• Hello Guest, welcome to the initial stages of our new platform!
    You can find some additional information about where we are in the process of migrating the board and setting up our new software here

    Thank you for being a part of our community!

Boost and iac issues

pigdog

Member
Joined
Aug 25, 2016
hey guys .

so i have a weird issue ive never been able to figure out .

i have a turbo swapped lh2.4 240: B230fk,19t etc etc etc all the usual stuff .

when snapping the throttle shut fast after hard acceleration it stumbles hard till sorting itself out, if at low speeds it will stall.

now this has happened for about 2 years, 3 different turbos , multiple mafs, iacs, tps etc etc if it could be changed it has been .

now we have finally figured out the problem but it creates more questions then answers .

if i unplug the iac , no more stumble , i can snap the throttle over and over and it recovers perfectly.
with the iac unplugged idle is at 1200 and hunting every now and again .

it seems the iac just cant react fast enough after hard pulls .

now the short term solution is delete iac completely, install a ball valve in its place and manually set the idle ,that should sort the high idle and hunting while still fixing the stumble

its far from an ideal solution though .

what could be causing all my iacs to not react fast enough ?

ive seen numerous people running high boost thru big turbos with no issues .
but ive also read heaps of threads with similar issues .

does anyone have any ideas? things to check? a more elegant solution then running a plumbers ball valve and going full ghetto haha

any help would be awesome this issue has been driving me mad for way to long
 
You're not alone in the fight, but I will clue you in on my findings over the last 6 years of fighting this.

It's actually not IAC related, but rather mixture related. LH goes into fuel cut on decel if you're over 1500rpm. If the system goes into fuel cut on decel, once you drop below that rpm it brings fuel back on slowly, things react normally. If however, you do NOT fit into those particular conditions, it will actually go pig rich when you close the throttle, then stumble and die unless you catch it. I've been able to map this out as a 100% consistent occurrence on mine, haven't been able to find a way to address it in the LH mapping, nothing for idle control seems to help. Changing cam timing on my IPD cam did make a notable difference, but I don't like the way it pulls when I have the cam setup so it doesn't stall as much coming to idle. Bigger turbo's and cam's mean more air moving thru, even off boost in some scenarios, and LH simply doesn't seem to be able to react quick enough, especially once you start moving to bigger MAF's or injectors.

My personal solution...switch to MS and allow me to tune things far better. lol
 
if i unplug the iac , no more stumble , i can snap the throttle over and over and it recovers perfectly.

Thinking out loud here . . .
If you made it so the IAC was disconnected above idle (using a microswitch or something), would that create the same effect as the unplugging experiment you did? Would it recover ok once the IAC got power again? Would it send any codes?
Dave B
 
The IAC is already disabled off-idle by LH.

LH2.4 keeps the IAC open at all times and adjusts the opening up and down as the rpm/load change. It also bumps up the IAC by ~5% when the A/C kicks on (not shown below).

Here's a graph of '937 LH2.4 IAC PW% (bottom white line) in operation over ~10 minutes of my drive to work (click pic for bigger):

 
Yeah, what ^^^^ said. It depends on conditions as far as where it was when OP unplugged it, but it should have been open to some degree or another. How far open though, that will have variables.
 
You need to make sure the base idle speed and idle switch are set correctly, then make sure a second time.
 
we actually have an interesting idea I would like to run by you guys along the lines of what dbarton said.

the car doesn't stumble when the iac is disconnected but obviously that comes with a high idle and random hunting.

we are thinking about using a timer relay (NARVA 68076BL)
setting it to cut power to the iac when the tps hits the closed position for a pre determined amount of time ie 3,4 5secs or however long we find it needs.

our theory is, cutting the power to the iac when the throttle shuts will go into lh2.4s fail-safe idle mode of 1200rpm for those few seconds to compensate .

in theory it should eliminate the stumble while retaining lh2.4s normal idle control all other times.


any thoughts on this?

it's abit out of the box but it seems like a viable solution and would be simple to setup
 
Last edited:
Yeah, what ^^^^ said. It depends on conditions as far as where it was when OP unplugged it, but it should have been open to some degree or another. How far open though, that will have variables.

Obviously there are variables, since if the ECU simply cuts the signal from pin 33 when the TPS is above idle, then the IAC *SHOULD* act as though it was unplugged. But not in this case. So logic suggests the ECU is doing something different from just cutting the signal. And it turns out is DOES:

From the LH 2.4 Greenbook:
The IAC valve has two tasks. Firstly it must hold the engine idle speed constant no matter what the load from the automatic transmission, A/C, FC, power steering or GEN. Secondly it must supply air to the engine when the engine is braking the car so that the partial vacuum in the intake manifold is kept at a permissable level. The valve is only operative and controlled by the control module when the idle switch is closed. If the idle switch is open the valve goes into an open standby mode while still still receiving signals from the control module.

So this particular issue is allegedly happening while the IAC is in open standby mode, whatever that actually means. So how does the ECU know you're engine is braking the car? One way it knows is the speed signal from the speedo. Could that have been interrupted?
Dave
 
I found a further explanation of what open standby mode is.
When the current is off, a spring sets the idle valve opening for an idle speed between 1,000 and 1,100 rpm.

Once the engine is running, the control unit ensures that the idle valve is more or less open at all rotational speeds in order to prevent the development of unnecessary high negative pressure in the intake manifold when the throttle shutter closes suddenly during deceleration.

"When the current is off" . . . this would explain your high idle when you unplug it.
 
That first video behavior is the exact same as mine when my (+t) throttle stop and tps was mis-adjusted. IIRC I opened the blade prop screw like a quarter turn or two more than the closed tps switch spec and it helped considerably. Have you started by setting the throttle stop/tps to green book spec? got the feeler blades out? Winging it doesn't work well.

What it looks like to me is lh is making the fuel rich during the pull as per usual, then when you tip-out the throttle the lack of air and too much fuel kill/almost kill it.

Is the MAF/AMM pre turbo?
 
Last edited:
I agree, the first video is exactly what mine is doing as well, and watching AFR's, it goes pig right, engine almost dies, and as the AFR's lean back out it comes back. I've just learned to baby it coming off the throttle, for now.

The IAC is controlled based on TPS setting, so if the engine rpm is above idle speed, and the TPS is open, the IAC is deactivated. Once the TPS closes, the IAC becomes active again. You can set how many steps the IAC stays open in it's 'off' position, but I've not found that to be of much help in this issue.
 
Do any of your setups recirc the blow off air? I thought that with my lh2.2 setup that with the blow off valve venting to atmosphere, it was letting the turbo still draw air through the amm and make the car rich until the throttle plate opened again or the turbo slowed down. I think if the blow off air is recirculated back into the turbo intake it would slow down the air coming through the amm to prevent that happening. I ended up with MS and no longer have that problem.
 
Obviously there are variables, since if the ECU simply cuts the signal from pin 33 when the TPS is above idle, then the IAC *SHOULD* act as though it was unplugged. But not in this case. So logic suggests the ECU is doing something different from just cutting the signal. And it turns out is DOES:

From the LH 2.4 Greenbook:


So this particular issue is allegedly happening while the IAC is in open standby mode, whatever that actually means. So how does the ECU know you're engine is braking the car? One way it knows is the speed signal from the speedo. Could that have been interrupted?
Dave


that was one of our first thoughts but the speedo signal is intact and working fine
 
Back
Top