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Old 10-16-2012, 02:16 PM
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Severe problems accelerating 2003 Grand Cherokee 4.0L

It makes a rattling noise when I start it after it has been sitting for a few hours. It will run ok for the first minute or so and when I come to a stop it rumbles. After the first minute it has a really hard time accelerating. The max it will get up to is 40 mph (unless I am on a downhill) but sometimes just getting up to 20 mph is a struggle. It is getting terrible gas mileage. For 20 min the check engine light came on and I was able to pull the code for a speed sensor off of it, but I am afraid it is the Catalytic Converter. This started a few weeks ago and my friend replaced the throttle position sensor, fuel filter, air filter, spark plugs, cleaned the throttle body and checked the altenator (the day after he fixed it the battery was completely dead). It seemed to be running fine, 5 days later I noticed it was guzzling gas and the next day the problem was back and seemed to be worse. If I try to push it over 40 mph it gets jerky and seems to lose power. If I am going uphill then it hardly has any power at all. I paid $110 to get diagnostic from a jeep dealer and they couldn't tell me squat. They wouldn't even pull the code for me. I'm a blonde female and I feel like every place I go to is just going to rip me off. Does this sound like a clogged converter, speed sensor, or something else? I appreciate any help!
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Old 10-16-2012, 07:22 PM
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Re: Severe problems accelerating 2003 Grand Cherokee 4.0L

Welcome to the garage.
I know how you are feeling, it is hard to thrust garages nowadays.
After your friend replaced the parts, did you check for fault codes again? Do you remember which fault code you had before?
Eventhough there could be a problem with the cat, there could be several other things as well. For example a low fuel pressure due to a pump which is going out.
I don´t know the 4.0, but most probably there is a valve on the fuel rail, where you can check the fuel pressure. When you don´t have a pressure indicator, you could do a quick check as follows: with a cold engine (to prevent fire), switch the ignition in on and then press the schräder valve in on the fuel rail. With a good pump, fuel under high pressure will come out. Watch eyes and clothes and use a rag to catch the fuel. If the pressure is low, or the pump is bad, or a seal in the pump unit.
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Old 10-24-2012, 10:14 AM
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Re: Severe problems accelerating 2003 Grand Cherokee 4.0L

My thought was going to be fuel filter/pressure regulator but since you changed that already I'm starting to wonder about the pump. Try as Frank suggested by checking fuel rail pressure, there should be a port on there to hook a guage to. A store may let your rent one otherwise they aren't too expensive to buy.

I'm thinking fuel pump, if the converters were that bad I would expect the downstream o2 sensors to throw a code.
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