Here's the quick background on the problem: Car was idling rough a few weeks ago. I changed the oil (just because it was scheduled) cleaned the throttle body and the IAC valve. Put the throttle body back on with a new gasket, throttle raced to ~3500 rpm

I just got back from pittsburgh this afternoon, and found my new throttle switch and other assorted parts waiting for me on the porch. I replaced the throttle switch, and the idle dropped down to about 2000 rpm. Here's the weird part: It stayed at 2000 for a few seconds. As long as the seat belt reminder light was on, to be exact. Then it dropped to almost stalling for a few seconds, and shot back up to 2000. Fluctuated like this for a while. Then it stayed at the high end (2000 - 2500 rpm). I had a spare IAC valve handy, so i put that in. no change. I screwed the idle adjustment screw in the whole way (the big black plastic one), and the idle lowered, but not enough.

So after all of this, is there something else that could be causing these symptoms? None of the gaskets/seals are leaing on the intake, and my only other guess is maybe there's a hole in the vacuum line to the ignition control unit, but i doubt that would cause such a massive idle response. A last ditch effort might be to replace the ECU (i have a spare), but i want everyone else's opinion before i go taking the inside of the car apart.

Thanks

Is it damaged? Within spec? Did you knock the wires off of it?

- alex

'85 244 Turbo
'84 245 Turbo

If you block off the IAC and the idle goes to normal as was said in previous posts one thing you might want to check is the temperature sensor. I'm not 100% sure on LH fuel injection, but on Kjet if the control unit does not get a temp signal the idle goes up to about 3500 rpm.
Actually your idle will probably not be normal when you bypass the IAC because you messed with the base idle setting screw.

and, more importantly, is there any way to test the temp sender without pulling it out of the block? also after i pull it out of the block?

where is the temperature sender? is it the coolant temp sender under the intake? or that other thing under the intake that looks similar?

I have not had to deal with the CIS on my 89 so my experience is limited. I am pretty sure there is only one coolant temp sensor for LH fuel injection (The Kjet system has 2 sensors). But they work the same. The sensors resistance drops as it heats up. When the engine is warm the resistance should be low (200 ohms approximately with reference to ground). Cold the resistance should be in the thousands.
I have tested my kjet system by disconnecting the temp sensor wire and grounding the wire. If the system is working the idle shoots way up when you unplug the sensor wire, and drops to warm idle with the wire grounded. I have not tried this on the LH system though so I would not recommend it. I wouldn't want to be responsable for you frying your LH unit.

I'm not certain of how the sensor works resistance wise. The easy way would be to messure when engine is cold, heat it up and messure again. The 700/900 forum has a good article on how to messure and what the resistance should be at what temperature, it is very hard to reach however. Make sure you messure the one that is hard to get to, not the easy one, that is for the temp gauge in the instrument panel. The one for the engine management is waaayy underneath the intake manifold, closer to the firewall than the other one.

It might be an idea to remove the inlet hose to IAC valve and tape the fitting on the valve. Fire the engine up. If the idle is then low, the IAC valve is open, and that proves that everything else is OK. If the valve is commanded to open it may be that it is receiving power from a bad ground someplace - just how to find that I don't know. Otherwise it may be stuck in the open position. Best to prove that the excess air is entering through the IAC first. If the idle remains high while the fitting is taped off, then you have a second source of air - no amount of ignition timing will give you 3000 RPM. That means the air controls for the HVAC, or the brake booster, or intake manifold.

I second that, and would add:
Remove the valve completely and plug both hoses. This way you can see exactly what is happening in the valve. When you take the engine off idle, the valve should open completely to maximise full throttle air flow. Back at idle, the ECU should control the valve position to attempt to correct the idle speed. At idle, adjust the idle speed set screw to make sure the valve does in fact move to try to correct the idle speed. Actually, it is simpler to tape/wire/whatever the throttle switch in the idle position, adjust idle to say 500RPM, then use the throttle to increse the "idle" speed and look at the valve. If the valve does not respond as expected, it is either a dirty valve, broken valve, broken wire, or broken ECU.

When you've cleaned the valve, did you disassemble it? The motor brush is trickey to get in place. If you just flushed the valve, perhaps you did not get everything out or you've actually washed dirt into the motor brush/contact. On my 760, I've disassembled it to clean, lightly sanded the commutator, used a dry contact cleaner on the brush, and also placed petrolium jelly on the bearing (and commutator I think).

Have you set the base idle? The procedure is described in Bentleys. With the car off you will be able to hear a click as you move the throttle off of the idle position (this is by hand with your ear next to the throttle body). If you do not hear that click you need to reset the base idle. This base idle is lost if someone tries to adjust the idle speed by adjusting the linkage rather than cleaning the throttle body and setting it correctly. Check for the click just as the throttle begins to open (this will be a micro switch like click- faint).

Randy

Incorrect ignition timing can cause this. Have you checked yours?

how would one check the ignition timing? i would guess by taking off the timing belt cover and making sure that it hasn't slipped a tooth, but i don't know. Sounds like it's worth a shot!

No, the spark timing. You need a timing light to check it.