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Old 04-10-2012, 02:55 PM
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power loose and coils and catalytic code

Hello guys, I have a Jeep Grand Cherokee 2005 3.7 v6 4wd 80,000 miles

My jeep is giving me headache, first it turn on the check engine a couple of weeks ago and the code was to change the oxygen sensors so I change them, then it turn on the check engine again and it never go off becouse the computer is reporting a misfire. I change the spark plug and now it tells me that I have to change the catalytic and coils and still having misfires problems. The check engine only turn off when the mechanic erases the codes whit the code reader but turn on again later.

This is the problem: When I turn on the jeep in the morning or when I turn it off and start it an hour later, it starts to lose power and it get to 500rpm and tremble a little bit until I push the accelerator a few times and it turns back to normal and everything is fine again until I turn it off again.

My mechanic said that we should change the coils first before going for the catalytic. I don't know if all this came from the oxygen sensors change. Any opinions?

Regards,

Jorge Jaquez
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Old 04-10-2012, 02:58 PM
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Re: power loose and coils and catalytic code

What brand O2 sensors did you use.....use NTK/OEM ONLY
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Old 04-10-2012, 03:04 PM
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Re: power loose and coils and catalytic code

X2 ^^^ and also, if this has been going on for any length of time (with or without you knowing about it) your catalytic converter(s) may be toast. Also, shoot us some codes, so we can help a little more. Your info. above, I am sure, is not giving us the complete picture.
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Old 04-11-2012, 12:29 AM
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Re: power loose and coils and catalytic code

Yes fix the misfire before you fix the catalytic converter, while the P0420 code your probably getting for catalyst effiency MAY be cause by crappy O2 sensors (ALWAYS use OEM O2 sensors). With a misfire at 80k miles and you driving it regularly with a misfire it is VERY likely actually failing. A misfire will dump fuel into the catlayst and melt it down.

DO NOT REPLACE THE COILS! DO NOT REPLACE INJECTORS, maybe replace spark plugs if you need it. The first to WILL NOT fix your problem. PERIOD. The 3.7l is known for carbon buildup on the valves causing problems, I would suggest running something like mopar combustion chamber cleaner through the intake (following the destructions on the can) or something like seafoam (again following the destructions on the can). Your likely dealing with a cylinder integrity issue, specifically a valve sealing concern. If you want to spend a bunch of money on the exterior stuff that is not going to fix it just know you MIGHT be looking at having the cylinder heads pulled off, possibly replaced, possibly just getting new valves lapped in.

Slightly off topic but to prove a point, I had a customer in the other day that came from an independant shop where the O2 sensors were replaced (pacifica), It had an O2 sensor heater performance code, I did my tests and deturmined the heater to be getting power and ground and it to be dropping all the supplied voltage across the sensor so it had to be the O2 sensor. I got the new OEM O2 sensor and measured the resistance across the heater circuit compaired to the brand spankin new bosch unit I pulled out and the Bosch was at 3.XX ohms and the Mopar sensor was 5.XX ohms.. that is 40% less resistance (resistance being the creator of heat) in the brand new aftermarket sensor... So again, only use OEM O2 sensors.

Also, I am sure funds are and issue, I feel for you, but ANY technician that would suggest replacing coil packs to repair a misfire without swapping them to known good cylinders should NOT be trusted. Especially on a motor that has such easily accessable coils, in some cars its a matter of removing an intake manifold but not in the WK. When I dont diagnose something right, some customers seem to think that I am an idiot that uses velcro shoes because I couldnt figure out loop, swoop and pull if my life depended on it, but after watching my family deal with their independant shop and hearing others, it seems that they have their customers thinking that there is no way to actually diagnose anything and they are given a list of parts to start replacing... AND PEOPLE ARE OK WITH THIS... must be nice.
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Old 04-11-2012, 03:22 PM
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Re: power loose and coils and catalytic code

AMEN, and RIGHT ON, 2005JGC, on everything you've said ^^^. Well done.
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