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Jeep Grand Cherokee Backfire
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.