I'd like to understand how the evap system works on the factory TBI setups. Looking at the emissions diagram for a 1991 (I think), there is no electronic control or monitoring of the canister or anything. There's the vent line from the tank and a line going to port "A" on the throttle body.

So, what does port A do? Presumably it sucks air from the evap canister sometimes. If a person didn't have an evap system and just plugged port A, would they require a different tune, or would it mess anything up?

My canister looks like it just vents through to the outside world (I could suck air through it or blow into it at the tank port), so I don't quite see the point of it. Without thinking too much I always figured it let a certain amount of pressure build and purged it into the intake at controlled intervals.

My tank collapsed once due to venting issues. I'd like to make sure it doesn't happen again.

 

There is an ECM controlled solenoid on the canister. If not then it was swapped for the wrong one. Port A to the TBI is what draws air through the canister to purge it of fumes. It is also ported vacuum so that no purge can take place with a closed throttle.

The vapor control valve (flat disk Y shaped device in the line to the tank), opens when tank pressure is about 1 psi. The valve opens and allows fumes into the canister from the tank.

The vent is required to prevent pressurizing the canister when the tank vents into it. And to prevent fumes from being drawn directly from the tank during a purge cycle.

As for a collapsing gas tank, that is the job of the tank vent valve. Located by the rear axle, drivers side. It allows air into the tank and will vent out excess pressure. Such as if the canister fumes line to the tank is plugged.

RBob.

 

Being an f-body site I assumed this was for a TBI Camaro or Firebird. Trucks didn't get purge solenoids. They either purged whenever the throttle was open, while some have a therm-switch that won't allow purging until the engine temperature is up.

RBob.

 

Sorry, yeah, this is a truck application. The diagram shows only the canister with line to port A and another line going to the tank. I'm not looking at it now, but there was some other thing between the canister and tank that said "if equipped".

With that setup, it looks like there is nothing to help you if the canister, or the line to it, plugs up. Maybe the cap does something? It's obviously not just a solid plug but it's also "non venting". Does it allow air in but not out, or have a max pressure before it opens or something?

I was poking around the 7747 parameters and didn't see anything related to purge. I'm assuming that ECM was used in only in applications like I'm describing, where the evap system just does its thing. But then is it all set up assuming there will be a small vacuum leak through port A? What's the right way to hook it up without emissions, plug it? Thanks.

 

High altitude versus regular. High altitude received a vapor control valve along with a different canister. This canister has a vacuum controlled valve on it for when to purge.

The '7747 ECM doesn't support CCP.

RBob.

 

hi I've been following your posts about the ccp and how the 730 controls it. I have a '91 350 TPI in my '55 and have had the gas smell and the wife is complaning. I don't have the speed sensor hooked to the data so I don't think I can get it to signal the canister to purge. I need the part number for the fuel vapor pressure control valve because I don't have one and GM has stoped selling it, and how all the hoses hook up to the canister. I was thinking of useing the canister like the truck where it's not controled by the ecm. what would I need to do that and where to hook it up? The help would be appreciated thank you Mike

 

Here is the purge solenoid. Port A on my new remanufactured Rochester throttle body doesn't seem to create vacuum all the time, so I'll assume it does it whenever needed by pressure/vacuum.

This is that purge valve the other guy was asking about.
So this is off my 92 350 tbi, my 92 Camaro I haven't looked at but it should be similar as they have similar throttle body's. My purge solenoid takes a LOT to blow through in the correct direction and port A hardly if at all pulls vacuum but every time I pull the gas cap, does it ever hiss at me. The reason I'm mentioning it is because I got a new remanufactured Rochester throttle body with flow matched injectors to "cure" a problem. That problem is when the truck is cold, it runs perfectly fine until it warms up. Once warmed, holding throttle at 5-10-15% creates what seems to be misfires at rpms of 1,000-1,600. Under more throttle application, it's completely fine, but it doesn't really go "normal" until around 2,000. It does this jerking feeling in 2nd gear and 3rd, and low enough rpms when torque converter is locked up as well so it's not a tranny issue. Also, while maintaining a 5-15% throttle, suddenly letting go it will nearly stall out so I usually tap the gas pedal while it's on its way down to prevent this. New fuel filter, new throttle body, new vacuum lines, new EGR solenoid, but I'm wondering on this 32 year old beast if the EGR is shot...?