This car had mild surging at highway speeds when we bought it several years ago. Now, after a tuneup (shortly after we bought it) fuel filter, and throttle body cleaning, the surging is still slowly worsening. It is worst when accelerating moderately to highway speed. Ideas? No Check Engine light. It idles fine. Fuel economy is ok.
If you aren't using Top Tier gasoline I'd run a couple or three tankfuls through and see if that doesn't help or fix the problem. Read up it, I recently helped my neighbor clean the injectors on his chev pickup that would not start. If he had been using Top Tier I doubt he would have had any problems. The chev has a different type fuel injection and is more susceptible to injector clogging but deposits on intake valves and buildup on injectors can be a problem with all vehicles. If this doesn't help it's back to basics, the usual stuff, ignition, fuel delivery, engine codes etc.
In view of things you have done: It could be the ignition is not advancing in step as load increases on the engine. Get hold of a timing light, Hook it up to the ignition, and see if the engine advances as you free rev the engine. Any non-linear increase of advance could be the vacuum-advance being nutzed..
Jason
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