I wrote another post on the topic of rough idle before and everyone gave me good tips on things to check. A little background again 2003 KJ 3.7 with only 39,000 miles. It was my moms car and hardly ever driven and when it was it probably never went over 40MPH and 3-5 miles at a time. Runs great but seems to idle a little rough.

So far I have changed the spark plugs (proper gap) , Idle air control, PCV valve, took the throttle body off and completely cleaned, found 2 cracked lines going from the valve covers near back of engine up to the air intake housing and replaced. Now it actually seems to idle a little rougher. The idle isn't consistently rough. Sometimes it seems like it idles perfect and then a few stoplights later I think nope, didn't fix it. I have a couple questions. First I replaced the spark plugs with champion double platinum spark plugs because I couldn't remember which ones you guys recommended. Have any of you had problems with them? Were they a good choice? Second, the vacuum lines coming from each side of the back of the engine and going to the air box (the box that covers the throttle body) had some sort of plastic check valve or venturi about half way up the hose. when I went to get new hoses the parts guy pulled them off and I didn't see which way the arrows were pointing. He said both were pointing toward the air box( throttle body). Is he right? And third, what would be your next move if the idle ended up being a little worse after doing all of that?

Sounds like you've covered a lot of bases, so kudos to you for doing all that. What you've done makes a lot of sense. Here's what else I can think of, if it were mine.

If only ever driven at slow speeds on short trips, and with it never getting a chance to warm up, your engine might have a lot of carbon build up (you would have seen some of this in the Throttle body and the IAC, probably). I'd try some Lucas fuel treatment in several tanks of gas (or try Shell V Power gas, which has extra cleaners) to try to clean out your fuel injectors and upper cylinders. Be sure to blow out the cobwebs by accelerating quickly so your RPMs get over 4.5 or so, to dislodge those carbon deposits. Then go back to regular 87 gas after a few tanks.

In terms of spark plugs, the plain old copper cores work the best in the 3.7. I would say try those and see if it improves the idle over the fancy platinums you have in there now. (Will only set you back $12 as they are $2 each)

In terms of the vacuum lines, I think the mechanic is probably right, as the air flow would be towards the air box / throttle body where there would be less pressure due to air being sucked into the engine.

I'm assuming there is no check engine light illuminated, but it might be worth seeing if there are any pending codes in the computer for misfires. A code reader is a good tool to pick up and have on hand, or you could have an auto parts store check it for free. If a particular cylinder is misfiring, move the ignition coil for that cylinder to a different one to see if the code follows. If yes, you have a bad coil.

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Thanks for the reply. I did forget to say that I have ran a few bottles of techron fuel injector cleaner though. And I've put it through a few 90MPH long highway runs. Hey it needed to be blown out! The throttle body and IAC were loaded with carbon. I cleaned the IAC at first but then thought about how much carbon it had on it and then replaced it. I was thinking about doing the sea foam thing so maybe my next moves are to get the codes read when I go to buy sea foam and grab some regular spark plugs since I will need to change them after the treatment anyway. seems like the best next steps.

P.S. the plugs all looked like they were burning good when I replaced them. They were the originals and were worn but burning good.

right on with what lfhoward said about lucas! run 92 oct for a tank and run her hard. also, i didnt see that the air filter was replaced. make sure as well the maf sensor is clean.

brake clean does wonders on every engine part.

techron does no help to our 3.7. its gas hungry. fuel additives/cleaners are better then "mpg increasers"...

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2012 Jeep Liberty KK 109k miles and stong
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I did replace the air filter. Forgot that one. Didn't check the MAFS yet. That sounds like a good idea too. Haven't had a chance to work on it recently, been too busy. Next weekend I should be able to try to figure it out again. I'll let you guys know how it went.