96 Tacoma rich fuel trim

HELP! My 96 Tacoma has recently started running rich. Symptoms are poor fuel mileage, loss of power and very black/sooty exhaust. Engine seems to run fine except the top speed is down about 15 mph. Best quick dyno I know of. It has 150,000 miles on it. I have checked the TPS. Thought it might be telling the ECU that throttle was at 100%........OK. Checked MAF meter per factory service manual........OK. Checked engine coolant temp sensor............also OK. Replaced #1 oxy sensor with a new Toyota part.........didn't change anything. I might mention at this point that it hasn't provided any codes and that three shops have already looked at it, including the local Toyota dealership. I personally checked the above parts. I am a transport reefer tech by trade, so I am familiar with electronics as used in these applications. The first shop said the exaust manifold being cracked was allowing air into exhaust and fooling (reading a lean condition) the oxy sensor. he confirmed it was indeed rich. I replaced the stock manifold with a CARB approved header from LC Engineering. The manifold had been cracked for years and it ran OK before. Nope.....fixing exhaust leak didn't cure problem. And........yes, the air filter is a new paper Fram part. Timing varies when I throw a light on it, so it doesn't appear to be in the "limp home" mode. I could simply start throwing parts at it, like the new oxy sensor I bought, but that seems a little too "shade tree" at this stage. Haven't checked injectors yet since it doesn't miss and starts/runs fine. Also no vacuum leaks after MAF sensor. Any techs out there with Toyota experience have any input?

Reply to
Ratman
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What is your fuel pressure?

Don

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Reply to
Don

I have seen where a shorted (melted to exhaust) oxygen sensor wire caused the same problem you describe.

Reply to
M.Paul

Reply to
Ratman

Reply to
Ratman

Yes, if its only marginally high. Your Toyota will require a special adapter to get fuel pressure tested.

Don

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Don

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Ratman

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