I was just sitting here thinking about plenum leaks. I was in the process of posting on a mustang forum I used to frequent about my upcoming mods to my Ram. It got me thinking about what the leak actually does and why it causes mis-fires. I have come up with 2 things but not sure quite how it would happen or be a cause for problems. If this is old stuff which it probably is I'm sorry but just wanted a little decision on the topic.

The things I'm thing of are the fact that the leak is sucking in "unmetered" air. How is that possible though because the hole would be below the heads in the lifter valley. Its not like the air is "outside" air getting in.

The other thought is that with the leak sucks in oil. The oil is then able to mix with the air that is being drawn into the combustion chamber. That in turns makes the combustion less or not at all causing the mis-fire

Are either of them thoughts close or am I way off?

 

A clogged cat will cause a misfire on cylinders #6 and/or #8.

 

I think it's mainly the oil getting sucked into the chamber, which in turn destroys the plug(s) and builds major carbon and then an incorrect reading by the 02 sensor adjusts the A/F ratio to compensate and eventually ruins the cat. And when that gets clogged the first cylinders to misfire will be 6 and 8, because they are closest to the cat and will be taking most of the heat.

 

I had a #8 one reading years ago and never had a problem again. My pain in the *** one was #4

 

It can be a combination of the 2. I used to do these under warranty all the time when I was a Chrysler tech. So glad I'm not a tech at all anymore... lol. But yeah, the broken lower plenum gasket can cause a misfire or generally make it run like crap because of the leaking air. Like a leaking vaccum line.
The leaking oil can fowl the plugs if it goes unnoticed for too long. Had one kid with a R/T dakota come in with one. When he called about it the service rep said to "fill it up with oil before you drive it in"... he did... all the way till it ran out of the valve covers... lol I don't recomend that at all...

The oil can throw off the 02 sensors, gum them up, and clog the cat. Which would cause the misfire as well.

 

"Unmetered air" is air that is added after the trottle body (who meters air). As the plenum is below the trottle body, it won't be able to "meter" that air and can't compensate with extra fuel. So that means that there will be more air than fuel and the engine will run on a lean mixture. Maybe it's not direct "outside air" but it's still air, so that's no good.



Absolutelly correct