My 1998 Explorer,4.0,sohc,has a very sooty exhaust.The engine check light has never come on.The air cleaner is a clean K&N. There is 58k.miles on the vehicle. The MAF sensor is like new. Is there any way of telling if the O2 sensors are going bad? I think there are 4 of them,and at $60 each,I can't afford to guess.
Also,when I change the coolant,should the thermostat also be changed,and what temp. thermostat?

 

Usually if the o2 sensores are going bad the check engine light will come on. As far as changing the t-stat, not quite sure, but you really don't have to do it unless you are running hot. But somebody else might be of help for that.

 

If the thermostat went bad, in the closed position the EEC V engine control increases the RPM to lower the temp and kills the AC. I am not sure what it does if the thermostat fails in the open position. Another thin to check is the PVC valve. You could be sucking oil into your intake if the PVC is clogged. Also you said you checked the MAF sensor, this is a hot wire air flow sensor. Carbon does build up on it. Use Gunk intake cleaner and spray it down (it is in a nipple inside MAF section of intake tubing). Let dry thoughly before using. I found this helps sometimes, as when the MAF sensor has carbon build up, curtousy of the K&N air filter oil, you air fuel ratio is skewed off a bit. I had a 97 Explorere, same engine and filter, same problem. rreplacing the PVC and cleaning the MAF and throttle body one a year fixed the problem. Hope this helps.

 

I know you said the CEL light hasn't come on, but have you pulled the codes? It can throw a code that could help you diagnose the problem, without lighting the CEL.

 

Yeah I agree. get a real time scanner, check for codes and then run a real time test and watch if all 4 O2 sensors are switching above/below .45 V. If you are seeing some low, then it is either lean or just thinks it's lean. If it is not actually lean but a bad sensor return, the PCM will richen it up even further causing it to run actually rich - the blake soot you see in the tail pipe.