mechanic says its IAC, I don't believe him

[Warm Pudgy]

For the last few months I've been taking my 92 sc400 to mechanic after mechanic, after mechanic, with no one able to figure out whats wrong with my car.


The current mechanic says its the IAC causing all the problems

I do not think the IAC will cause the cause the engine to hesitate under load.

IAC should not cause this to happen:
http://yfrog.com/n6vid20100909163306z
http://yfrog.com/mqvid20100909164022z


After this post I have the IAC wired correctly, and it idles just fine.


I've replaced ALL sensors related to air/fuel mixture

Fuel pump is fine, fuel pressure is fine (new filter as well)

There are NO CODES, NO CHECK ENGINE LIGHT

I've eliminated every possible vacuum leak.

Fuel injectors are clean and working properly.

Plugs and cables are new.



I'm at a complete loss as to what to do next. I'm temped to commit insurance fraud and just burn the damn thing or drive it over a cliff.

 

[Bushwhacker]

Have you checked/cleaned the AFM?

 

[Warm Pudgy]

Got one from lextreme

 

[Bushwhacker]

Oh yeah, sorry, turned on my speakers too late.Have you tried running the engine with the IAC unplugged?

 

[Warm Pudgy]

I don't see how that would effect anything. If the plunger is closed then the car won't idle, and if its open then it would idle at an rpm according to how much airflow can get thru. If its open all the way I think its around 2k rpm

 

[Warm Pudgy]

And how would the iac effect how it drives. Its only purpose is to control the idling rpms

 

[Bushwhacker]

True, but unplugging it won't hurt and if nothing changes, then you can rule it out.

 

[gloverman]

One idea for you. Mark up your present position of the tps and try moving it a little bit. If you have a multimeter and some feeler gauges set you could set it up properly ie idle switch changes with 10thou of throttle opening. Ensure you use the lower throttle body stopper. I would just try bringing the revs up and moving the sensor slightly first. Cheers

 

[Warm Pudgy]

TPS works fine, one of the first things I checked out.

I had an idea. I read some where around here the fuel pump has two settings, high and low.

Would the high pressure mode of the fuel pump failing cause the engine to struggle under load and acceleration?

How is this controlled?

 

[sideshow]

crimp return line slightly
raise fuel press by 10 or so psi
see if hesitation goes away
if it does then motor is running lean
problem with borrowing other parts is do u know they work perfect
best thing is to find an sc400 same model as yours and try your parts in it
air flow meters can be fkt and u would not even know it
if tps was a problem u should be able to hold it at a certain opening and it might play up
hesitation is normally fuel enrichment or air leak
have u run a gas thru intake to make sure u have no leaks
running fuel pump on 12vs should be fine all my looms run batt voltage to fuel pump with no probs

just decided to look at your vid
sounds similar to ecu still thinks your on the idle circuit but revs are over 2000rpm
so it shuts the engine down for a sec but then it doesnt seem like a harsh cut like it should if it had that prob
i also had same prob when i had a stuff air flow

unplug the air flow see if it does same thing

 

[mshawari1]

change the MAF.

 

[LP1]

Iac

The ECU is way too responsive with adjusting the rpm to be a simple vaccum leak, it is adding fuel as it senses a lean fuel mixture or erroneously stepping up rpm to what it thinks is idle. You told us the IAC is wired fine but is it working fine? It will step the idle during the cold start, step down the idle when it is warm, and step up the idle when the air conditioning switch is on.

Did you try disconnecting the IAC as suggested above? I would warm up the engine then disconnect the electrical plug for the IAC, that should freeze the valve in its current position (unless it goes to a default position - don't know). Then run your test. Next, plug the air supply line to the IAC, you'll have to open the throttle to get it to run/start, then run your test. If the surging stops, your IAC is malfunctioning either because it is broken or being commanded by the ECU to do stuff its not supposed to do (ie cycling above idle speed).

PS The engine will run okay without a MAF. I disconnected mine for fun, idle dropped slightly, engine light came on, but it ran fine.

 

[sideshow]

it sounds like rpms are chagning to fast for it to be iac
im not sure which model 1uzs but alot of guys on here say they have run theirs without maf plugged in and its ok

but i have had some and including my 1uz run crap as u rev it up with a faulty maf
mine was an early crown

might be worth even looking inside ecu just incase
duzza on here had a tiny flatspot on his 1uz
unplugging air flow ran better but used bit too much fuel
ended up being his ecu needed repairing
once that was done air flow plugged back in it runs great

as u rev it the motor adjust things and at that rpm where it cuts at the ecu is getting wrong or no signal
so its cutting motor for a split second
rpm drops ecu thinks all is ok lets run ok again
rpm rises ecu sees wrong signal again
bang cuts motor
and so on motor hunts up and down badly

just quickly test the tps

theres 4 wires
brown on one end is earth
next one is idle
idle wire probe it should have earth when throttle is closed
and around 12vs just as u crack throttle
then 12vs all the way to ful open

next wire along is tps signal
probe it
0.6v or close at idle or shut
and gradually climb up to 3.8 or 4 votls at full
if at any time that voltage doesnt climb gradually as u slowly open the throttle then double check it
should be linear

last wire think its blue/red
should be 5v all the time

 

[Warm Pudgy]

Thanks for all your input guys, its still at the shop, I haven't had the chance to even touch it yet. I'll let you know what I find out.