ive got a 90 dx hatch with a 91 ls swap just rebuilt it with p30 pistons and a port and polished head. ive done the conversion wiring for mpfi. i can get the car to fire up and it runs fine for a few seconds then it goes into high idle pretty hardcore. im not sure what the problem is but im afraid that if i let it run im gonna break something and have to rebuild it again. any suggestions?

 

Could be a few things... Vaccume leak... throttle cable set to tight... fitv... iac...

 

I had an issue with idle around 3k, I had and obd0 ls manifold before I changed to an 0bd2. The problem I had was with the fast-idle thermo valve (under and attached to the throttle body with coolant lines going to and from it on an obd0 manifold). Coolant temp controls your idle throught this valve when the car is warming up. It was totally eliminated when I changed manifolds, you can still operate in obd0 with a newer manifold. Also check the idle adjustment on the throttle body itself.