I have a 1987 GTA that draws a vacuum when shut off.

Asked by Apr 06, 2009 at 12:01 PM

Question type: Maintenance & Repair

My GTA has a 5.7l engine. When the engine is off the car continues to draw a vacuum in the fuel tank. It's so bad the tank begins to collapse in on itself and then at some point it gets air and pops back out. The canister,mechanical purge valves,hoses,cap and the tank ventilation valve have all been replaced and has made no difference.Whe I bought the car a year ago it did not have this problem. It did have a raw fuel smell that I found to be a dry rotted seal at the pump.Fixed it and it was good for a while until this problem occured. I think once I completely closed the system again It caused something else to go. I'm looking for any little bit of information or suggestions on what this may be. I'm at my wits end. Thanks

11 Answers


allright well maybe you just need a gas cap that allows a certain flow of air. and if that dosent work unplug the vacuum hose from the vacuum canister under the hood, it's only an emissions control system, it won't stop the engine from working.

Will that take car of my vacuum problem in the tank? Will I then have a fuel smell or engine light come on if I detach the vacuum line?


it souldin't create any problem, but to know for sure, you'd have to try it!

Thanks I will.

I would like to switch from using conventional motor oil to full synthetic. My T/A 5.7L has 128000 miles all on conventional oil. Is this a good idea or no?


replace your gas cap the vent is pluged


I wouldn't. Synthetic is supposed to be used when you first buy a new car or get a new motor. Sure you get more out of the engine and all, but if you decide to use it now, it could damage a lot of parts. All of that sludge will be removed very quickly, which will go against moving parts and cause damage. There is a program where you should slowly introduce synthetic oil into your engine before using it alone. Go with that and start using it little by little before converting over to synthetic completely.


you will be ok if you switch to a synthetic blend at first. do NOT go straight to full synthetic. the basic rule of thumb is ten years or 80k miles on switching to full synthetic. anything above that and you could potentially start a leak. the different oils will cause swelling or lack of swelling in the seals. also, synthetic oil tends to run cleaner than conventional oil. this could clean out any dirt in the seals that are preventing leaks. and once you have a full blown a leak switching back will not stop the leak. by switching to a blend you can monitor the situation and any start of a small leak can be switched back to conventional oil.


you can switch but it is recommended you switch out the seals for new ones that will only be used with full synthetic


Conventional tend to swell seals and that swelling can go away if you switch to a synthetic. IMHO this is not a reason to avoid switching to synthetics. With synthetics the oil package needs fewer friction modifiers so a larger percentage of the additive package can be used for more beneficial purposes that fitting a square peg into a round hole (bad analogy but it fits). If it leaks after switching it means the seal was already worn and is at risk of failing. going to a synthetic blend won't change the fact that the seal has swollen and worn, so when the swelling goes down it will leak if it's going to leak. And if it's such a catastrophic leak that it's not manageable it would already be an issue. We're generally talking about hundredths or thousands of an inch here due to swelling. If you want more information then I recommend researching the issue yourself, and be careful what you read and make sure it's from a reputable source. There are more myths about oil than anything else.


when was the last time you changed the fuel filter ?

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