Mechanics

"WE HAVE NO IDEA WHAT'S WRONG WITH

1998 Jeep Cherokee

Engine Performance problem
1998 Jeep Cherokee 6 cyl Four Wheel Drive Automatic 175000 miles

This problem started over a year and a half ago. I noticed my 98 Jeep Grand Cherokee Limited 4x4 4.0 I6 had been idling rough. Soon the check engine came on, and after a little more troubleshooting realized that it only starts this after the engine has gotten to operating temperature, and then runs so rich that it blows black smoke out the tailpipe, and it'll make your eyes water. I took it into the shop, and it showed a PO171 code, bank 1 lean. So it stayed in the shop for 1 month. Computer changed, and the upstream o2 sensor. Still did it, but then slowly tapered off. Well come to find out that I was getting a crack in my exhaust manifold(not uncommon on these jeeps). I drove it like that for a year, until last week when I finally got the time to change it. Used the aluminum gasket it came with, no rtv or any cleaning solvents. Just a wire brush, and a putty knife to clean the surface of the block. Got it installed torqued correctly, and running again. The problem returned. Same code, and conditions. I took it to the jeep dealership, and they kept it for two days, and said they have no idea what it is. The diagnostic scan shows that the engine thinks its running lean, so it is dumping fuel and air into the motor to keep it from dying(hence the rough idle, and rich smell, and smoke) The fuel shouldnt' be dumping over 2.5 or 3.0, and it's at about 8.0 or 8.5. The o2 sensor is showing.16mV and doesnt fluxuate, except when the throttle is pressed. No vaccuum leak shown on the scan. They said the upstream and downstream O2 sensors would be the first step, but they are not at all convinced that this will fix it. They said the next step would be the exhaust manifold to pull it off and make sure there is no problem, but I have a hard time believing this because when the manifold was cracked, the pipe broke all the way around, just before the lead donut gasket. So that would have let tons of air into the exhaust. Why would it do it when it is a much tighter system now? Help me please.
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CoastieGHI
August 9, 2008.




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