Gas pressure in tank?? Didn't happen before...

[TNMGB]

Okay, when they repainted my 79 B, they took everything off as they should have, but now when I take off the gas cap, I get the same little "whoosh" but also a big pop of the gas tank itself no matter how slowly I try to remove the cap. They did repair what I guess is a gas vent line in the trunk. It was broken off before and one time allowed gas to spill into the trunk. So now I wonder is this the way they all do and I was spared this jolt every time I fill up? I just wonder too if that small hose vented the gas vapors and they just didn't connect something back correctly?

 

[Banjo]

I think they blocked your vent. There should be a round silver tank mounted up twards the top of the right quarter panal inside the trunk with two hoses going to it. That's part of the vent (on a 79 it's all linked to a charcoal canister for a vapor control system). Look in a shop manual to get the whole system setup and determine what's not there or what's hooked up wrong

 

[TNMGB]

Thanks for the info...I'll check it out and have them fix it back at the shop since I've got to have a few minor things taken care of anyway. Is this harmful in the meantime? I have a tendency to think the worst will happen, i.e., the car will blow up.

 

[Banjo]

if the pressure builds enough it can force fuel past the float valve in the carb and cause it to flood over, but I doubt that would happen. You could just disconnect the hose for now. Or leave the gas cap loose (but you might loose it)
It may also be an internal blockage or a plugged canister. Meaning that the shop hooked up everything correctly but the system is bad.
BTW I saw your other post. Woodstock looks great! Love the wheels.
Best of luck Guv'na

 

[TNMGB]

Since it didn't do any of this before, I'm assuming they just need to make sure of how to hook the dang thing up.

I drove yesterday and had no issue such as flooding so I hope that's all right.I just filled up the tank before the price of petrol goes up (???) but again I say, any prob with me leaving it the way it is for now? I won't really be driving it again until I go to the shop but there's that fear factor building. I'm not really thrilled with messing with the hose...I am a big chicken.

 

[Steve_S]

Re: Gas pressure in tank?? Didn't happen before..

Are you sure it's positive pressure? If it's negative pressure, your fuel cap could be not venting properly. This will result in fuel starvation after a few miles of driving. The more fuel in the tank, the faster it will starve. Removing the cap will cure the starvation until the "vacuum" builds up again.

 

[piman]

Re: Gas pressure in tank?? Didn't happen before..

Hello TN,

I agree with Steve, unless the tank can breathe, the fuel pump can't pump to it's full capacity as the air should fill the tank at the rate the fuel is drawn out.

Alec

 

[Banjo]

Re: Gas pressure in tank?? Didn't happen before..

Yes, thats a very good possibility also. Sometimes I forget to look at the other angle of a problem. Good call guys.

 

[TNMGB]

Re: Gas pressure in tank?? Didn't happen before..

Yeah, I'm really wondering if they crimped the hose that is supposed to vent the tank when they attached new one. I've driven 3000 miles with it and no prob until they replaced the line with a new one. They'll repair it but I just want to make sure it will be okay to drive to the shop in the meantime.

By the way, I've driven it the past two days 75 miles or so just fine before I knew this was a problem.

Thanks!

 

[TNMGB]

Re: Gas pressure in tank?? Didn't happen before..

Okay, so now here's a thought....now that I have a "closed" system as opposed to the "open" system before the broken hoses were replaced, is the original gas cap supposed to be vented or not? It seems that the hoses are correctly attached and I'm just wondering...

I'd be happier if it were the electrical system that I had dealings with...I just don't want to get blown to kingdom come or have so much built up pressure that I have other issues.

sigh...

 

[TNMGB]

Re: Gas pressure in tank?? Didn't happen before..

AHHHH!!! Ha HA! I did forget to mention that I have no, zip, none of the smog stuff on the car. If that's the case, then I do need a vented cap??????

I saw in another forum about drilling a couple of small holes in the cap to allow the pressure to release.

Please advise...I cannot be trusted on my own...

sigh...

 

[TNMGB]

Re: Gas pressure in tank?? Didn't happen before..

Does it matter that I don't have the smog stuff hooked up?

 

[Steve_S]

Re: Gas pressure in tank?? Didn't happen before..

Not sure since I have no experience with smog equipment, but try running the car without the gas cap and see if it acts differently. You may want to run under half a tank to prevent gas splashing out. If the problem goes away, it's a venting issue.

 

[TNMGB]

Re: Gas pressure in tank?? Didn't happen before..

BOOM!!!!! Just kdding...

I figured out that when the smog system was taken off they blocked up a piece up in the engine. If they hadn't, I wouldn't have had the pressure build up and it still would have vented through the front of the car. So I took the gas cap to my MG guru and he did drill a small hole in the center of the cap on the inside so all should be well. If not, I'll drill another hole in it.

He said this was a common prob in earlier Jaguars and the only real concern was that if the tank was full and you pulled off the cap it would splash back on you.

WHEW!!! All is well again...for now.

 

[TNMGB]

Re: Gas pressure in tank?? Didn't happen before..

HA!! Again...HA!!

It works fine now. Drove it around a good bit and took off the gas cap...this time, it didn't blow me into the next county.

Ah, progress!

 

[Guinn]

Re: Gas pressure in tank?? Didn't happen before..

Well, now you have "cured" the symptom. Might be a good idea to find the cause.

Guinn

 

[TNMGB]

Re: Gas pressure in tank?? Didn't happen before..

Well, the cause was the blocked smog system that is no longer in use. When the emissions stuff was taken off, "they" literally blocked up a part with some "goo", even though they just disconnected the hose from it. When the shop fixed the split hose from the gas tank to the vapor cannister they unknowingly closed off the system which would have normally vented to the engine.

Another mechanic said these are the sort of things that rear their head when people begin removing parts they think are unnecessary. So, since I did pass emissions without the smog stuff on there, I don't really have reason to fix the problem. Now it just vents to he outside like older cars used to do.

 

[Bret]

Re: Gas pressure in tank?? Didn't happen before..

You answered your own question already. But for cars with all of the smog stuff intact & functioning properly - you should have a sealed gas cap. The closed design was meant to capture fuel vapor rather than letting them escape to the atmosphere.

I did notice that I had a slight whoosh when I opened my gas cap and even had a slight gas smell in my trunk. The smell turned out to be a few old cracked rubber vent lines to & from the separation tank that needed to be replaced.

Addressing the whooshing noise. I checked all the vapor hoses from the Zenith Stromberg carb to the charcoal canisters and on back to the fuel tank. I did find a slight clog in the line from one charcoal canister back to the separation tank and just used compressed air to clear it. I’ve heard that this is a common problem.

While the whooshing noise worried me a little, it wasn’t excessive and barely noticeable until the tank is almost empty. Thinking about it – it almost seems normal for a closed system as I’ve never had a fuel starvation problem.