I've been trying to chase down the source of an apparent vacuum leak. It's an FE 390 with a Motorcraft 2150. Everything is stock. Carb professionally rebuilt about 18 months ago. My basic trouble is poor idle after I've driven 20-30 minutes. Until then it idles fine.

I fixed a couple of small leaks by replacing carb and EGR gaskets. That just bought me a few more minutes of driving until it starts idling poor. So there are no more leaks around the carb, and apparently no leaks on the intake gaskets. (I covered both the PCV/breather holes with duct tape, and neither side pulled in.) I've checked/replaced vacuum lines around the top of the engine.

Where else would I check? I'm fairly certain it's a vacuum leak, since I've addressed all fuel and spark issues, and once it's warmed up adjusting the fuel mixture screws has almost no effect. I've also checked and double checked choke operation which seems perfect. Also, once warmed up and idling like crap, closing the choke valve greatly improves idle. Gotta be a vacuum leak right? Or maybe not. Again, it's fine when cold/warming up/just reaching full temp, only after driving for 20-30 minutes does it start getting grumpy.

 

This is just taking a guess but check the ported vacuum switch(s) that control the EGR valve. If one of them is bad, it could be allowing the vacuum to leak through and make the EGR valve crack open.
Like I said, just a guess.
Easiest way to check would be to disconnect and plug the vacuum line at the EGR valve so it can't open and then drive around until it warms up.

 

Ported Vacuum Switches are working?

 

Thanks for the quick replies. I haven't looked into that. I'll test that out tonight and report back.

 

So here's what's happening with the vacuum. I found a line that was open and pulling good vacuum. Don't know where it is supposed to go so for now I capped it. There are two other plugged lines. I'm curious what they are.


Here's potentially a bigger problem. I pulled the fuel filter which is about 3 weeks old. If was full of sediment. What's pictured is about half of what eventually came out.