I have a 2003 F350 5.4L Auto. My symptom is Truck starts a little rough like it hasn't caught on all cylinders right away, give it a little gas and it's fine after that. What I noticed today is when I shut it off the Tach goes to zero then it jumps back up to about 350 rpm and sits there until I turn the key to the on position again and it goes back to zero again...it's just strange.

 

I forgot to mention, it's about 5mph slower than it reads, so if it says I'm doing 60mph, I'm really doing 55mph. Also the ABS light is on but I don't think it has anything to do with the speed or the rpms, am I correct in this thinking??

Thanks.

 

Your rough start could be due to a bad IAC (idle air control) valve. ?? Just throwing out ideas, I'm not a pro. The tach issue is strange, I have no idea. Speedometers are typically off a few mph, I haven't owned a vehicle yet that I've found the speedo to be spot on.

 

I'm not sure in the 2003 but at least in later models I believe the speedo uses the same wheel speed sensors as the ABS. So one of the wheel sensors could be having issues and affecting both systems. For the rear there's only one sensor mounted on the differential.

 

Thanks for the info guys, I have to drop an oil pan on a Scion tc this weekend and check the bearings, so I'll get the truck in the driveway too and putz around with it as long as I have to get dirty.

I already put two new sensors on the front wheels when I changed the wheel hub assemblies a few months ago, so unless the problem is in the wiring before the new sensors, I guess I'll concentrate on the rear sensor by the Diff.