Adiaz8813

So, this is the 2nd Sc300 I've had that I have a high idle on. This is a 1996 5 speed SC300 Non-VVTI. When I start the car it typically idles at 1000 RPM. as it warms up, most of the time it will drop to about 850. As soon as I touch the gas it will go no lower than 1250 and will sometimes stick at 2k. I know this sounds like the throttle body plate is getting stuck but I have looked this over a thousand times and it is clean as can be and is closing completely. The idle air control valve is also 100% clean. I have smoke tested the vehicle and found only a few vacuum leaks and got them repaired. I reset the ecu and I see no changes. MAF sensor has been replaced. The ECU capacitors look as if they are all in perfect condition. TPS, I tried swapping with a properly idling sc300 and made no difference. I tried messing with ignition timing but I cannot get the idle low enough to get it timed properly if it isn't already. With the idle at around 1250ish, it's sitting at around 14. But I cannot rotate it any further to lower the idle any further than that. Most of the time when it gets stuck at 2k rpm, if I hit the throttle really quick it will drop back down to the 1250 range but it is not a throttle plate or linkage issue as I've tested many theories on it and even tried to pry the linkage back carefully to make sure the plate is closed completely. Throttle stop screw is also backed off so that it's not keeping it any kind of open. Please help.... it's driving me crazy... ALSO, never had a single check engine light since I've had the car for the past couple months... Thank you!

Tallyhoe

I'm no JZ expert nor operator, maybe a collapsed vacuum hose? Maybe someone with more JZ experience can help you out better than I can.

Edit: See if anyone near you has a spare JZ ECU, or can borrow theirs for a week to test on your car. Sometimes ECU's take a couple days to adapt and settle down. When I got my UZ ECU rebuild it idled high for like 3 days and went back to a normal ECU and has been good ever since.

Adiaz8813

That is an interesting idea.... I've never thought about that. All vacuum lines were replaced with silicone not that long ago but I will definitely look them all over while the engine is running just to see. Appreciate your input!

RudysSC

High idle can definitely be a vacuum leak as well... You may have to go through and check vacuum hoses closely and perhaps try some brake cleaner to spray around the vacuum lines, intake, and intake manifold to see if you can find any leaks.

Adiaz8813

Yeah in the very beginning I thought it was just going to be a simple vaccum leak causing this weird high idle but I've made sure every line is connected and properly routed... not a single one is old or brittle or have any cuts in them. I've even tried spraying every vacuum area with brake clean/ starting fluid and I'm not getting any fluctuations in the idle at all... Thank you for your input as well!

RudysSC

You may have to try and adjust your TPS perhaps? There's a few threads on how to do it by feel. It might also be worth checking to make sure you're throttle cable isn't over-tightened and that when the throttle body closes it's hitting the stop screw. Do those have an IACV? I think they may - that could also need a clean. Just a few ideas - hope it helps a bit!

GAsc300

RudysSC has good idea to check throttle cable in engine. Those can stick over time or spring mechanism can lose tension. And can be moved out of place doing other engine work. You probably checked this out already but sometimes we skip the simple stuff

I don’t know if the SCs have throttle body relearn procedures other than maybe disconnecting battery after doing throttle body work. I can’t recall if I had to do this when cleaning my throttle body in the past on my SC300. On my rx330 the idle was crazy low after doing throttle body work until i did that car’s relearn procedure

​​​​GAsc300

RXRodger

iTrader: (2)

If your steering low speed vacuum actuated switch is still in service try pinching off on that vacuum line. Sometimes those switches go bad and let air in causing idle issues.

pishta

My 92 SC400 went high on idle (1800 right off start) and I was able to drive it home but it was pig rich. I took the IAC out and the 2 skateboard bearings inside were almost locked up. I took then out and lubed them back to normal operation and it completely fixed my idle speed , Rock solid. Most of the plastic hose connectors will break so be careful. Just cut lines off at connector and slice the remaining ring off the connector , don't pull it off or it's sure to break the plastic nipple.