Unfortunately, there are too many variables present to give you a straight answer. Might be 100k miles, or it might be 10k--depends on what kind of driving you do, and how often you do it

 

The engineers at Chrysler probably didnt intend to put a steel plate on a aluminum intake manifold. It was the bean counters who figured out that putting on a steel plate would save Chrysler money, just like they did with the head gaskets on Neons; Used a single layer gasket instead of a MLS gasket to save money.

Its one of the contributing factors of why these trucks were priced several thousands less than Chevy or Ford trucks at the time.

 

Given that many, like me, have had blown plenums for a while without realizing it - how does one know if their cat is clogged as a result?

 

Pull the pre-cat O2 sensor out of the pipe, and see if the truck runs/drives better that way. (It will set a code.)

Or, get a gauge for just such a purpose, drill a small hole in the pipe in front of the cat, plug in gauge, start engine. If you get much more than about 2 pounds of pressure at idle, replace the cat.

 

so just so I understand, 2psi is about normal back pressure for our trucks? and if you have a recommended gauge, I'd be grateful

 

2psi is pretty much normal for anything with a cat.

I have an old cheap, no name brand gauge that is probably older than I am. Check and see what the local parts stores have. Pick one ya like.

 

Excellent thank you! And if anyone else is interested, this guy shows the hole drilling thing.

[youtube]IzyvL5tQLzU[/youtube]

 

OP amended to add cat failure tests, thanks guys

 

a while Back a mechanic who had repaired blown plenums all the time mentioned he would just replace the gasket and use the shorter bolts since the real problem was the fact that the longer bolts would bottom out and not sandwich the gasket with enough pressure causing it to fail.... He mentioned he never once had the issue come back after repairing in this manor... of course their are those who are religious about it being the plate.... funny thing is their are lots of places where aluminum and steel are bolted together and experience hot temps and this is never an issue... like the aluminum heads that are bolted to steel sleeves on my magnesium block vw without a gasket or the steel throttle body plate that is bolted to my aluminum crossfire manifold on my vette and makes up the whole top plate of the manifold.

 

so i got a spark knock and havent been able to fix it yet, so it could be the plenum but thats the only symptom i have ,
and when i look down into the throttle body how am i supposed to tell if its oil there or just other build up over time i cant exactly tell if its oil at the bottom or not

no loss of power that i know of, easly go up hill very little oil loss over 5000km oil change only symptom is spark knock?