Severe speed loss uphill

Tiny
PEGGYCREWE
  • MEMBER
  • 1996 MERCURY MARQUIS
  • 135,000 MILES
Problem occurs mostly while going uphill at a high speed. Going up at about 70mph and it will drop to 40mph. Also rough starting and has some hesitation and stuttering from a stop. It was throwing a code for fuel being lean so I replaced the 2 upstream o2 sensors. Replaced plugs and wires, checked fuel pump, ran some seafoam through and checked fuel injectors. Someone said it could be TPS or clogged catalytic converter.
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Wednesday, May 1st, 2013 AT 11:05 AM

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Tiny
CARADIODOC
  • EXPERT
You need to get a mechanic involved before you needlessly waste more money. The oxygen sensors reported the lean condition which proves they're working. They just report what they see in the exhaust. You replaced the messengers because you didn't like the message.

The rough running and hard starting are most likely related to the lean condition. A misfire will send unburned gas and air into the exhaust system where only the unburned oxygen will be detected as that lean condition. In response the Engine Computer will command more fuel but there will still be that unburned oxygen. The clue is you will smell the unburned fuel at the tail pipe.

A vacuum leak will result in the same lean condition and low power. Fuel pressure that's just a little low will result in too little fuel being sprayed from the injectors, and a lean condition.

The throttle position sensor has about the least say in how much fuel goes into the engine, and it won't cause a misfire, rough running, or low power. It can cause a stumble or hesitation on acceleration.

A plugged catalytic converter will cause real low power but the engine will run uncommonly smoothly at idle and you'll hear a hiss from the tail pipe instead of the normal "thump thump" sound.

A plugged injector will cause a misfire and low power but that will only affect one side of the engine. You will get a lean code for bank 1 or bank 2, not both.
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Wednesday, May 1st, 2013 AT 2:00 PM

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