[bryan23uh]

I have a 99 Mustang V6 automatic. My check engine light came on out of nowhere about three weeks ago. I got the code pulled and it was a P0455 (Large Evap Leak). I've been searching threads on this issue since then and I've checked all the lines (replaced one cracked hose), gas cap, fuel tank, canister, IAC. I wanted to get some feedback before I go spending too much more money on parts thataren't going to help any. I'm putting new plugs and wires on this weekend and I'm going to clean the throttle body.

I also came across something interesting today while I was out and about. As soon as I turn the A/C or heater on the idel changes (normal I know)...however when I come to a stop say at a stop sign or traffic light or just sitting in traffic...the engine starts idleing super rough almost like it's about to die then goes back to normal...it only does this when my A/C or heater is on and I'm coming to a stop or stopped.

If anyone has any clues or info on where I should start diagnosing this I would appreciate it.

[shinerstang]

It is a vacuum leak somewhere. Gotta start tracing vacuum lines......

[bryan23uh]

I appreciate the reply but I've already checked all the vac lines. Could it be one of the valves i.e. EGR valve or Solenoid valve? Or could it be a sensor? Would a faulty sensor cause the engine to run differently? Also, I have never had the fuel filter changed but I was told that it shouldn't need changing unless you get some bad gas through your lines. My car has ~130,000 miles on it so it's getting up there. I've read many blogs concerning this type of issue and they all say different probabilities but I don't want to spend tons of money taking it to a dealership or repair shop and not have the problem fix. At the same time I don't want to spend tons of money replacing every part on the car. If there are any other possibilities I might be missing any help would be greatly appreciated.

[bryan23uh]

Okay here's an update. I took the EGR Valve off and gave it a good cleaning along with the throttle body and replaced the EGR solenoid Valve. I also replaced the plugs and wires. I also seafoamed the engine. The engine seems to running better but I'm still having idle issues with braking. I started thinking...could it possibly be something with the brake booster that would be causing this? and would this cause the computer to throw up a P0455 (Large Evap Leak) code?

[shinerstang]

Mass air sensor would be the next thing to check, but it wouldn't throw an evap code...... Check your canister and all vacuum lines. Spray some carb cleaner on the vacuum lines with the car running. It could also be an intake gasket......

[bryan23uh]

Well it is throwing an Evap code (PO455). How exactly do you check whether the MAF is good/bad?

I've already spayed carb cleaner over everything in the engine compartment and the idle pretty much stays steady.

Also, what should I look for as far as the intake gasket?