Loud backfire after releasing accelerator...
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From: Virginia Beach, VA
Loud backfire after releasing accelerator...
Sorry if this is a redundant post...I did a search and found a similar topic but can't seem to find it now...anyway, just replaced the MAP sensor...that wasn't it...I seem to remember a post I read during my search about air flow being diverted from the Cat to the exhaust manifold to prevent backfiring? While changing my oil today I didn't notice any air hose to the cat...is there supposed to be one? Thanks in advance for any thoughts/observations/assistance...peace
Oh yeah, I forgot to say I've got an '84 F150 w/4.9L I-6 1-bbl feedback carb...all vacuum lines seem to be present and in good shape...just the above mentioned issue...thanx
Oh yeah, I forgot to say I've got an '84 F150 w/4.9L I-6 1-bbl feedback carb...all vacuum lines seem to be present and in good shape...just the above mentioned issue...thanx
Last edited by jschwen73; Feb 5, 2008 at 05:57 PM.
MSEE
Joined: Apr 2004
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From: Austin, TX
First off, this is the first time I've heard of a carbureted engine using a MAP sensor, so if you could further expand on that that would help.
Anyway, Ford used an air injection system to dump fresh air into the exhaust to reduce emissions. However, if there is a sudden jump in intake manifold pressure with this setup, you'll backfire very badly. This is why there is an "air bypass valve" downstream of the smog pump that cuts off the air supply in this case. Typically you will have a check valve downstream of the exhaust manifold - it could be in the carb spacer plate with an exhaust pipe, or elsewhere. The check valve hooks to the smog pump via hose, and the air bypass valve splices into that hose between the two. The valve then connects to manifold vacuum. If you are missing this valve and your smog pump is otherwise hooked up you will experience this problem.
Anyway, Ford used an air injection system to dump fresh air into the exhaust to reduce emissions. However, if there is a sudden jump in intake manifold pressure with this setup, you'll backfire very badly. This is why there is an "air bypass valve" downstream of the smog pump that cuts off the air supply in this case. Typically you will have a check valve downstream of the exhaust manifold - it could be in the carb spacer plate with an exhaust pipe, or elsewhere. The check valve hooks to the smog pump via hose, and the air bypass valve splices into that hose between the two. The valve then connects to manifold vacuum. If you are missing this valve and your smog pump is otherwise hooked up you will experience this problem.
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From: Virginia Beach, VA
Thanks for the reply...yeah, I'm pretty sure it was a Manifold Absolute Pressure sensor that my mechanic replaced...he said if that did not fix the problem it was probably a carburator issue...I don't think he knows about the air bypass valve thing you mentioned though...that's the info i was looking for---I'll check into that next...thanks.
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