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Another lh3.1 Idle Thread

L

Lg_Ryan_

New member
Joined
Dec 27, 2019
Hello everyone, I've been working on chasing down various issues with my 1991 240 wagon, and I'm having trouble diagnosing my issue at this point. I have a very slight stumble at idle that is visible if you watch the engine, but isn't a big enough change to register on my small tach. While investigating that, I determined that the IAC valve was not working correctly. I could pinch both hoses going to the IAC all the way flat, and there was no change to the idle. Also it looks like someone had messed with the little adjustment screw on the valve at some point in the past. I replaced the IAC with a new one from IPD, and now the car idles waaaaaaay worse than before. According to my small tach, it idles at about 1-1.2k, but sometimes it will get hung up after revving around 1.5-2k. Also sometimes it will surge from about 1.2-2k. Usually I can get this to stop by revving it back up and the idle will rest back at about 1k. I have visually checked for vacuum leaks and found nothing. I have also installed a throttle body gasket, intake gasket, new throttle position sensor, new crank position sensor, and coolant temp sensor within the last month. My next idea is to check out my MAF and maybe get the car smoke tested to see if I might have missed a vacuum leak, but I'm running out of ideas.

Is there anything I've possibly overlooked that I should also check out?
In the past, I have gotten code 232 for the car being too rich/too lean at idle, as well as the code for the speed sensor. I visually checked out the wiring by the speed sensor, and it doesn't look bad. Also sometimes my speedo will cut out, but it can be fixed by smacking the top of the dash.

Thanks in advanced for the help,
Ryan
 
^The 3.1 doesnt have an idle adjustment screw on its throttle body and there is no click because there is no switch.
 
I can adjust the stop screw by the spring for the throttle body. I'll give that a shot today and see what happens.
 
it has a stop screw for the valve/control rod interface, but no dial appendage for a switch. the throttle position sensor does all that work.
78949272.png

(took pic from here)
 
seanile said:
it has a stop screw for the valve/control rod interface, but no dial appendage for a switch. the throttle position sensor does all that work.
78949272.png

(took pic from here)
Click to expand...

That's what I thought. That's why the OP needs to follow the first part of those directions on the brickboard. If the base idle isn't set with the IAC hose pinched off, the ECU can not control the idle. The system kind of freaks out trying.
 
Today I attempted to adjust/set the throttle. I followed the instructions from brickboard as best as I could. When I pinch the hose that goes from the accordion intake tube to the IAC, nothing happens. I adjusted the stop screw so it just barely rests on the valve. I don't really have a way to adjust the idle as far as I can tell though. Now, when I start the car, the idle starts at about 800, then slowly rises to about 1200. After a bit of a drive, it got up to about 1500, but I blipped the throttle and it dropped back down. After I adjusted the stop screw, pinching either IAC hose had no effect on the idle. The very slow idle creep seems odd, could it possibly be a very small vacuum leak somewhere that gets worse as vacuum builds?
 
Lg_Ryan_ said:
I have visually checked for vacuum leaks and found nothing.
Click to expand...

Brake booster's vacuum hose to intake - remove at intake, and plug intake port

Higher idle RPM requires this stuff called "AIR," so either throttle plate is allowing it in, or some other entry point.
 
I've replaced the line from the intake to the booster recently because it was looking a bit old. The booster itself could be leaking I suppose though.
 
Alright, I was able to plug the nipple that goes to the brake booster, and there was no change in idle. This week I also realized my thermostat had failed and replaced that, as well as installed a new MAF because my resistance reading was a bit off. Neither of these had any effect on the idle. The idle is still very erratic, sometimes it is low, around 500, sometimes it is around 1200, and sometimes it will surge from about 1200-2000rpms. With the idle fluctuating so much, it seems like more than a vacuum leak causing my problems. I believe I have replaced or checked every related sensor, except for my O2 sensor.
Does this seem like a good next step?
I currently have the IAC unplugged, because none of the erratic issues happen, the car just idles a bit high, around 1200rpms. Is this where the idle should be with no IAC hooked up?
 
Lg_Ryan_ said:
IAC unplugged...round 1200 rpms
Click to expand...


2. Checking the Throttle Body (Plate and Throttle Position Switch) Adjustment

Connect an accurate tachometer then warm up the engine. Let it idle in Park with a/c off. Pinch off the hose between the air intake and the IAC valve (don't damage the hose with something sharp!) Idle speed should drop below 500 rpm, or the engine may stop: both are normal. If idle speed does not drop, adjust as below.
 
I don't have any way to adjust the idle as far as I can tell. I do not have the adjustment knob, and my tps is not adjustable. Lh3.1 has the potentiometer tps, and it is supposed to handle all of that as far as I know. I have adjusted the stop screw for the throttle plate so it is just barely touching the lever when it is fully closed.
 
He said he adjusted the stop screw for the valve, nothing else he can do with the throttle body
 
have you checked the accordion hose for holes? Pull it and look very close. they wear through on the strut tower!
 
seanile said:
He said he adjusted the stop screw for the valve...
Click to expand...

This can't be confirmed....if throttle plate is fully seated, and high RPM exist, then the problem is elsewhere.

Throttle plate is suppose to be a pinch opened...Main Purpose...for when driver's foot is quickly removed from accelerator pedal, the engine will run smoothly.

High idle requires air...either too much via throttle plate...or elsewhere. If plate is seated, then one can nail where to look.
 
I have checked the accordion hose, and even swapped with another one, and there was no change.
 
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