rough idle, surging, hesitation, check engine light on

[93redxlt]

About a week ago, the check engine light started coming on my 93 xlt, and the engine has no power when first starting out. Just goes flat. A little scary pulling out into traffic. I also have a lot of pinging under acceleration. Also, at cruise every so often, the engine will stumble for a second, then the engine light comes on. While the engine is idling rough, the exhaust stinks and is producing black soot.

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 tried the test several times, because the brake peddle and goose codes were probably my operator error.

I have cleaned my MAF sensor...very clean already. I am suspecting that my MAF sensor has gone south, which I believe would explain all of my symptoms, plus the codes. I think if the cat were plugged, the truck would be under powered, and it is not (for a 4.0). I am not getting any O2 sensor codes. I am also going to check & clean my IAC valve.
The MAF sensor costs $100 at Autozone, so I want to be sure. I there a way to check the MAF sensor with a volt meter? Does the brief stumble at cruise speed sound familiar to anyone? Your help is greatly appreciated!!!!!!!!

 

[MrShorty]

First thing I would do is verify that the wiring between the MAF is good (no breaks or shorts). Get a wiring diagram and verify that the MAF has battery power where it should. Sensor output can be checked with a voltmeter. I don't remember all the pin assignments, but you should be able to get that information from a wiring diagram. You should get a baseline voltage KOEO. Then a higher voltage with the engine running that varies with engine RPM. Note that the absence of KOEO and KOER codes indicates that the problem only shows up while driving, so you will have to adjust your diagnosis accordingly.

If the MAF seems to check out ok, then check the TPS output. The computer may be seeing inconsistencies in the two sensors and choosing to blame it on the MAF rather than the TPS. Same could conceivably be true of other sensors like the O2 sensors.

Do you have access to a scanner? '93's seem to have teh ability to output live data to a scanner. Would be interesting to get the comptuer to tell you what it is seeing.

 

[93redxlt]

I will try to experiment with the MAF voltaged today. I have taken my Explorer for a drive with my borrowed scanner hooked up and in "live" mode. Most of the available functions appear to rise and fall smoothly with throttle or RPM. There is no where that checks the MAF sensor specifically during this. The TPS voltages seem to be smooth and non-irratic. One thing I do notice is the EGR vacuum regulator (duty cycle %) never comes off of 0%, at any RPM,sitting still or driving. I do get EGR Feedback voltage which seems to change smoothly with throttle position. Autozone does not show a EGR regulator available, but do of course sell an EGR valve. Could a faulty EGR system cause the engine conditions I am experiencing, and if so, what component feeds the computer the EGR duty cycle percentage? Thanks for your help!!!!

 

[MrShorty]

I don't know for sure how the '93 4.0 EGR system is supposed to work.

Interesting that the scanner doesn't show MAF voltage? You'll have to get it from a voltmeter, I guess.