Rough idle, check engine light on, black soot

[93redxlt]

My 93 XLT started running rough about a month ago. Upon start up the engine idle would surge up and down and almost die, then rev to about 1600 RPM, then almost die again. This would last a couple of minutes and then it would seem to stop after the engine warmed up. This was not every day, just every third day or so. About a week ago, the check engine light started coming on, and the engine has no power when first starting out. It seems to be fine at cruise and highway speed, but is most noticeable coming off of idle...just goes flat. A little scary pulling out into traffic. The check engine light will go off then come back on, sometimes at idle and sometimes at cruise. I borrowed a friend's code scanner. I get zero hard faults during the KOEO test, but do have code 157 (MAF sensor below minimum voltage)in the memory and once got a code 158 (MAF sensor above maximum voltage), but that hasn't come back. I ran the KOER tests several times and would sometimes get a 412 code (Unable to controlRPM during high RPM Self-test) and a 536 code (Brake on/off (BOO) circuit failure/not activated during the KOER) I was trying to determine if I was testing wrong, and was able to get the 412 code to go away, but never could get the 536 code to not show. After resetting the engine light, it will almost immediately come back on. Also, the exhaust stinks something fierce, and is producing a lot of black soot, which tells me rich. Shouldn't I be getting a hard fault code if the light keeps coming on? Another thing. My wife drove our Explorer on the freeway, and on two occasions she heard and felt a large bang from under the truck, as though the tranny was doing something violent. Could the computer sending a wrong signal to the tranny cause this? Maybe this isn't even related? If any of what I have described sound familiar or if you know what my problem or problems might be, PLEASE HELP!!!!!!!! THANK YOU!! Dave

 

[ExplorerDMB]

Your idle problem is probably the IAC (Idle-Air Control) valve. It has been talked a lot on this board, it should be able to help you out by searching for IAC or Idle Air Control, etc. The check engine light may be coming on because of the IAC. You say there is black soot coming out huh? Maybe you have a diesel ha -- check your exhaust. If you have a laser-point temperature gauge, point it at the inlet end of the converter and then after. If they are dramaticly different (say 200 on the inlet side and 120 on the outlet side) than your converter is good. If the stay somewhat the same, then your converter has taken a crap out on ya. Good luck. Hope you'll get a lot of help on it. But I'm sure the IAC is your problem on the idle!

-Drew


EDIT: Welcome to the site!!!

 

[GSEREP1]

I would check the MAP sensor. In some vehicles, the MAP sensor is controlled by a vacuum line off the intake manifold. If that vacuum line is leaking or disconnected, black smoke will be present, and the engine will run like crap. If the vacuum is present and not leaking, then I say change the MAP Sensor. This is especially true IF you get a high MAP reading one time and Low MAP reading the next time.