1uzfe rapid idle issue.


New Member
Alright I've been fighting with getting the kinks out of my project 89' cressida with a 93' 1uzfe.
Its been idling from 1000rpms to 2000 rapidly. I thought it was the iacv so I replaced it with a used but newer iacv and now it rapidly idles from cold start.

Was curious could my ecu be causing this, being the "fuel capacitor" problem?
Or do you think its mainly the iacv and I should get my hands on an earlier model iacv and rebuild it?

Has all new cap, rotor, plugs, wires, belts, pumps, seals etc...



Super Moderator
Staff member
I'd open up the ECU and check for leaking capacitors.

I had a few driveability and idle problems with my 92 SC400 ECU and it turns out one capacitor fell off the board when the repairer first looked at it.

May explain why it wasn't running as well as it could have.


New Member
Air has to be coming in somewhere. If it's a rhythmic fluctuation, then definitely IACV. If it's inconsistent, then it's a leak somewhere else, and that can be aided by a bad plug, wire, or injector.

You say the after replacing, there is a rapid idle, that is certainly indicative of a vacuum leak.


New Member
question about the IACV vacuum connection? when you delete the egr and then loop the vac lines back on them selves is the the IACV not getting a vacuum reference?


New Member
I had the same problem with a 94 1uz I put into a 4runner.it turned out I had ecu wired incorrectly. I did not use the mrel terminal of the ecu wired to switch on my main efi relay. I just had a relay switched on by the ignition feed.i wired it correctly and the problem when away.


New Member
Really deegree?? I have a 93' but since 94' and 93' UZ's use the same ecu's that could be it, I have my fuel pump hard wired to come on when my key is turned to accessory like you. How exactly did you wire your efi and mrel?


New Member
So i double checked my already triple checked wiring and my efi relay/ MREL is hooked up correctly.

i took a can of wd-40 and sprayed it all over my intake manifold to check for idle changes (leaks) still nothing.

Checked my ecu seems to be just fine no signs of burn marks or leaks on capacitors.

My guess its IACV so i picked up an earlier (rebuildable) one. it has to be either that or my throttle body isnt closing completely shut but IDK.


New Member
long story short rebuilt IACV, idles great.
sometimes stalls at stops because it idles so low but i think its still learning.