Whenever I "hit" the gas or lug my 5.4L, the cab are vents change from blowing out the panel vents to the defrost vents. I believe I have a vacuum leak somewhere, and thought I would check in for some pointers before "diggin' in"

My truck is a '97 F-250LD 4x4 with the 5.4L engine.

Cheers from Kansas, KK

 

I would trace the vacuum lines to find one that goes into the passengerside fenderwell. I beleive, that is where the vacuum canister is.

 

There is also a vacuum check valve in the line between the motor and where the line goes through the firewall. These can and do fail, which will let the vacuum bleed back out of the canister when the engine is pulling hard.

I had one go bad this week, which caused the same symptoms as yours are. I was afarid of a vacuum leak, but after checking the valve, I found you could blow through it from either direction.

The rubber flap seal inside the check valve goes bad after a while. They are cheap and easy to fix.

 

hey, thanks yardbird. I haven't had time the last couple of days to work on it. I didn't know about the check valve. I'll look there first.

I'm sure I can find it, but where does the vacuum line go through the firewall?

Thanks again. KK

 

I don't know on your truck. I should be a small, hard plastic, colored line, that enters somewhere around the heat a/c unit.

The check valve will be between there and the vacuum adapter on the engine. Someone who owns a 97-03 model can tell you more about the location of the vacuum lines. The only modular engine I own is a 4.6 in my T-Bird. My truck is an 87 with a 300 I-6. Everything is easy to find on it.

 

I just repaired my truck for this problem. It was a line behind the battery. The one way valve you are asking about is located in the same place. I had a split line going into the firewall. Good luck finding it. It was a pita on my truck.

 

Thanks lariat97; that will help. I hope to get to it in the next couple of days. KK

 

I too am having the same exact problem with my 98 expy. I checked the valve that was behind the battery and it seemed fine if it was the correct one. One mine it was a tee type of valve and I could blow both direction through the top 2 connectors but only one way through the bottom one. Does this sound like it is the right one and is it working properly? Any help would be appreciated.

 

Thanks all. I finally got around to taking a look under the hood. It was like you said, right there behind the battery. It was fairly obvious, luckily, that one line had simply cracked where it came out of a T shaped connector. I broke it on off and pushed the good part of the line back into the rubber connector and all is working as it should.

 

bomberchamp, I don't know a lot about it, especially since mine was simple to fix. About all I can say is what was allready said.....Find the line behind the battery, find where it enters the firewall, and trace it back to the origin.....

good luck!