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Lq4 running rough

This is a discussion on Lq4 running rough within the General Help forums, part of the LSx Technical Help Section category; I just finished installing a 2004 lq4 out of a silverado 2500hd into my wrangler. My issue is that it ...

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    Lq4 running rough

    I just finished installing a 2004 lq4 out of a silverado 2500hd into my wrangler. My issue is that it seems to run rough and not quite have the power most of the time. But every once in awhile for a second itll run perfect and extremely smooth... Just wondering what some of the experts over here think of it


    Thanks in advance

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    welcome to the site!
    can you tell us more about it? is it a stock lq4? direct swap pcm, harness, and tune? are there any codes being set that you know of?
    Last edited by My00Z28; 10-22-2011 at 11:09 PM.

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    None Shall Pass Knight's Avatar
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    Could be a number of issues - MAF, vacuum leak, fuel pressure, ect.. Check the easy stuff first to rule that out, then move on.

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    Quote Originally Posted by My00Z28 View Post
    welcome to the site!
    can you tell us more about it? is it a stock lq4? direct swap pcm, harness, and tune? are there any codes being set that you know of?
    thanks, yea its a stock lq4, the pcm and harness are off a 2003 of the same truck with a stock tune. the pcm and harness were modified by novak to work with the jeep. i checked the codes last night and there was nothin coming up

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    Quote Originally Posted by Knight View Post
    Could be a number of issues - MAF, vacuum leak, fuel pressure, ect.. Check the easy stuff first to rule that out, then move on.
    i just fixed a vacuum leak yesterday. one thing i have been noticing when i hit the throttle it creates a really loud sucking sound from the intake. ive heard that only once before and it was on a friend who we swapped an l98 into his s10, what caused it was an improper tune. as for fuel the lq4 has a return less system and the jeep has a return line as of right now its ziptied off to the side, could that potentially be an issue? ive been trying to find what the lq4 requires for fuel pressure but im failing to find it... i know the jeeps system puts out approx 55 psi

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    I just retested my fuel pressure, I was, wrong my system sits at 65psi

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    im not sure off the top of my head but 65psi should be more than enough to run that motor! whats the pressure drop to when you start the engine?

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    Quote Originally Posted by My00Z28:2712541
    im not sure off the top of my head but 65psi should be more than enough to run that motor! whats the pressure drop to when you start the engine?
    The pressure doesnt hardly drop at all, it comes down maybe 1 or 2 psi

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    Quote Originally Posted by Knight View Post
    Could be a number of issues - MAF, vacuum leak, fuel pressure, ect.. Check the easy stuff first to rule that out, then move on.
    Quote Originally Posted by Ripper3494 View Post
    The pressure doesnt hardly drop at all, it comes down maybe 1 or 2 psi
    like knight stated above! make sure your MAF sensor is clean, you already found one vacuum leak make sure there are no others, fuel pressure seems to be fine.

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    Checked the maf and it was clear, couldn't find any more vacuum leaks. Is it possible that my air filter is limiting how much air my engine can draw in? Its the middle size cone filter from autozone... Just a thought

    Honestly it feels like a cylinder isnt firing but that would throw an error code faster than hell

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    It's possible you may be restricting too much air. You could remove the filter temporarily to see if things don't improve. I would expect a code from most things I'm thinking of, but that doesn't always mean you'll get one. Also, just because the MAF looks clear, doesn't mean it's reading properly. Pick up a can of CRC cleaner and clean it good. Follow the instructions on the can. If that doesn't improve anything, you could spray some starting fluid around to check for vacuum leaks.

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    Quote Originally Posted by Knight:2712676
    It's possible you may be restricting too much air. You could remove the filter temporarily to see if things don't improve. I would expect a code from most things I'm thinking of, but that doesn't always mean you'll get one. Also, just because the MAF looks clear, doesn't mean it's reading properly. Pick up a can of CRC cleaner and clean it good. Follow the instructions on the can. If that doesn't improve anything, you could spray some starting fluid around to check for vacuum leaks.
    Ill grab some cleaner tomorrow, and what do you mean spray starting fluid to check for vacuum leaks. Sorry if its a dumb question but I've never heard of doing that

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    Spray a little starting fluid around the vacuum hoses and intake. If the idle raises, you have a vacuum leak. I'd do it with the filter installed though, so as to not get a false idle rise. It's an old trick.

    And there's no dumb questions here. We're all learning, some of us are just a little ahead of others

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    Lol thanks, I just did a quick run with no air filter and saw no significant difference, will clean the maf and look for vacuum leaks tomorrow. the engine puts out a ton of power for the jeep but I have a silverado with the same engine and I know it should be running better...

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    Did some reading about maf sensors today. one article said to unplug it and run it like that. It says the pcm will create a value for the sensor and if it runs better without it thats the problem. If it runs worse then the issue is somewhere else. I figure ill try that first after school and if thats not it ill look for vacuum leaks. If its not that then I'm stumped

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    I feel dumb, it was a loose plug wire

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    Lol been there done that

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    At least it was a simple fix.

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