90 crx dx automatic, dpfi to mpfi swap with the original dx cam. Auto 9091 ex ecu, crx si intake manifold, ex sedan engine harness with an auto ex ecu.

Everything seems to be working correctly besides one strange problem. The best way I can describe this is that the spring on the throttle body is weak. Tap the gas pedal to shoot it to 4k and it goes back down to 750 very slowly... usually hesitating around 1500 to 2000 rpm for a while before going down... or sometimes even sticking up around there. The same thing when going lower, tap the throttle body closed (slightly, not hard) the rpm stays down about 500 or even lower and the even obviously hesitates and sometimes turns off...

One problem may be the timing? TDC was put at the normal DX location...

Mechanical? Electrical? I have no idea anymore... My friend will be bringing another si throttle body today... hopefully that is the only problem.

EDIT:
Strange thing with the rpms tonight again.

Driving in the automatic at 30 mpg, take the foot off the gas pedal and the car doesn't slow down on its own. Wtf... this is really freaking pissing me off.

 

Sounds like either weak throttle body spring or vacuum leak (be it "internal" or external). To test the spring is easy - close it all the way by hand then open it half way and let it go. It should go back to/ very close to all the way closed. If that is not it, then check for external vacuum leaks by whatever method you prefer. If that's not it - it could be a failed IACV, FITV, or something of the sorts. If you have a vacuum gauge put it on and give us readings for around 700 rpm.

These are easy things to check out... do some checking and get back to us.

 

Did you reuse your original throttle cable? Or did you switch to the appropriate short style throttle cable? It could be the difference in cable or even just a cable adjustment. Originally when I had this conversion done a decade ago, I had similar issues and found that the problem was with the throttle cable. Next time you are driving it and experience your problems/symptoms try lifting the pedal up by putting your foot underneath it and pulling.

 

combination of si camshaft without a y8 cam, throttle body sticking, throttle cable to big...