I have a 1999 Ford F250 Super Duty with a 5.4L Triton engine with about 180,000 miles. The vehicle is in overall good shape and has been well taken care of. For the last few months, there has been an intermitant misfire upon light acceleration between 45 and 50 MPH at about 1700 rpm. My mechanic could not find a specific problem and his diagnostic code reader indicated a bad ignition coil on cylinder 10 - I have only 8. His thinking was that it was an intemitant problem that was not severe enough to correctly register a trouble code.
Now this morning, after heavy rains last night, I jumped in my truck and started it up. It ran very rough at idle and at speed. I did not see any black smoke. It continues to run rough even after being driven for 20+ miles.
I noticed two small signs. I have no idea if they are apart of the problem or not. First, upon acceleration, I can hear (barely) a high pitches squeal - maybe a vacumn leak. Secondly, after I turned the truck off and got out, I could smell a slight sulpher odor.
Could very well have a vacuum leak, check the PCV line at the throttle bore, and all other vacuum lines. The sulfur odor is most likely the cat converter not being happy with the mixture that it is getting because the PCM is trying to compensate for the misfire. Have your frien scan it, light or not it should have something stored if it runs that poorly.