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Allow me to preface this by saying this is not my truck, it belongs to my father but I use it almost as often as he does, and only I have noticed this problem. It started a few months ago, at around 55 mph it would shudder gently, every few seconds. At 50 it would stop, and at 60 it would stop. The problem gradually worsened and now it does it at 55, 60, 65, and 70. It also will sometimes do it at 45 and 50, but not nearly as often. The shuddering is also becoming stronger. The longer you maintain the speed, the more intensely and the more frequently it shudders, eventually becoming one constant shake. A deviation in speed or revs of even 2 mph or 200 rpm will make it stop. It generally happens when you have maintained one of the above speeds for over a minute, and only once the RPMs have settled to about 1500-1750. This truck runs and drives like brand new in every other way, including when accelerating and decelerating. All of the above leads me to suspect the overdrive. On the other hand, a trusted friend of mine who rode in the truck and experienced the shaking believes that the torque converter is going out. He said that he had to replace one on his old F-150 because it was having similar symptoms of only shaking when the overdrive engaged. I’d like more opinions though, so I thank you for any and all help you can provide.
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Thanks for the input. I won’t have much access to the truck this week, but next weekend I intend on putting on some jack stands and seeing if it still vibrates, which would pinpoint the problem as being drivetrain related.
Eric, I would suspect the tires too, but the specific symptoms are just too strange. Any change in speed or revs, or using the cruise control and the problem disappears.
I probably won’t have any more information until next weekend so I’ll report back then. Thanks again to everyone, this forum is one of the most helpful I’ve ever been on.
I highly doubt this is related, and sorry for the double post but I thought I should report that at low revs now, there is a burbling gurgling sound coming from either the rear of the engine bay or the transmission. It is especially audible when you rev the engine in park or neutral, but can also be heard when driving. It only does this when cold so I’m guessing its just oil or trans. fluid that is getting sloshed around but I wanted to be thorough and mention every little thing.
Well I just drove the thing a hundred miles with less shaking than it has been in the recent weeks. I’m not sure what’s going on here, but I am almost 100% certain it isn’t tire or steering related. When using the cruise control it didn’t do it even once. When maintaining a speed manually, it shook very lightly (definitely coming from the rear end) after about 30 seconds, and then it only did it twice throughout an entire hour or driving between 55 and 65. I’m starting to suspect the rear differential. The pumpkin has never been opened so changing the lubricant in there could be beneficial. In other news, it is in extreme need of oil, to the point that the check engine light came on. This engine doesn’t need anything more than regular gasoline, but it’s beginning to ping under load. Could the low oil cause that?
Engines don’t like to be low on oil that’s for sure, low oil can cause a bunch of problems, I would top it off and recheck. Vibrations at speed are almost always caused by a rotating assembly, which includes the driveline, I suppose there could be a problem with the rear differential but I haven’t known that to be very common I see more problems with noise from the rear diff when the get worn but not vibrations. I have however seen drive shafts that were out of balance causing a problem similar to yours. Lets not ignore the obvious however, if you have low engine oil and a vibration it could be an engine performance issue that you’re having as a result of an engine problem.
I got the oil up to a satisfactory level and the CEL went away, so at least that’s one problem gone. Revving the engine in neutral doesn’t produce any vibrations. For having the kind of mileage its got, without ever being rebuilt, I’d say it runs exceptionally well. Like I mentioned I plan on putting the truck on some jack stands this weekend and seeing if I can reproduce the vibrations and more accurately figure out which situations they occur under. I had wondered about a slight misfire causing the vibrations under a load, as it doesn’t do it in neutral, but I haven’t noticed any loss of power or other symptoms of a misfire. The only thing left to do then is get some injector cleaner, because when started cold it does sputter somewhat.
P.S. The engine is almost changing it’s own oil at this point. I wish we could afford an engine rebuild because it’s burning oil very rapidly, and until recently it didn’t burn any. I don’t think this is related to the original shaking problem, but I’m not the expert.
The plot thickens… I wasn’t told until now, but yesterday my mother was driving the truck on the freeway at 65 with cruise control. While going up a fairly large hill, it kicked down and revved up to about 3000 like normal… then it kicked down again, with a very rough jolt, revved almost to redline (5k) and the CEL began flashing. After turning the cruise control off, the CEL stayed on but she drove the rest of the way at the same speed and no problems happened. Oil and trans oil are both fine.
I’ll see about getting a local mechanic to pull it for me hopefully tomorrow. Nobody I know has a scanner, which is odd because we all work on our own vehicles…
Given that the engine is burning so much oil and what you’ve described up to this point I would be looking to a mechanical issue. You might do a compression test but perhaps you have some worn cam lobes or something causing the misfire.
I hope you update this thread once you sort out the issues. Overall it doesn’t sound good for this very old truck…. I always find it sad when it’s finally time to go to the junk yard. My elderly suburban gives a pronounced shudder, whenever I drive by the metal scrapyard (grin).
That would deeply sadden me, even more than my dad, and it’s his. I learned to drive on this thing and I’m going to inherit it soon as it stands. I picked up a bottle of injector cleaner, which definitely helped the engine performance a bit, it was sputtering and pinging occasionally, and that’s now fixed. It’s worth mentioning that it also seems to have stopped burning oil. I put in 2 quarts a little over a week ago and it’s all there, and is the right color. I also got a bottle of high mileage transmission treatment, which I just got done applying. Unfortunately I can’t drive it anymore until tomorrow, but by Thursday at the latest I should be able to let you know if that had any effect. In any case, I’m going to drive it until it falls apart.
Interesting it’s not often things are solved by a ‘mechanic in a can’ but keep us posted on your results.
Yeah, I know. But I figured what the heck, its cheaper than actually having a full tune-up done (which is is very much due for, but funds prohibit it) so it’s worth the possible savings to try it out. I don’t feel qualified enough or comfortable with doing it myself, or I’d happily save the money that way.
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