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Little to no vacuum Port A

I

INChubbs29

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Salt Lake City
Hello all, I have a 1991 K5 Jimmy, with the 5.7. The issue I am having is I get a great deal of pressure build up in my fuel tank. I have cleared the line from the EVAP to the tank and I can get compressed air at the tank. The valve seems to work just fine in that it opens when vacuum is applied and air will pass through when it is "open", and no air will pass through when it is "closed". I am stumped on this one because at Port A, the vacuum Port on the far right side that goes to the EVAP charcoal canister seems to have no vacuum at all. The line going from the Port to the canister is clean and open, it just doesn't have any vacuum there. The Port A in question seems clean and clear "compressed air will pass through". All of my other ports show strong and healthy vacuum for as many miles are on the engine. Why is there no vacuum at Port A?
 
The vapor canister purge valve, on my 91 it is located on the fuel tank, and goes into the fuel pump vent line.
 
Does a gauge show any vacuum at all at that port? And did you check it while raising the RPM of the engine at all? I would think that there wouldn't be full manifold vacuum going to the evap canister as there wouldn't be a way to meter any fuel that may possibly end up in the line from the tank.
 
The gauge shows a slight fluctuation from about zero to 1 slash 2 at idle. When raising the rpm it barley hangs on around 1. The only thoughts I have is a busted throttle body gasket. I smoke tested the engine today with no signs of vacuum leaks or a visible throttle body gasket leak. The other 2 ports show about 15 on the vacuum gauge without fluctuation*Also note that I live in Salt lake city*. The odometer quit working at 130000 miles but the engine still has "decent" compression with the amount of miles.
I am stumped and didn't want to remove my TB if I don't have to. Also know that excessive pressure build up is dangerous.
 
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Well, like I stated before, I don't believe that you want full manifold vacuum going to the charcoal canister, but I haven't actually checked my truck.

I would be suspicious of the gas cap. I am not sure, but I thought that they had a pressure relief valve to keep the pressure below a set amount.

I wouldn't pull the TBI for this problem, I look at things around the tank. As @Bent77 said, look at the exhaust and see if it's to close to the tank. Then check into a new gas cap, is what I would do.
 
I could be way wrong, but I thought that the charcoal canister was the only way to vent the tank on newer vehicles, due to gasoline vapors entering into the atmosphere, because I thought they pulled the vapors into the canister and then into the engine to safely be burnt?
I do know, because I tested today after "6872xtc mentioned full vacuum", that if I hook up my full 15 vacuum pressure it opens the "vent" valve on the canister. Which if I understand that correctly only opens if to much vacuum is drawing on the canisters. So you "6872xtc" are correct that I don't want full vacuum, I am just wondering now how much do I need?
My exhaust I think is the factory original, so I wouldn't believe it to be to close, but is there a spec I can measure?
 
A gross thought occurred to me that I don't really want to deal with either, could the vent line on my fuel pump be clogged?
When I blew out the line from the canister to the tank I made sure to unhook the line from the tank so I wouldn't just blow a bunch of trash into my tank, and dangeroursly fill it with air. I didn't even think to check if the pump line would be closed. Thoughts?
 
No vent line on the pump, since it's in the tank.

I thought that you meant fuel tank when you said pump earlier.

As far as how much vacuum you should have to the canister, I don't know.
 
sending_unit_fittings.jpg

I am a little confused on "no vent line on the pump", runs all the way to the canister
 
INChubbs29 said:
sending_unit_fittings.jpg

I am a little confused on "no vent line on the pump", runs all the way to the canister
Click to expand...
That is the vent for the tank, not really the pump. Mostly this is just wording, as the pump is mounted below that, in the tank.
 
6872xtc said:
@ZooMad75 do you have any ideas?
Click to expand...
Couple.

Normally excessive pressure happens because the tank is sealed and without purging the vapor pressure builds. Heat can accelerate the pressure building but if the vapors don't get purged it's going to build.

Does the purge valve hold vacuum when applied? Reason for asking is if it won't hold vacuum it can't keep the valve open and won't purge.
 
I will double check the purge valve, and leak down test the entire line, also make sure that my vent line on the pump housing is clear. I will also test to see what amount of vacuum pressure will open the valve. Because I still feel like there simply isn't enough vacuum pressure to open the valve or adequately purge the vapors.
If my purge valve is bad, where would I get a new one? I was on most all the auto parts websites and am having trouble finding the darn thing.
 
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Might only be available as part of the whole charcoal canister.
 
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