What would cause an '87 to run slightly rich?

[movin]

My '87 seems to run a little rich. Idles fine, runs smooth with no misses, just seems to get slightly poorer mileage than before. Replaced distributor cap, rotor and plugs, air filter, and set valves. It has 125,000mi. I also ran 2 bottles of Techron thru it, no change. What's up? Leaking vacuum line? Worn injectors? Worn rings?

[Rufblackbird]

O2 sensor?

[movin]

Replaced the 02 sensor about 40,000mi ago. About time for a new one?

[Jim Smolka]

An aftermarket chip that is not worth a hoot. Could also be a Fuel Pressure Regulator going bad. Had FPR on a bronco leak fuel via the vacuum line into the intake manifold. Check Fuel pressure and rate of Fuel pressure leak down after you shut off the car.

[carnut169]

Make sure the O2 sensor is plugged in, inside the engine compartment. On the left side there is a plug.... often prev owners will have unplugged it as it can make the car run better. My old car (87) *sigh* had it unplugged and it was fouling plugs. Did not make a difference when I plugged it back in- on my 1st 911, an 86, it did run better unplugged.

[stlrj]

The air flow meter barn door is spring loaded and with time it will fatigue causing a rich mixture. I would say that most 3.2s could benefit from a bit more spring tension to make them run like new with better throttle response and better gas mileage.

Cheers,

Joe