Engine surge tank

Mine had the same issue a couple of years ago. I think Motorhome Magazine had a maintenance article about it. If I recall, the article said it was caused by the plastic surge tank being exposed to direct sunlight for extended periods when the rig is parked. I had the surge tank replaced at my local Freightliner dealer, but after seeing how straight forward the job was, I think it might be a DIY job. I think the article said there was a "fix" (probably from Freightliner) that was basically an aluminum foil shade that could be pasted onto the exposed side of the surge tank to keep it out of the sun. Another option I heard about, but can't verify for sure it actually exists is an after-market surge tank made of metal instead of plastic. Good luck.