Pontiac Repair: Loud Noise from rear of vehicle after parked, fuel filler cap, crankshaft position sensor

Question
Hi Todd, I have a 1998 Pontiac Grand Prix w/3800 Series II, Supercharged 6 cyl. engine that has begun exhibiting a handful of different issues that I feel may be related to one another. For the past month or so, it has been starting and running fine in the morning but on my way to work at random times it will have a slight hiccup and the tachometer will go to zero and stay there. The car continues to run ok after that. A couple of evenings ago, it did the same thing only it actually lost power and I had to pull off the highway. After about 10 mins. of attempts to start it, it finally did and I was able to get home. This morning, it did the same as others but I also noticed that the gas gauge also went to "E" and remained there. It had a half tank of gas in it. It still ran, when I got to work, parked, and turned it off, I heard 2 loud clunking sounds coming from the rear around the gas tank area. From that point forward, it is doing this everytime I park it. The sound it makes is like a build-up of pressure and then a loud "clunk" or 2 to relieve it. Sounds dangerous to me. Hope with your expertise and advice I can figure it out before something bad happens, if it hasn't already.

Thanks much for any advice,
Mike

Answer
Hi Mike:

well you coulds have a number of things going on here. As for the tach dropping off and the engine loosking power that could be a crankshaft position sensor going bad or an ignition module. more so the crank sensor.. as for the noise you hear it could be an issue with the evap system in the car it captures the vapors in the gas tank and burns them in the engine. If for some reason the vent control valve on the system sticks cloased it could build up a vacuum in the gas tank and that could be the poping noise you hear. next time you drive it for a while keep the engine running put the car in park and go take the fuel filler cap off and see if you hear a air rushing noise back into the tank if so that's what is going on.. If this were the case then the engien computer should trip a code and turn the service engine soon lamp due to not being able to vent the vacuum out of the tank it's self....  I have seen a few RARE cases where so much vacuum built up in the gast tank it affected the operation of the fuel pump and that could cause you stall... That is a BIG long shot though.. Good luck :)