Does an auto or manual ecm for a pk3 have any difference? Got high rpm's while rolling in neutral. Either the ecm or throttle sensor?

 

you can use an auto or manual ecu in a manual 3g prelude, and both will operate exactly the same. but if the car is auto, the ecu has to be auto.

TPS has no effect on rpms.

high idle, whether it's while rolling or sitting still, is usually due to a vacuum leak, ignition too advanced, or a bad FIV (fast idle valve).
3g preludes are notorious for the fiv failing or coming loose internally, causing high idles or bouncy idles. the fiv is located on the rear of the intake manifold on pk3 (b21) engines. take it the cover off it, I forget if its the top or bottom cover you need to remove. but you're looking for a white/yellow/brown plastic circle thing that can be screwed down. screw it ALL the way down, then put the cover back on it. if that doesnt fix it, start looking for a vacuum leak, and/or check ignition timing (distributor adjustment) with a timing light

 

Wow Les you asked this question on HT? What happened to PZ or PP?
Both 3rd gen specific forums....lol

 

there's some knowledgeable prelude guys on HT still Sean

 

There used to be.

 

Hey now, I'm back lol

 

I was not referring to you bro...lol
Most the old school 3rd genners have gone dark or moved on.

 

I know, you know I love messing with you sometimes though lol