2006 Ford F150 4WD with the 5.4L V8
150,000 miles (oil change every 5000 miles)
In the fall, I started having problems with my truck. It had a rough idle and would shake at certain mph or when under a load. I ended up doing some research, and determined that it was my timing chain guides and tensioners. I ended up taking a weekend to pull the front timing cover off and fix the problem; however, this fixed my rough idle problem, but did not fix my shaking problem, revealing that there was more to the problem than I originally thought.
To give more details about the problem, my truck has had a very bad shake, as if you were giving over pot holes, but I can't really pinpoint the problem. The shake usually always occurs at very specific mph ranges. It will first happen around 45-50 mph and again at around 60 mph It also happens when under additional load, such as driving at a constant speed up a hill. I have been able to mitigate many of problems and lessen there frequency by adapting my driving style around the problem, such as accelerating/increasing my rpm when I'm near the problem areas(45-50 mph, 60 mph, going up a hill). Also, this problem does not happen when overdrive is turned off, so I often find myself switching into overdrive when going up hills or accelerating around 45 mph.
Something else that is strange is that the truck has not thrown a single engine light.
This is not a rough idle shake, this is a very noticeable, rough, shake. It doesn't feel like any kind of shake I have ever felt before. You can feel it in the steering wheel, the seats, the floor. It is as if something hitting underneath the vehicle.
My first thought was that it was transmission failure, but it doesn't feel like any transmission failure I have ever encounter. I also thought that it could be engine misfires, but certainly the computer would throw an engine light if that was the case, right? I less convinced that it is an engine problem. I think it is more of a transmission or drive train.
Also, during my initial attempts of trying to diagnose the problem, I tried to use a product called Lubegard Instant Shudder Fix to see if it would fix the shaking. This help for a day or two and then went back to the way it was operating before.
If anyone has had any experience with this problem or knows what it is, please let me know. Any advice or guidance is encouraged.
Edit: I totally forgot to give and update on the situation... so I was able to figure out at least one of my problems. The cause of the rough idle was the timing chain, timing chain tensioners, and timing chain belt rails. I was able to take the front timing cover off the front of the motor and replace the broken tensioners and rails; however, this only fixed the rough idle.
A month or so had went by until I noticed a comment on a similar shaking issue, stating that it could be caused by bad spark plugs, so I ended up changing them and the shaking went away. This didn't last forever, the shaking came back after about 10,000-15,000 miles...
Because the spark-plugs seemed to fix the problem, I think I can rule out the issue being a transmission or u-joint problem, but instead something like the coils