Need help identifying vacuum switches
 

See this is the problem with having more than one problem at once. I fixed the issue with the truck stalling on hard left turns and jackrabbit starts. That is fine. No stalling, But I'm still getting hesitations. Specifically it starts to buck when is shifts into 3rd gear. It is prevelent when the truck is hot. Not so much (not at all) when it is cool.

Now If I continue to drive after it starts doing this, eventually it will throw up the P0701 code, Map (or barometric) Sensor low input voltage. Now I've been tracking this problem for a while. It definitely is not fuel issue. That is DEFINITELY fixed. And it is NOT ignition. Now I come to my question.

I'm looking at the driver side of the upper plenum. There is a vacuum line that connects under the front of the intake, it then runs back along the plenum bolts and hooks into a relay. And then this relay is in turn plugged into (with a short vacuum hose) another relay just behind it. They are both mounted on the upper plenum side bolts. Each has an electrical plug. And the rear switch in turn has a vacuum line that runs to the back and back to the tranny. I've seen it go down to the tranny when I had the plenum off, though I do not know exactly where it plugs in.

My question is what are these two switches. Now I've replaced the front one with one from a junker. No change. But I have found that if I unplug the electrical switch from the rear switch the bucking is no longer there and it doesn't throw a code as if something is missing. I'm going to drive it a while and see if the bucking starts again. But would like to identify this as I see many of the junkers I have checked haven't even HAD this switch. I know it's meant to be there as it has a plug going to it but just don't know what it is or what it does but it does seem to be malfunctioning. I'll post pics of the switches tomorrow. Unless someone has some info for me. But as I said, for now I'm going to leave the pisser unplugged.

Could you snap a picture of the items in question and post it?

Sorry for the size. The two switches are the ones that are behind the oil fill tube in the picture. The front one is partially blocked by the fill tube. But the one in question is just to the right of the fill tube. You can see the plug jack on the switch. The tube from the intake is sitting in front of the wire bundle and goes under the radiator hose. The plug that goes to the front switch is behind the fill tube also and has a red plug. The second one for that back switch is the blue plug sitting just below the jack. The tube that goes down to the tranny goes out to the right and is partially covered in the pic by the brake booster hose.

OK Now I'm confused. Driving today. The truck ran perfect. No problems whatsoever. However with that switch unplugged I get a code. P1442 Evap Vacuum Switch Circuit. OK So it looks like that is actually the evap line. I thought that line went down to the tranny. Guess not. But it keeps the truck running great. But I haven't done a long trip yet. Gonna do that and get back to ya.

Started a new job so don't have the time to dig into it at the moment. But I'm going to check out that evap canister this weekend. It is now sticking in my mind how unplugging that line when I was fixing the tank issue caused gas (fuel) to come out of the evap canister. I'm glad that it is this and not the tranny but quite confused now.

As you have found out, that is the EVAP Purge solenoid. It sounds like you have a leak in the system or possibly a faulty purge switch.

That line will run from the purge solenoid back to the charcoal canister located behind the driver rear tire.

I think I've figured this one out (finally) I drove to work fine. I drove home fine. but tonight we had take out pizza. Before picking it up I plugged the solenoid back in and then unplugged the evap hose. I wanted to see if the problem was AT the solenoid and isolate the evap canister. Well, sure enough, driving to get the pizza, a 5 mile drive, it started to slightly buck a little. Not bad but again, it was not hot but warmed up. So It looks like it's a trip to pick a part one more time to find one of those solenoids. If I can. Every time I go up there those are always gone. So until this weekend I'm going to leave the bugger unplugged and deal with the SES light.