1998 4x4 expedition
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Change out the rear sway bar, and maybe front one too. This will make a huge difference in handling. I just bought a Helwig front for $180 with free shipping from www.carparts.com. I also just ordered a Eibach front sway bar ($260) and can't wait to install. All my shocks were blown out on my 98 Nav. All 4 were blown (dry-rotted) too.
Get a performance air filter kit (get rid of the snorkel), throttle response will improve and slight mileage improvement too. I did a cheap (free) custom air filter mod. Take off the the snorkel and air filter, get a flat piece of aluminum and trace out and cut out a circle slightly larger than the orange air filter . Then cut out the inside portion. The outside diameter will be slightly larger than the air filter and the inside diameter will slide over the air filter. I used a jig saw to do this. Then slide air filter with this aluminum adapter over the stock filter and clamp in into the large circular clamp. IF cut correctly, you will have a tight seal between the filter and plastic instake. Then use 1-2 tywraps to secure the circular clamp at the bottom. Presto, you have a performance setup. I did this on both my F150 and Nav and noticed moderate improvement (free too). There may be better posts on this, just do a search.....
Get a performance air filter kit (get rid of the snorkel), throttle response will improve and slight mileage improvement too. I did a cheap (free) custom air filter mod. Take off the the snorkel and air filter, get a flat piece of aluminum and trace out and cut out a circle slightly larger than the orange air filter . Then cut out the inside portion. The outside diameter will be slightly larger than the air filter and the inside diameter will slide over the air filter. I used a jig saw to do this. Then slide air filter with this aluminum adapter over the stock filter and clamp in into the large circular clamp. IF cut correctly, you will have a tight seal between the filter and plastic instake. Then use 1-2 tywraps to secure the circular clamp at the bottom. Presto, you have a performance setup. I did this on both my F150 and Nav and noticed moderate improvement (free too). There may be better posts on this, just do a search.....