I've done a search but still need more help. I've checked the vaccuum lines at the evap control valve in the engine bay, I've changed the gas cap, but still haven't had any luck.

I suspect some kind of switch that is vaccuum operated but don't know which one or where.

The engine runs fine when it's cold. Once it gets up to operating temp it misfires and hesitates really bad under acceleration or when climbing a hill. It runs fine at idle. makes me think that there is some kind of vaccuum advance related to the evap emmission control system and that it has failed in an open position for the computer to say it has a major leak. Maybe I'm wrong, I dunno!!

Anyone have any helpful ideas on how to fix this.

thanks,
Dave

99 Expedition, 5.4L

 

P0455 is a gross evap leak, which is usually the cap (which you changed) or a leak at the cannister connections, hoses, or you may have a vent valve not sealing. It's on the cannister, I've had a few plugged with dust and dirt, clean it out and the hoses and sometimes they will work again. Take the hoses off the cannister and see if they are full of dust and dirt.

 

on which cannister? in my manual, it says there is a cannister under the vehicle near the spare tire, but i cant find it. what should I be looking for to find this cannister/vent valve?

 

It's above the frame brace above the spare tire. Look for a round black cylinder approx. 4-5 in. in diameter and about 12 in. long with a solenoid on it. Hope this helps.

 

Ok, so now I've replaced the gas cap, checked vaccuum lines at the evap valve in the engine bay, took off the cap on the evap canister and cleand the vent valve at the back of the vehicle.

the "service engine" light has turned off, but the vehicle still runs bad. It's fine when it's cool, but once it warms up it hesitates and misfires real bad. It feels to me like a vaccuum issue, but I can't find it.

any other ideas?

I don't want to have to take it in to the dealer and pay out a whole lot of money that I don't have right now.