shutter at steady 40 miles per hour

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#1
I have a 88 Chevy celebrity it has a 2.8 automatic trans axle it has over 100000 miles .It has no service codes showing and when you jump it the only code that blinks is 12 and that is normal.when you are cruising at a steady 40 it shutters stops as soon as you go above or below that magic number.trans oil is old but clean my daughter inherited the car. i don't know if it was ever changed .It does not feel like a slip it does it on hills and on the flat it doest matter .was told the tcc will do that but that should set a code. any ideas would help thanks jay
 

billr

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#2
I'm surprised something that old even has a TCC. Does it do the same shudder if you keep it manually in a lower gear? Neutral? You might try disconnecting the TCC control wire to disable the TCC, see if that affects it. Even with much more modern stuff it is quite possible to have a failure that doesn't set a code (for the TCC as well as many other systems), so "no code" doesn't mean all that much... Also, check motor mounts and half-shafts, it seems like there is a resonance point there at 40.
 
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#4
What slipping into neutral and killing the engine at that speed? Sounds typical of a secondary harmonic resonance that exhibits a very narrow speed range.

In line with what Gus said, did have spark plug insulator leak problems with that identical symptom. Will find a varnish type of stuff where the insulator contacts the metal base. I put these plugs in my tester, was weird, above and below a certain test pressure, spark was okay. But at the pressure, the spark will kill that would correlate to a very narrow misfire at a very narrow speed range. Which was the exact symptom of that vehicle.

Stuff like this ventures into the twilight zone.