# Vaccum Leak



## Raland271 (Feb 10, 2008)

Can anyone explain this? When I am driving and I let off the gas it seems if the truck will start missing? It only happens when I let my foot off the gas, what could this be.


Thanks


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## boogyman (Jan 24, 2005)

It sounds like the timing is off. What is the year and model of your truck?


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## tacosteelerman (Jan 20, 2008)

Raland there is an emission part named the AB valve (Anti-backfire valve)..."The opening and closing of the exhaust valve creates vacuum pulses which draw in fresh air through the filter and valve assembaly. The reed valve only admits air to be drawn into the manifold,and prevents exhaust back-pressure from forcing the gases back out.In some systems,this fresh air entering the exhaust manifold can cause back-firing during deceleration due to an over rich fuel mixture.The AB valve,which is actuated by intake manifold vacuum,corrects this by routing additional air from the air cleaner into the intake manifold during deceleration..
AB valve test ,remove air cleaner and disconnect the rubber hose from the ABvalve.Start engine and allow it to reach normal operating temperature.Whenthe engine is thoroughly warmed up,have an assistant run the engine at about 3000 RPM while you hold your finger near the AB valve outlet fitting.Have your assistant abruptly let off the accelerator so that the engine quickly returns to idle.As this happens,you should feel a pull or suction at the AB valve fitting.If so the valve is functioning normally.If not,It should be replaced with a new one." This info is word for word out of a Haynes manual It's worth a try GOOD LUCK


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## boogyman (Jan 24, 2005)

Learn something new everyday. Thanks Taco.


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## mmiller2002 (Sep 29, 2006)

tacosteelerman said:


> Raland there is an emission part named the AB valve (Anti-backfire valve)..."The opening and closing of the exhaust valve creates vacuum pulses which draw in fresh air through the filter and valve assembaly. The reed valve only admits air to be drawn into the manifold,and prevents exhaust back-pressure from forcing the gases back out.In some systems,this fresh air entering the exhaust manifold can cause back-firing during deceleration due to an over rich fuel mixture.The AB valve,which is actuated by intake manifold vacuum,corrects this by routing additional air from the air cleaner into the intake manifold during deceleration..
> AB valve test ,remove air cleaner and disconnect the rubber hose from the ABvalve.Start engine and allow it to reach normal operating temperature.Whenthe engine is thoroughly warmed up,have an assistant run the engine at about 3000 RPM while you hold your finger near the AB valve outlet fitting.Have your assistant abruptly let off the accelerator so that the engine quickly returns to idle.As this happens,you should feel a pull or suction at the AB valve fitting.If so the valve is functioning normally.If not,It should be replaced with a new one." This info is word for word out of a Haynes manual It's worth a try GOOD LUCK


What years, models, etc. are you referring. Where is the ABvalve located? What does it look like. I may have a similar problem.

Thanks


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## tacosteelerman (Jan 20, 2008)

The info that I have is from a 1980-1988 Nissan/Datsun book LOL sorry I still live in the caveman era.But the AB valve on my model is located on the front of the intake manifold directally in front ot the throdle body,but new ones need something to to equalize the pressure also.If nothing else maybe this can give you a place to start some research GRAB A $15.00 MANUAL IT WILL EASILY PAY FOR ITS SELF SEVERAL TIMES GOOD LUCK


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## Raland271 (Feb 10, 2008)

Thanks for all the info, the anit back fire valve sounds just like what my problem is.. I have a 2000 frontier crewcab with 75k miles on it. How much does this valve cost and is it easy to replace.

Thanks


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## tacosteelerman (Jan 20, 2008)

The AB valve on my 88Pathy is 1 bolt and 2 large vacuum lines very easy to replace but 12 years later on your 2000 frontier I could not tell you...But another very likley place to suspect is the exhaust system If the cat converter has been messed with lack of back pressure will also cause back firing during deceleration I can not stress enough to get a model specific auto repair manual. There are downloads of factory service manuals and other manuals if you look enough you can find free manual downloads including looking on this site..GOOD LUCK


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