93' f-150 shudders in OD/cruising speed

Old 01-07-06, 07:23 PM
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Question 93' f-150 shudders in OD/cruising speed

hi, I have a 93' Ford F-150, with a 351 engine, electronic 4 wheel drive, automatic transmission, XL, single cab...anyway, at about 135,000 miles it developed a minor "shudder". It only shuddered when I shifted into overdrive and when cruising at about 55-65 mph. The shop said it was my transmission, so I just left it and figured I replace the transmission whenever it croaked. Well, it ran another 20,000 miles, with the shudder getting worse and worse, sometimes doing it while in 4th gear, but usually just in overdrive. At about 155,000 miles, the transmission finally died, so I replaced it with a used one from a wrecking yard. Totally different transmission and torque converter, and the shudder is still there, as bad as ever. It's not a u-joint or rearend because it is directly linked to the shifting process. Would it be the transfer case? I thought too it could be something in th engine, like it shudders when the engine is under a certain load point, like when it reaches overdrive? I'm stumped, Its been in and out of the shop, i've looked for that shudder off and on for 2 years. Any ideas?
Old 01-08-06, 07:54 AM
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I was in charge of a fleet of Ford pickups in early 90's, and encountered what I think your describing, on a couple of the pickups. Having years of background in auto repair, I naturally assumed that the tranny's were on their way out too, just by the way they acted. Shudder, shifting in & out of overdrive etc., but always under load in O.D. I finally sent one to the local Ford dealer, who called back about 2 hrs later to say it was fixed. I was pretty shocked to find out that it was a combination of carbon build up & a bad plug wire & spark plug. The guys that were driving these trucks in particular,were older and never drove it over 50 mph., or gunned it once in a while to clean some of that out either. Those engines were turning so slow due to overdrive, that they carboned up almost weekly. It happened a couple of time's so I started by using a spray into the throttle body of combustion chamber cleaner, following the directions on the can. ( I used Chrysler combustion chamber cleaner spray, and let it sit all night before firing back up & revving within safe limits) . I then changed all the plugs, check all the wires, and took it for another drive, being sure to go full acceleration through the gears, so as to let engine rev with loads as much as possible within the speed limit.(ha-ha).
Sure enough, the shudder & shifting erratic was gone. According to the Ford mechanic, it was a common problem, because when the engine began to carbon & misfire, sensors were giving the electronic shifting of the transmission, mixed signals, thus causing those symptoms we were experiencing. I know it sounds far fetched, but if your due for new plugs and wires anyway, what have you got to lose? It worked for us, and all I had to do was trade the truck to the young guys for a day or 2 and let them run them hard, and the problem never re-occured.

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