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So I got a full exhaust set up was fine until I put the down pipe on and now my car stalls. Its automatic and stalls only at a stop.

 
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What size piping? Also, any other mods done to the engine?

 
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4.5" I have Ram intake but that was on before also got my head chaved because their was a head gasket leak

 
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severe lack of back pressure is causing your issues. The only thing that should require that large exhaust piping on a D-series is a massive F/I setup. For an essentially stock engine, 2.25" piping would probably be the largest you'd want to go

 
Rep Power: 210

Where is th eoxygen sensor located?