Join Date: Sep 2006
Location: Westminster, CO
2010 Genesis, 2009 Kia Sorento, 2008 Sonata, 2004 Jeep Cherokee
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You have confused the issue with the possible wheel alignment problem, so you may have two things happening.
1. Water in the gas: this is highly likely. The Elantra engine can accomodate some mild spark misfiring, and this probably becomes severe at higher rpm. You can test this theory by noting the speed, rpm and gear you are in when the car starts to buck violently. You did say over 70 mph, but the rpm is more important.
2. Now that you have the rpm figure, shift to a lower gear like third, and drive the car under load to the SAME rpm in a lower gear. This should be a lower speed, say 45 mph, but the engine bucking should be the same. If yes, get three cans of HEET gas deicer/water remover. You will have problems until the bad gasoline is used up.
When was your last timing belt change? We have assumed the cause-and-effect was your fill up in MS. Unless you know that your belt was changed at 60k miles and 120k miles, remove your upper timing cover, have a helper rotate the engine crankshaft bolt clockwise slowly, and inspect for broken/missing teeth on the timing belt, and inspect your static timing...line up the crankshaft timing mark, and verify that the two camshafts are also on the timing mark.
If your engine does NOT buck at the same rpm under load in all gears but only 70 mph, then your problem is independent of engine rpm...and likely not engine related. Get the wheel alignment and tire balance. If your front brake rotors are vibrating, get them inspected and turned at a brake shop.
P>S> you did not mention a Check Engine Light, but misfiring will cause a CEL code.
Where's my bailout?