1999 Ford e450 v-10, 63k miles.
On a recent trip I noticed a vibration in the engine area and steering wheel when accelerating under load (like uphill) at highway speed. With no load there was no vibration (flat or downhill areas). New tires, alignment ok, no other issues except I found it was about 3 quarts low on trans fluid. Oops! I heard it could be a misfire, but had no check engine light, so wasn't sure.
After much research and deliberation I decided to start 'small' and go with a basic tune up, just changing the spark plugs to see if it was an ignition or misfire issue. Sure enough, that solved the problem. Apparently, the thermostat gasket was leaking onto the number 6 and 7 plugs and making problems. All the plugs were pretty bad, too, must've been originals. New plugs made a big difference in performance, too! Cost around $500 for everything, happy to invest that money into fixing the problem.
Just passing this along because there were lots of other posts with similar issues. BTW - thankfully there were no issues changing the plugs, broken plugs, etc.
Safe travels!
