so what is the right plug for a 91 4.0
the number I have is RC12LYC to replace the platinums that were in it from the PO

 

That number sounds right... I would doublecheck but I don't have access to a catalog right now.

 

Does this mean my cheap champions won't work well with coil packs????

 

ive had autolite plugs for a couple months...is it possible for them to just fail all the sudden and cause a choppy iddle...?it studders while idle and reving up.

 

They are fine. MOPAR switched to platinums to meet the 100K-mile emissions regulations for tune-ups.

Pull a couple of plugs and look at them. It's likely you have a different problem.

 

100K mile emissions regulations???

I just want to know which is the best to put it in because my mileage is blowing chunks lately.

 

1998... the EPA required vehicles to go 100K miles before needing any emissions-related maintenance. Some manufacturers (like MOPAR) stepped up to the plate a little early to test if their engines could last that long without emissions-system maintenance.

Tune-ups (spark plug changes) were considered by the EPA to be emissions-related maintenance.

 

Oh I see but that doesn't mean that the platinums are the best for the vehicle right?

 

Copper plugs need to be changed every 30K miles... that's actually a good thing.

Plugs are cheaper than coil packs. As the plugs wear, there is more strain on the coil pack to fire through the higher resistance.

 

Exceeeeelent. I'm hoping it will help some of my hardstart and mileage issues.

 

Any one ever use an SP 445 motorcraft plug?

 

i have some autolite Platinums im mine...11K and good so far. the +4's Just dont do good on alot of vehicles from what i hear.

 

i have a 97 cherokee sport. need to change the plugs, should i go with champion? i know this is a stupid questions but can anyone tell me exactly how to change a plug?
thanks

 

ONLY CHAMPIONS...if you want your 4.0 to run right with best mpgs and not burn a hole in your piston dont use those crossover junk autolites, look around till you find champions for it

 

I ran into a problem with the Bosch platinum after a couple months I had a dead cylinder it was the plugs I put in. for the year I have I just stick to Champions.