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Jeep Grand Cherokee Backfire

dansjeepdansjeep Member Posts: 3
edited March 2014 in Jeep
95 Grand Cherokee Laredo 2WD, 4.0, auto trans, no security

I start the beast every morning and for the first 15 minutes or so there is no power. RPMs will not go above 1500 and hills are deadly. If I stop and turn it off for a couple of minutes, then when I start it it's much better. Note that it must be driven for 15 minutes; if it just idles in the driveway it never gets better.

PCM does not throw any codes (using the check engine light and counting flashes).

I have checked crank position, cam position, and MAP sensors - all OK. I have changed O2 and coolant temp sensors, and even the brake light switch (suggested by mech).
This weekend I changed exhaust, both muffler and cat. Muffler was cracked right in the middle, cat is COMPLETLY hollow - nothing inside. My guess is all the backfires blew it out.
I checked the timing today. At first start, it is all over the place. I had to make sure my timing light was working.
When it 'gets better', timing is pretty stable. I checked vacuum and it's at 17 Hg, pretty steady.
I checked compression and all 6 are OK.

What gets me is that after the first start, stop, and start, it runs pretty good the rest of the day. A mechanic wanted to do a timing chain, but I said no. Why would it have slipped, but gets better later in the day.

Any help will be much appreciated. I have to do something with this soon, I'm late for smog test because it already failed.



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    dansjeepdansjeep Member Posts: 3
    Took the beast to see how it does on smog test with new exhaust.
    Here's the comparison at 15mph (OLD / NEW):
    rpm 1426 / 1426
    %CO2 13.9 / 14.6
    %O2 1.2 / 0.1
    HC (PPM) 125 / 28
    CO(%) 0.60 / 0.08
    NO(PPM) 2306 / 766

    And the comparison at 25mph (OLD / NEW):
    rpm 1402 / 1384
    %CO2 14.0 / 14.6
    %O2 1.1 / 0.1
    HC (PPM) 123 / 25
    CO(%) 0.59 / 0.12
    NO(PPM) 2156 / 730

    The NO at 15mph is what fails.

    The tech also pressurized the fuel system and detected fuel vapors, saying maybe the evap container is leaking (couldn't put the probe on the canister because it is totally not accessible).

    Two questions:
    1. Any idea on what would lower the NO so it passes?
    2. Does any of this new info help to understand the underlying problem from my original post.

    This one is a challenge. I'm hoping this new info lights up somebody's memory.

    Thanks for the help.

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    tuggajbtuggajb Member Posts: 646
    if the engine has a egr valve that could be sticking NO is high if exaust is nit hot enough to burn off nit ozides egr helps at idel and slow speeds
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    5213252132 Member Posts: 62
    It sounds weird, but the 2.5L and 4.0L engines are two of only a handful of gasolene engines that don't have EGR valves.
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