back fire through the intake question

90stang5speed · Jun 29, 2011

I have had vacuum leaks & a low fuel pressure problem at separate times. both have caused crappy idle and back firing through the intake if I tried to rev the engine. My question is, why will a lean fuel condition cause a back fire through the intake? I understand that a rich condition will let unburned fuel ignite in the exhaust & cause an exhaust back fire. how does the intake back fire happen if your ignition & cam timing are correct? just a question out of curiosity.

LarsD · Jun 29, 2011

I have had this happen to me before, but it was the timing being out of whack that was the cause.

90stang5speed · Jun 29, 2011

i understand that timing being "out of wack" would cause an intake back fire. spark plug wires in the wrong order could also cause it but, if your timing and wires are correct, how does a lean AFR cause a back fire through the intake? I can only think of one explanation. Extremely Lean fuel mixtures burn slower because the fuel molecules are farther apart & if the the fuel hasnt all burned off by the time the intake valve opens again, the incoming charge will be ignited and blow back out the intake. This seems unlikely so, does anybody have a better explaination?

jrichker · Jun 29, 2011

90stang5speed said:
i understand that timing being "out of wack" would cause an intake back fire. spark plug wires in the wrong order could also cause it but, if your timing and wires are correct, how does a lean AFR cause a back fire through the intake? I can only think of one explanation. Extremely Lean fuel mixtures burn slower because the fuel molecules are farther apart & if the the fuel hasnt all burned off by the time the intake valve opens again, the incoming charge will be ignited and blow back out the intake. This seems unlikely so, does anybody have a better explaination?

You haves stated the correct answer.

90stang5speed · Jun 30, 2011

Wow! That was just a wild guess. I didnt think the combustion of a lean mix would burn all the way through the power stroke, continue to burn through the exhaust stroke, & still be burning when the intake stroke starts. Well thanks for confirming what was just a guess on my part.

aar0s · Jun 30, 2011

90stang5speed said:
i understand that timing being "out of wack" would cause an intake back fire. spark plug wires in the wrong order could also cause it but, if your timing and wires are correct, how does a lean AFR cause a back fire through the intake? I can only think of one explanation. Extremely Lean fuel mixtures burn slower because the fuel molecules are farther apart & if the the fuel hasnt all burned off by the time the intake valve opens again, the incoming charge will be ignited and blow back out the intake. This seems unlikely so, does anybody have a better explaination?

would the octane of the fuel have an effect on that?

90stang5speed · Jun 30, 2011

my guess is no because I only run 93 octane. from what i understand, octane is a fuels resitance to detonate due to heat. I would think 87 octane would ignite quicker even in a lean mix and there for have a little more time to burn up before the intake opened again. thats just a guess though.

Rick 91GT · Jun 30, 2011

Do you have a open element breather on the valve cover or is it closed cap and hose to the tb like stock.

1bad67sbf · Jun 30, 2011

i have a vented one, is this bad?

90stang5speed · Jul 1, 2011

My pcv set up when I had the vacuum leak was an explorer intake with a gutted pcv valve with a tube that ran to the passenger fender well. the tube on the filler neck had a tube on it that hung down beside the block connected to nothing. I know that is some unsafe red neck riggin so, my new set up has fittings i each vc with a tube that connects the two VC's and then from the passenger VC down to my homemade catch can that will have a clamp on filter real soon. I now have a knock off version of the vic jr intake modded for injectors that doesnt have a pcv port. that back fire issue was caused by low fuel pressure due to a bad ground wire to the pump & bat. That has been corrected too.

Rick 91GT · Jul 1, 2011

1bad67sbf said:
i have a vented one, is this bad?

I have found it is hit or miss with a vented breather on the valve cover, Ive had car have the "intake pop" until I put it back to a closed/stock like system....others run just fine.

It is letting in and out unmetered air which can cause issues.

back fire through the intake question

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