Another Surging Idle Thread

[Shaggs2Dope]

I have searched the numerous idle issues on the forum, and none of them seem to make a difference. I have checked the entire intake, and it is a tight system, no leaks, no vacuum issues. I was driving my girlfriends 90 LS400 and it just started accelerating on me with my foot not on the gas. If I let it go it gets up to about 50mph by itself in gear. In Idle it revs at 1500rpms or so at startup, and then switches to 2krpm jump-down to 1k, back to 2k every half second or so. It really pulls and surges at stoplights like it wants to launch into the car in front of me. I am just glad I upgraded her rotors and pads to cross drilled slotted with metal masters, or it would have rear ended someone.

The only thing I can think is that the car sat for a long time, and has some old fuel and marvel mystery oil in it. Plus, I washed the car off, including the engine bay a lil bit but kept it away from intake, mainly rinsed the belts since they are screechin a bit, even with belt dressing, which usually means they have someantifreze on the belts.

Anyway, there is no check engine light, the car has ran fine until this point at 245k miles. I know on hondas it is usually Idle Air Control valve or vacuum leak, and most imports its a gross leak after Mass Air Flow sensor. Do the Air flow sensors go out a lot on the lexus LS, because I have seen a TON of threads on here about them being faulty. I had the MAF go out on a mitsu I had and it would hunt because it is stuck in a loop.

Anyone else find a remedy for this. I really hope its not MAF because they are usually expensive. And I dont have the lexus code reader, even though there is no engine light, I can't plug in the diagnostic port.

 

[sideshow]

if it hits 2k something is letting more air in
when u say tight no leaks how do u know
only wya to chekc is with smoke checker
or run enigne and remove air filter and try block intake into air flow and see if it stalls
most of time its the idle motor
see the ecu see the throttle clsoed so why the heck are we reving at 2k
so it shuts the eu down
then when revs drop to 800 it says oh its all ok now then lets it rev back up so thats y it goes up and down quicker than a 2dolar hore
so try blocking off the small pipe feeding the idle motor
if that doesnt drop idle then u might have another leak

 

[Shaggs2Dope]

I have worked in car/racebike biz my whole life. I took out the Air flow sensor and hit it with some parts clean, and now it idles at about 1500. Checked Idle speed, vaccum hoses, and broke one in the process, as it cracked at the fitting when the hose clamp came off. The MAF is definitely what is causing the problem. Any way to just order the black plastic part that screws in rather than the whole housings? And anyone know the best place to get MAF for lexus where its not 500 bucks? like maybe an aftermarket unit?

 

[sideshow]

cheapest way is to look at jap import shops for 2nd hand ones
only prob is ive had afew 2nd hand ones play up
luck of the draw i guess

 

[Peewee]

Highly unlikely the AFM is causing a problem, it doesn't have much to do with the idle circuit.

The hunting happens when the revs get to about 2500, AND the TPS is in IDL mode.

ie, the ecu is unable to control the idle (via the ISCV) and cuts fuel when the revs get to 2500 and turns the fuel back on at 1500 (give or take).

You need to make sure that;

• the ISCV is working correctly (fit one from a working mates car)
• the ISCV is wired correctly (check for voltage and continuity to ECU)
• the ISCV resets itself when you turn the car off (it should go tick-tick-tick)
• the throttle plate is 100% closed, and the IDL contact on the TPS works only when it should
• there are no intake leaks between the throttle body and the head (with the exception of the ISCV and power steering idle up)
• the ECU is working correctly (swap with a mates one).

If you have done all that, are are 100% sure its none of the above, panic and/or cry.

 

[sideshow]

99% of time its an air leak or idle motor sticking open

to tell if its idle motor stick a nut with 8 mm hole in middle down the idle motor intake hose the small hose bout 16mm diameter

this restricts max rpm too about 1300 at idle
this is god as sometimes these engines rev to hi at cold start
**** design and **** programming hehehehe
they idle so smooth u never need over 1200 rpm at idle when dead cold

anyway if this cures yr prob then its idle motor

if its still idling too hi u have air leak