I wasn't sure where to post this, but this is the closest place i could find. I have a freightliner box truck with a caterpillar motor that is driving me nuts here. It has about 320,000 miles or so on it and it doesn't want to start sometimes. It cranks over good but just wont fire up. I took it to the freightliner dealership before the holidays and they had it three days and it was ok for about a week and now is worse than ever. They put a new fuel line on it thinking it was deteriated and drawing air in somehow. They claimed the shut off solenoid was working ok and that that wasn't it. If i put a strong mixture of a diesel additive in it like fuel injection cleaner then it seems to help, but to me that doesn't make any sense. It doesn't help 100% ,but it probably helps 80% or better. Everybody tells me it is probably one of two things. The shut off solenoid or it is drawing in air somehow. I can take the primer pump and pump it really hard a few times and it will start, but there is pressure on the pump. Not like it is pumping up or anything. Anybody got any ideas? I can't afford to let those clowns at freightliner have it again.