Back in 1990 I bought a 1987 IROC, with the 5.0 automatic from a dealership with only 40,000 miles. I loved the car (this was my first of 5 iROC's) One thing I loved about the car was, when cruising around 70 in cruise control, if I tapped on the brake to knock it off cruise for my upcoming exit on the freeway the engine would go from it's RPM's in the 2,000's down to 700-800 and stay there while i coast or until I gave it some gas.. I always thought this was a "gas saving" design from GM.

I've noticed that none of my other cars do this. but they have all had some kind of work done to them.. mod wise and such. When I drive my 5 speed, naturally I go to neutral and coast there until it's time to shift. My question is.. did that tranny do what explained by design...? I never had ANY transmission issues with that car the 4 years I drove it. Any info on the subject would gladly be appreciated.

Stone

 

Sounds normal to me, if I understand correctly.

JamesC

 

I guess the part I left out was.. None of my other cars do this. All my other cars, the RPM's match my speeds at all times. Only this unmolested 87 5.0 could be coasting 70, 65, 60 and so on.. and all the while be sitting at a nice low 700-800 RPM (as though it was in neutral).. I can see where it would be good for gas mileage. My 91 Convertible RS 5.0Tbi is getting 25.1 mpg.. and in these days where gas is $3.50 per gallon, just trying to squeeze every mile I can before the old lady says "honey, gotta get rid of the Z, it's not good on gas anymore" Don't worry guys, I'll get rid of her BEFORE I get rid of my car(s).. I'm the MAN in the family, just don't tell her that!..
Thanks for your help.. gotta run, got to do laundry before she gets home..

Stone

 

Sounds like in yer first car the lock up torque converter was disengaging as it should when the brakes wer applied. I have no idea why the other cars would not do this. However i do have a tiny bit of advise belive it or not when you engine disenages and the car coasts it burns more gas then if the engine is at higher rpm and not under throttle. I can explaine it best in case of a manual trans if you take it out of gear and coast the engine runs at idle speed and burns gas to do this. However if you leave it in gear and let off the throttle the engne turns at a higher rpm but it is turned over by the force of the car going forward not by burnign gas. It may seem it would have to be burning more gas because its turning faster but because the throttle body is closed it is taking in less air and burning less gas as well. Although it would seem at idle at lower rpm it would be getting even less air because the throttle body is still closed but the idle air cntrol valve opens and lets more air in and the computer adds more fuel to match. I know it seems hard to believe but its true they even did a thing about it on mythbusters. Ps this applies to fuel injection cars not sure if it would also apply to a carburated motor though.