i have a 87 rx7 with a 91 motor in it and im not sure what is wrong with it but it wont start and it never has since i put the motor in, well it will start but rev real high for a little then die. i talked to my mechanic about the problem and he said it may be that i need a new ecu, i have the same wire harness that i used for the s4 motor but i just spliced it on some place. the question i have is, could a new ecu fix this problem and if it could can somebody help me pick one out and do i get it for the s4 model or the s5 model. thanks

 

S5 electronics used with a S4 harness, or what? List everything you changed. Series swaps can be done all sorts of ways.

 

are you using an S4 ECU? That's a bad idea. I'm pretty sure it's because you're using an S4 ECU with an S5 AFM.

give us some more info on how it's set up.

 

Tell the mechanic to get another job. Maybe run for president, or some other no knowledge required type job.

 

okay, all i have done is put he s5 motor in the car but it came with part of the wire harness because the people i bought the motor from decided to just saw through the s5 harness. i have only changes the parts of the harness that are close to the throttle body because thats the only part of the s5 harness i have and plus it looks as if thats all that needs to be changed because every thing else fits. could i take the old ecu out and just plug a new s5 ecu in or will it not match?

 

So you have half of the S5 harness cut off and somehow spliced into the S4 harness, then back to a S4 ECU? You realize the S4 ECU has no way at all to deal with things like a completely different AFM, TPS, OMP, etc..? The pin outs are totally different on S4 & S5 ECUs, so you can't just line up wires that seem to match and cross your fingers.

Take off all of the S5 manifolds & electronics, and strip the motor down to just the block. Swap on an S4 front cover, OMP, manifolds and all electronics. Fix your hacked up S4 harness or buy a new one, and connect it up like it would be on a stock S4. The motor will be run as an S4, but the rotors will be higher compression.

You didn't say whether this is a turbo or NA motor either, so if NA, you can look into adapting the S5 manifolds back in to make use of the VDI system. Get it running first, then worry about this conversion.

Rotary Resurrection has a small writeup on this kind of swap too: http://www.rotaryresurrection.com/2n...o_s5_swap.html