Hey guys,

1984 CFI Injected / stock Corvette. Daughter and I drove the Corvette around today after getting the brakes fixed... and it runs pretty decent. The long and short story of this car is... it's a 52k original mile car that sat for a long time with the previous owner, and we're going through it now. After driving it around the neighborhood for about ~10 minutes. It basically stalls out when we come to a stop after a few minutes, and it's unable to keep running. I pulled it into the garage with a little bit of nursing, and then removed the gas gap. There was an ENORMOUS suction sound for like almost 10 seconds as it acclimated to ambient air pressure and relieved the vacuum in the tank.

This isn't an insurmountable challenge, but wanted to know if you guys know if there's a "common" issue with this. The fuel tank SHOULD have some way to properly vent. I know the fuel evaporative canister is supposed to consume fuel vapors from the tank and feed it back into the throttle bodies. The vacuum line to the purge valve and the fuel evaporative canister are both disconnected right now as we've been working on the car.

The question I have is... is the car supposed to vent only through the evaporative canister, or does the fuel cap also play a part in that? The fuel cap is some aftermarket STANT one, but it's definitely very old (probably 20+ years old). I'll probably just order a new one anyway, but curious where I should be looking to see why the fuel tank isn't properly venting?

Just for some background, I replaced the fuel pump and my daughter the fuel filter, and when we did it, we replaced all the rubber lines both at the pump and on the front right side of the car. So I know the tube / vent, at least to that point, is OK.

EDIT: Just wanted to confirm, it's probably a pretty safe bet that the reason why it's stalling out after like 5-6 minutes is because the vacuum is too great, and the fuel pump can't suck any more fuel out?

Thanks!!!

Todd / 84 Corvette

 

I can only tell you about 86 and 87, they have a venting cap to allow air into the tank to replace the fuel pumped out of the tank.

 

You could test this theory by leaving the cap loose and driving around and see if the stalling stops. If you are going to change the cap anyway drill a small hole in it so it vents. I just got a locking gas cap years ago that is venting and this suction went away. Dan

 

The vented vs non-vented cap question has been a subject of conflict and debate...

https://www.corvetteforum.com/forums...ed-or-not.html

 

Are you sure it is vacuum and not pressure ??
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The fuel tank sending unit vent is the outside line that's next to the pump feed line. The return line is on the side by itself

 

Hello all, Check the evap canister and the vacuum line to the purge valve near the throttle body. My 1993 was stalling out after the engine got to operating temp. The line from the evap canister to the purge valve was clogged with charchol, and so was the purge valve full of charchol. The purge valve was stuck open. If you can blow through it, the purge valve needs replacing. I replaced the purge valve and cleaned out the plastic line from the evap canister to the rubber vacuum line behind the front passenger side wheel well, replaced the vacuum line to the throttle body and now my car runs great. I did not replace the evap cannister because you have to take the back of the cars bumper off. I did not feel like doing that so someone suggested that I install a plastic inline fuel filter right at the canister to keep the charchol from clogging the vacuum line to the throttle body and so far so good it's working. Fixed my stalling problem. If that doesn't work, could be opti-spark.

 

This is where reading more than just the thread title comes in handy... 🤦🏻‍♂️

Our 1984 Cross-fires don't have an opti-spark, and the charcoal canister isn't in the rear bumper, it's on the driver's side wheel well under the flip up headlights

 

There are 2 vents on an 87 corvette.

1. The gas cap, it lets air in .
2. The charcoal cannister vent line, it lets air out. These metal lines often get clogged from rust. Especially the last few feet in the metal line in the wheel well. There is also a one way (tank pressure control or restriction valve) in that wheel well. That was why I asked if he was sure it was vacuum instead of pressure. Cannister line clogged, you get pressure. Gas cap clogged you get vacuum.

The vent hose leading from the tank to the charcoal cannister is not a 2 way flow or else it would not work. Fumes from the tank flow toward the charcoal cannister and air need to replace the fuel as it is used comes in from the gas cap.
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IHBD was correct , I see no pressure control valve for 84.

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I don't think that the 84 has the tank pressure control valve. The tank vents directly to the canister. Ported vacuum opens the purge control valve on the canister, and the ECM allows purge under certain conditions with the purge solenoid.

While looking at the evap system schematic in my 88 FSM just now I saw something I've never noticed before. The fuel cap shows two check-valves. One in and one out. Interesting. The next time I'm at my smog guy I'll have to ask him if the machine that tests the caps is checking for vacuum or pressure? There is probably a threshold for pressure release, but I would think very low, less than 1 psi.

