Overheating Due To Low Idle

[M.Rad.]

Hi Everyone, I have a 1997 SC400. It runs great, and idles smoothly, but low (about 400-500 rpm). This causes it to overheat when waiting at traffic lights, etc. This is both in gear and in park, with the A/C on or off.

If I raise the rpm up to about ~700rpm, the temp immediately goes down.

I tried using the search function ("sc400 low idle", etc) but the site keeps telling me that I can't use words of three letters or less. That kinda eliminates "low"...

Any ideas? All help is greatly appreciated.

Edit: Today I tested the IAC and it seems to be working (clicks when engine shut down). I R&R'd it, disassembled and cleaned. No change in idle rpm.

I also unplugged the MAF, and the engine died, so I guess it's working (no other driveability issues). I'll do a proper pin-out later and report.

As for the overheating, I removed the large plug on the thermostat housing and added about 1/2 gallon of water. This probably means my coolant reservoir cap is bad, because the tank is full of coolant, but it wasn't being sucked back into the system. I left it running with the A/C on, and all seems well (but it's only 70*), so time will tell.

Chime in with any suggestions as to the idle issue....

Regards,

M.R.

[MrLatifa]

Sounds like your definately searching in the right spots. Not to sound simple, but did you clean the throttle plate and reset the fuel trim via battery? Looks like the maf is a VERY expensive part, so perhaps being obdII, you can check to make sure its showing a decent flow at regular idle rpms. Dont forget to look at the tps value while you've got the scanner hooked in. Hell, you should be able to check the long and short term fuel trims on the meter. Looks like the vehicle can be equipped with a few diff egr setups, visual inspection required to determine. If it aint electronic, its a cinch to test and clean. Remember to clean the passages as well. At least the ideas to start with DONT include spending much money... Let us know with anything else you may find.

*I dont think the overheating issue is a cooling problem, rather induced by low rpms. please watch ur temps until this gets figured out. *

[M.Rad.]

MrLatifa, thanks for the encouragement and additional ideas to try. I will definitely remove the throttle body and do a thorough cleaning (including any small passages). I did a re-set using the EFI fuse, per the manual instructions, but there was no improvement.

I don't have a scanner, just a multi-meter, and the (excellent)4,000 pg. :o Lexus manual. I'll see if I can "rent" one from AutoZone in the next few days and actually pull the codes.

Right now I can't get it to overheat! <_< That's ok by me, but I've got a new coolant tank cap coming.

The idle is still low, but it runs so smoothly that it's only an issue because it's not "right".

I also found that my "guibo" driveshaft flex-coupling is shot, so the list grows. Actually I am extremely pleased with this car. I haven't had to do almost anything to it in the 5 years we have owned it.

Regards,

M.R.

[UCF3]

Quick question. Does it overheat when you're in motion as well?