to stall say in the middle of an intersection. Also after I refuel, I start it and it will stall. Repatedly until it builds up vacuum in the fuel tank. Unless i sit there and hold the throttle for a 10-15 seconds.

Vacuum or pressure? Why would vacuum in the tank cause it to start?

Because the Evap solenoid is opening up, allowing basically a massive intake vacuum leak to open up from the fuel tank when cap is removed. Would a video of the issue help any? I can remove the gas cap with it running and it will lope and hunt and surge for idle, I put my hand over the gas filler and i can feel a vacuum, once it builds up the lope stops.

It really becomes a ***** when its been sitting in the sun and i try to start it, And it wont start worth a ****. Have to sit there and race it for a while.

Sounds like you've hunted down the issue to be a major vacuum leak. Any codes from it? Take a look at the FSM and learn how this system works and that will tell you if there's something malfunctioning. Which will lead you to the correct part to replace.

Originally Posted by RyanL

if this is actually your problem why dont you replace your gas cap?

Really wish it was that easy. The Evap solenoid is opening up full time when the ignition is turned ON. Allowing full manifold vacuum to be pulled on the fuel tank. Opening the fuel cap or expansion of fuel vapors from cold to hot creates a rush of air/fuel vapor into the manifold upon start up causing the car to die or lope. Thats the best way i can explain it.