Curious that the OP is reporting "significant vacuum" after a 10 minute drive. The volume of fuel consumed in 10 minutes is only a few cubic inches. (231 cubic inches per gallon.) Depending on ulage, this won't create much vacuum. If it is indeed vacuum, could it be engine manifold vacuum being introduced to the tank through the evap system? I have not a clue what malfunctions could cause this, but checking for vacuum on the line to the tank with the engine running would be a way to dismiss the possibility.

 

Buy a GM fuel cap with this correct part number and you can't go wrong. Many parts stores have incorrect information. They sold me a non vented cap and I had very high vacuum pressure in my tank.

 

To be fair... it was a 10 minute drive, but I think we'd had it idling for about 10 minutes also. Someone up above asked if it was pressure or vacuum. I am convinced it was vacuum because I did not feel any rush of air escaping. Can't say I felt anything rushing past INTO the tank either... but it sounded a bit more like a suction than the sound you get when air is escaping from something.

The odd thing, and I'm not sure we really did anything, was that it's no longer doing it. I let it idle for a bit, got some more gas, drove it around for about 5-6 minutes and it didn't make a sound at all when we removed the cap. Nevertheless, I purchased a new GM cap from Rock Auto for the appropriate year. It has a vent in the center that I guess vents up and around the rubber seal... or as you're saying, allows air into the tank.

My suspicion is that the charcoal canister is clogged, or something along those lines (literally) is. If it were me, I'd probably have just ripped everything out and re-installed all new parts or reconditioned them... but we're trying to really diagnose every single little problem (even if it's as simple as replacing a fuel cap) because I'm trying to teach my daughter more how things work and how to diagnose problems. Replacing parts is easy, but identifying why something isn't working has to be learned.

This car was really jacked with before we got it. Like I said in another post... it had one tire in the junkyard. The fact that the frame has no rust, and it's never been in an accident, and the motor has 52k miles on it, it's the only reason why I bought it at all. But the previous owner did all kinds of crazy stuff. It has two positive battery cables... and both of them work. Make that make sense. It also has two sets of horns (not two horns, but 4 horns), which also makes no sense at all. It also doesn't feel like vacuum lines are routed properly... everything is all over the place.

Thank you! That's exactly what I did... it matches with the one I bought from Rock Auto.

 

Adding fuel reduces the volume of air ('ulage") in the tank. Air can compress, liquid can not. A full tank can not pressurize at all other than the 1" or so of space above the fuel if the tank is not "topped off". Adding fuel reduces the propensity for the cap to hiss when removed.

I have an 85, it does not have the tank pressure control valve, but I drive it so seldom that I can't recall if it "hisses" or releases pressure with low fuel level after a drive. I think it does at times because the fuel heats up (expands) as the car is driven, thus 'pressurizing' the ulage.

My 92 will lift the cap out of the filler it has so much volume of compressed vapor with low fuel level after sustained highway running. When fuel level is higher, it doesn't. (The 91-up with the canister in the rear of the car has a different pressure residual valve that maintains appx .7psi in the tank, so behavior is a little different than the 90-earlier.)

 

Hmm... that's probably what made the difference. I may not ever know, but at least it was a teaching moment. I'm going to go through the fuel evap and alll the vacuum lines. We "were" going to do a tune-up, new plugs and wires, and then realized the AIR injection tubes were kind of in the way, so I bought new manifolds for her to weld and grind-off the AIR tubes... and then to get those off, I realized all the accessories were in the way, and I intend to replace the alternator with a newer style one, CS130 (I think, I forget my model numbers), and then also replace the A/C compressor... so we just started tearing everything off. We may never know, but I suspect when I get into the fuel evaporative canister, that I'll see the lines either collapsed or clogged.

I don't see any hard lines for the fuel evap system in the front... only the hard lines that run along the side of the car. I'll have to check the manual, but I may run a small hard line if I can (if I find there isn't really one there). Doesn't make a lot of sense in my opinion to run a long vacuum line that length... too much opportunity to collapse. I'll check the manual... my guess is it had one and the PO removed it. I have a bunch of really long-*** vacuum hoses... literally 3 feet in length under the hood... really doesn't seem like this would be stock, but maybe it is.

 

Yup. It's all soft line

 

Ugh, thanks... I'm going to have to re-think that. I think in my opinion, I may go with a couple of smaller (similar interior diameter) hard lines, and then just a couple of silicone transition lines to the components.