'79 280ZX idling went to 4000 rpm before turning engine off.

[Guest Rockin']

Last year while having a new muffler installed the muffler man asked if I wanted a small straight pipe out instead of the usual long bended pipe going along the backside and out the passenger side. I said yeah OK.

In a week or so my car started roaring to 4000 rpm in the driveway a few times and then soon blew a ring. Another mechanic has since rebuilt the motor with bigger pistons and cam but not any distributor work. The motor is not screaming to 4,000 but is idling to 3,000 before stalling out in the mechanic's garage. The car accelerates slowly and maintains 60mph after taking foot off the pedal as if on cruise control which it does not have.

The 280ZX still has the shortened exhaust pipe system. Please help.

 

additional: the ECU is stock on this non turbo fuel injected '79 280ZX.

[Thumper]

Need more details. Stock engine? Stock ecu? If so check the Idle control valve for being stuck open or more likely the throttle plate isn't closeing all the way. Could be from bent linkage, stuck throttle pedal, junk in the tb, misaligned tps, etc...