Help with fouled spark plugs and running rich

I'm hoping to get a few more ideas as I search through the posts...

89 Cherokee (stock engine) is running way to rich. Spark plugs are fouling up (black carbon deposits) within 100-300 miles. The O2 sensor seems to be working fine so I'm trying to work my way through any other culprits. I've checked the gap so I believe it's not the plugs themselves. It does run great up until the spark plugs foul, and at that point there's a noticable difference at higher RPM's. Seems to have started after starting it and driving in 20 degree weather. Any ideas would be appreciated! Thanks!

Had similar issues on a Ford F150 when the vacuum controlled fuel pressure regulator went bad. Supplier too much pressure to the fuel rail. Easy test would be to see if the diaphram holds a vacuum.

Make sure your CCV system is operating, has vacuum. Is there any blue smoke on startup? Are all the plugs fouling, or only a couple?

I cracked the vacuum line for my MAP. It would run rich and occasionally load up at idle so it barley ran. I periodically test it now, by closing off one end with my finger and lightly blowing (or sucking) on the other end. It was a hairline crack I could barley see.
Another time the injector harness where it runs up the front of the motor was cooking on the exhaust manifold. For some reason the motor would never go into closed loop and always ran rich like it was staying in warm up mode.
MAT sensor or engine temp. sensor have to be suspect.
Small vacuum leaks may make it run rich.
Check for weak spark, for various reasons the spark can get weak. A thickish yellow spark that sometimes splashes a little on the elctrode of the sparkplug, instead of a sharp blue spark you can hear go crack when it is good.
I can tell when my motor goes into closed loop, usually when the temp. is above one quarter on the temp. gage.

o2 sensor "seems Ok"? Test it.
Coolant temp sensor? Test it.
Exhaust cracked upstream of o2 sensor? Check it.
Condition of fuel injectors? Check them.
Heat range of spark plugs? Check them.
Thermostat of the wrong temp or stuck open? Check it.


As the others already posted: fuel pressure, MAP, MAT.

Everything you have ever wanted to know about Renix sensors: http://www.lunghd.com/Tech_Articles/Engine/Basic_Sensors_Diagnostics.htm

Check the hoses to the EGR solenoid on the driver's side fender next to the air cleaner, too. It will run rich if they come unplugged.