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?