98 Ford F150 Runs Rich Black Smoke

This is a 1998 Ford F150 4.2L 6 cylinder MT 5spd RWD with about 98,000 miles on it. This is just one of those crazy problems that threw me for a loop so I figured I would share the problem. Has anybody ever seen this before? Please read and comment below, thanks.

Vehicle runs rich, blows black smoke, b1s1 and b2s1 o2 sensors go up to 800-950mv when closed loop, fuel trim -20, b1 + b2 o2 sensors won’t switch so it goes into OFL loop and stops the fuel trim control. The downstream o2 sensors read rich also 600-700mv. I immediately thought stuck fuel pressure regulator since I have seen this before but fuel pressure is good. I’m thinking I should remove the fuel injectors to see if they are leaking fuel or spraying bad, fuel pressure holds KOEO (testing for really bad leaking injector) and there is no fuel in the vacuum line for the fuel pressure regulator. Last when you perform WOT throttle tests (the fuel cut comes on decel?) and the o2 sensors temporarily switch to lean then spike right back to rich so I’m sure they are working, besides constantly false rich o2 signal would cause lean condition


  • P1132- o2 reads rich b1s1
  • P1152- 02 reads rich b2s1
  • P0460 – fuel level sensor input


  • MAF signal @ idle: 1.0-1.1v | 6-7g/s
  • MAF signal @ 2500: 2.1-2.3v | 30-34g/s
  • MAF signal  @ wot: 4.2-4.3v | 150-160g/s
  • BARO KOEO 157hz
  • TPS @ ct: 0.9V
  • TPS @ wot: 4.7v
  • ECT 58°F – operating temperature 195-199°F
  • IAT 46°F
  • Fuel Pressure 32-35psi


  • I removed the intake plenum, tested EGR valve operation(ok) cleaned out individual cylinder EGR ports(ok)
  • I performed fuel injector balance test(ok) + inspect spray pattern(ok) + check for leaking injectors(ok)
  • I took fuel sample(ok,combustible) no water or ethanol visible(no seperation) fuel is good
  • I checked the evap purge valve operation(ok) not stuck open
  • There is no fuel in the fuel pressure regulator vacuum hose, diaphragm hold vacuum, fuel pressure is okay.
  • Scope all six fuel injector signals(ok)
  • Scope cam + crank sensor signals(ok)
  • Pin voltage test all power + grounds(ok)

98 Ford F-150 camshaft position signal + crankshaft position signal


Engine idles rough and PCM set P0460 fuel level sensor circuit, the fuel level signal is withing range 1.2v @ 1/4 tank, fuel pump module supposedly replaced by vehicle owner. I don’t think the code is relevant unless I have a failed PCM. The engine oil was fuel soaked and contaminated so I changed it and no change in engine performance, still runs really rich.

After testing all the o2 sensor circuits and performed complete PCM pin voltage chart testing could not find anything wrong. I finally came to the conclusion that the PCM o2 feedback control was completely inoperable. The fuel injector pulse width stuck at 6.5-7ms PID and showing actual pulse width of 17ms when viewing the fuel injector waveform on a labscope. Pin voltage chart shows a PID specification of 2.5-3ms closed loop, engine warm with normal fuel trim values. After replacing the PCM the fuel trim values were good and the P0460 fuel level circuit code went away also.


Scanner Values After PCM Replacement

  • o2 voltage switching good 50-900mv
  • zero fuel trim values
  • fuel injector pulse width is now at about 2.5ms at idle engine warm

This post was written by: Martin Hand


If you find this information helpful please consider a donation. These articles, questions and comments are very time consuming so even a small donation gives me motivation to keep educating automotive owners. Donations will allow us to continue open questioning/comments, automotive education and repair tutorials in the future as the business grows. All proceeds go to the expansion and maintenance mdhmotors.com. Thank You

Martin Hand

About Martin Hand

ASE Certified L1 Advanced Mastertech. Martin Hand has over 15 years experience in Asian and European Import Auto Repair. Specializing in electrical diagnosis, engine performance, AT/MT transmission repair/rebuild. Martin is also pursuing a degree in Computers Science & Information Systems starting at Portland Community College while he plans to transfer to OIT. Certified in Java application level programming, experienced with other languages such as PHP, Ruby, JavaScript and Swift. Martin has future plans of automotive diagnostic software development.

One Comment

  • Dave Parent says:

    Always on the net searching for tough diagnostic problem solutions.I think i hit the jackpot on your site. In regards to your post ” 98 Ford F150 Runs Rich Black Smoke”, i have the identical vehicle, mileage and customer complaint. But i didn’t dig deep like you did and instead replaced all 4 O2 sensors…..only to have the same problem. I have now read Ford’s explanation of it’s computer strategy ( on Mitchell OD5) for setting codes. Wish i had done that first. What’s the old saying?…’You can’t fix it if you don’t know how it works’. Have yet to check ERG and EVAP systems but am now leaning towards a computer issue. Very impressed with your detailed diagnostic report. Rare to find someone like yourself who takes the time and effort to use the tools at his disposal, PROVE the problem and post on a masterful site like yours. I tip my hat! Dave P.

Leave a Reply

Your email address will not be published. Required fields are marked *