I have an 86 fwd sedan deville with the 4.1 and about 105,000 miles and it runs great under 55mph. Most of the time when I'm going 55 or faster on the freeway, the motor will intermittently shake and it's rhytmatic, not sporadic.

It's definately not a bad tire balance either because when I let of the gas it is smooth.
I've never experienced a misfire but I'm guessing that could be what it is:hmm: That never happens under 55, even under full throttle.

This has been happening for more than a year and even after the plugs and wires were changed. The cap and rotor look fine too.

What could that be? Could it be the transmission, it shifts great.

Here is a list of things to check for your problem.

1) Worn or Damaged CV joints.
2) Tire or Wheel out of balance or out of round.
3) Loose or Worn Wheel Bearings.
4) Worn Tie-Rod ends.
5) Worn Ball Joints.
6) Excessive Wheel runout.
7) Blister or Bump on Tire.

The above are all things that should not disappear when lifting off the gas except maybe 1) & 3).


Other possibilities are exhaust system restriction (like plugged catalytic converter), fuel pressure problems, or an ignition coil showing its age.

It sounds like a fuel problem. I had this same problem in my buick. Change the fule pump and the fule filter and it should slove your problem that helped me.

I Don't know David..... BEFORE I would drop that tank and get out all that fuel pump stuff out, I think I would give those Inside tie rods a jiggle. My '85 FWD SDV had the same thing happen; last summer at about 80,000. REMEMBER, those front end parts, are just like blood pressure problems in humans- the Silent killer. They are ALWAYS being SLOWLY worn... and can really fool you, if, at least, you Don't check. OK ? The Doc . jcypher@aaahawk.com

I HAD THE SAME PROBLEM, i just changed my plugs, wires, distrubutor cap, and ignition coil, the 20 year old ignition coil was the culprit