Runs great for 15 to 20 minutes then BLAH!!! '83 245GL

[jgreenwood]

Hi fellas. I am the often proud owner (though not at present) of four 240's. My '83 245 GL is a beautiful black wagon for my youngest daughter. It came from a junkyard and I replaced all the wiring under the hood two years back. It has been running fairly well over the last 9 months until now. Now it has been hesitating and surging or bucking when accelerating, all after about 15 to 20 minutes. Prior to that it runs well and strong. I have needed to replace several things and have taken this opportunity to do it, but to no avail. I have replaced the air mass meter. The old was indeed bad and the new rebuilt one seems okay except the adjustment screw seems real loose. The car idles better and generally runs better now for 20 minutes or so. The car runs real rough when I unplug the AMM which makes me think it is okay. I do not know how to adjust the thing. I have replaced the fuel pressure regulator with a new one. I have replaced the main fuel pump and filter and boy was it clogged up with dirt. I have cleaned the throttle body thoroughly. I have replaced the temp sensor. I have replaced the fuel pump relay. Still no joy. I noticed that the in-tank pump assembly was loose and not sealed by that gasket. I had replaced that pre-pump awhile back and must have done a poor job closing it back up. I know that if that pump is bad then it will likely show up when the tank is half full or less. Now, if the tank is not sealed properly would it also not run well even if the tank was full of gas and even with a bad pump? What else could it be causing the car to behave this way after 15 to 20 minutes of driving? It MUST have something to do with pressure build-up after time or heat build-up after that amount of time. I suppose that pre-pump may be worn out due to having to help force fuel through a dirty main pump and filter. The pre-pump does run, I can hear it, but I have no idea if, after driving a bit under pressure in the tank, if it is then prone to failure. Wow. This is a lot. Could it be something else entirely. I am a novice and need help. Also is there any Volvo mechanic in the Raleigh, NC area that may be willing to give me some informal help here? I welcome any and all input from you forum guys. I am at wits end and getting poor. Remember, the car runs great from cold and driving for 15 to 20 minutes THEN starts running rougher and rougher and wants to surge and buck when accelerating and up hills. Thanks!!!

[wojeepster]

When it starts to run poorly have you tried opening the gas cap to see if there is a big vacuum built up? Just something to check..