Ok, i have run out of options here. I'm hoping someone on this board will have seen this before and tell me what to replace. I have a 1987 IROC 5.7 TPI auto. Ever since i bought the car (in 2000) it's had an odd problem. When coasting (no engine load) at speeds of 40mph or greater, the rpm's jump around. In other words, it won't free-wheel. You'll be coasting down a hill, and the rpms will go from say 1500 to 1300 and you can feel engine drag kick in. Then they'll go back up. It might do this 3 or 4 times, depending on how long the hill is. It does it in both 3rd and 4th gears. If you add a little gas, it quits doing it. Now everyone agrees that it's a TCC lock/unlock problem, but we can't seem to make it STOP. The list of components changed on this car is EXTENSIVE, and yet the problem continues. It's got
1/ new engine
2/ total tranny rebuild with new converter
3/ new brake switch
4/ new egr
5/ new maf
6/ new vaccum pot
and lastly, and ENTIRELY different ECM and chip.

NOTHING that has changed has made the slightest bit of difference. Does anyone have ANY ideas what else we can look at???

Thanks!

 

My guess is the vehicle speed sensor. My car did somewhat the same thing, but mine was doing it while driving and not coasting. Eventually the tcc would not lock up at all and the ses light would come on, plus when driving and I would let off the gas and my engine would go right back to idle. Have you checked for codes? Having a scanner hooked up to your car will help find out whats going on also. Thats how I found out that my vss was bad.

 

What happens if when you're going down a hill you put your brakes on just enough to turn the brake lights on. From some throttle eliminating the problem, I wonder if it has a deceleration fuel cutoff that's trying to work when it really can't because the engine is spinning too slow.