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Bogging/Tank Pressure


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#1 Mycroft

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Posted 30 May 2018 - 07:25 PM

As I get to know my new to me 70, I’ve been disappointed with a bog when I punch the gas for a little fun acceleration. Today, I went to fill up the tank and when I turned the cap I heard the longest, loudest swoooooosh of air pressure release. On the way home, the bog had completely vanished. Could it be that simple? I’ve seen clogged vents do similar to a lawn tractor.

#2 Paul Bell

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Posted 30 May 2018 - 08:19 PM

Did pressure release from the tank or was there a sucking vacuum?


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1999 Trans Am. Plenty bolt-ons.
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#3 MC1of80

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Posted 30 May 2018 - 09:19 PM

The 70 has a vented gas cap. Check and see if you indeed have a vented cap. Somewhere along the line someone probably, most likely changed it. If it is vented, I would get a new vented cap. Good luck!

#4 Mycroft

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Posted 31 May 2018 - 03:42 AM

Oddly, I’m not sure if it was releasing pressure or sucking air.
The cap has a small steel ball, looking from the inside. I’m assuming it’s vented. I’ll check closer.
Thank you.

#5 Paul Bell

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Posted 31 May 2018 - 07:12 AM

As the engine runs, fuel is drawn from the tank and air needs to get in to replace the fuel. If the tank has no vent (for whatever reason) and air can't get in, the fuel pump starts to cause a vacuum in the tank. The more vacuum there is, the harder it is for the pump to draw fuel. This would explain why the engine was bogging (fuel starved) then "fixed itself" after you cracked the cap on the tank.


1971 Monte Carlo SS clone, 496 CID, 600 HP 600 ft lbs, 9 inch rear.
1999 Trans Am. Plenty bolt-ons.
2003 Isuzu Ascender Limited DD


#6 Mycroft

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Posted 05 July 2018 - 06:12 PM

Hello all.
It’s been a while...
The very next day, the bog was back. I tried a new vented gas cap and still had pressure build up. I was able to determine that it is pressure and not vacuum. In the interim, I had posted elsewhere about what turned out to be an A/C canister. I have no A/C.
The can was connected to the intake manifold. I can only figure that it was just a vacuum leak at that point.
I took out the can and plugged the hose. She’s been running real good ever since.
I still get pressure in the tank and can’t figure why she ran good that one evening.




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