I am having a weird idle issue on my 90. After I set the idle by unplugging the iac valve and all that my car idles perfect! After I plug in the iac and start the car it idles high again at about 1000rpm. Ive checked for vacumm leaks, new iac, new tps (voltage is good), new oxygen sensor and I cleaned the throttle body and put all new gaskets. Any ideas as to what is causing this?

 

The computer will overide a low idle setting to help with emissions. I like my idle between 650-750 RPM and the computer overrides it, it is SO annoying. You should also notice it when you take you foot off the gas when cruising.....the idle doesn't drop right away, it holds at maybe 500 RPM below where you were to burn off the fuel and not spike you HC's. I hate it, but I don't know of any tuning places that can remove it, I believe a DFI setup may be the only way to truely remove it.

 

I hate to say this, but it might be your ECM. I've heard they can do this without throwing or storing a code, too. Makes it very vexing to correct, that's for sure.

 

Go here

http://www.thirdgen.org/tpimod2

I think you have another step to complete.

 

The idle rpm is controlled by means of a PID controller within the ecm. The primary input for the desired rpm is a table of rpm vs coolant temp that has a generally decreasing trend. This should be controlled within 25-50 rpm of the target

Here is the table from my 88 when it was stock

For temps between -40 and 8 C = 1200 rpm
20 C 850 rpm
32 C 800 rpm
44 C 800 rpm
56-152 C 700 rpm

Also there is a park position table for IAC counts vs coolant temp which is generally used when you are rolling and off the throttle. This table is not rpm based, but uses IAC position vs coolant temp, but is only active above 6 mph or so.

In addition there is a throttle following mode which opens and closes the IAC in response to throttle position. It Opens up when you're on the throttle and closes slowly when off the throttle to provide some dashpot function.

From your description I would guess that you have slightly > 0.54 v tps and are entering throttle follower mode which adds rpm. This begins around 3 % throttle.

Other things that can affect idle rpm are ignition timing and vacuum leaks.

There also seems to be a learning function to aid the controllers with historic values, you could try disconnecting the battery and restarting and allow the car to idle for a few minutes with the a/c on and a/c off to permit for some new learning of IAC position in the current state of tune.

 

Is this procedure for OBD I or II vehicles? Will it work on a '92 LT1? I have a high idle and I think it's either a vacuum leak or dirty throttle body/IAC valve.

Should I clean the TB and IAC first before attempting this procedure (if applicable?)

 

YES!

 

So that link is applicable to OBD I LT1 engines, too?

 

correct

 

The link doesn't really make it clear, but when you disconnect the wiring plug from the IAC. The key must remain "On" and the "A" and "B" terminals of the ALDL must still have the jumper attached while the IAC is being unplugged. When you "set the idle by unplugging the iac valve" it really shouldn't end up with "my car idles perfect!". The procedure you describe is for the minimum air adjustment. The minimum air idle speed is generally about 400-450 rpm, and doesn't leave what most would describe as "my car idles perfect!". The actual idle speed is burned into the prom. The only two ways to alter your idle speed is one, to have a chip burned with the idle you want, or two, do a hack job by setting the idle at a speed higher than the IAC will try to regulate the idle to. Chances are that will cause you to lose some of the benefits of the ECM controlled IAC.

RACE ON!!!

 

I had this problem on my '91. Took me ages to solve it.
I tried pretty much all of the above and some other stuff, but not a new ECM. In the end a new trottle body coolant passage plate and (another) new IAC valve cured it. Idles perfect now.

Could be your tach though. Can you actually HEAR the higher RPM's, it may be your tach is out which i hear is quite common. My tach reads 120 RPM's higher than it shoud be doing which can make it look like a high idle.

 

Thanks. I'll start with a good TB/IAC cleaning. That did the trick two years ago. Recently, though, my base idle's been off and I'd been wondering where to start.

I replaced my EGR valve Friday (Code 32) and noticed the rubber diaphragm on the back of it was ruptured. I thought that was causing my high idle, but the problem persists. I'll start with the intake couping and work my way back. Thanks again.

 

My tach is off but I do hear it idle way to high.

 

I did the whole wire jumping and tps setting. What i mean was when I unplugged the iac, set the idle with the screw the idle was what it should be. After I plugged the iac back in and set the tps voltage my idle jumped about 1000rpm.

 

So the idle jumped up from the 400 rpm minimum air setting to 1000 rpms? Cleaning the IAC and the IAC passages are never a bad idea, but when gummed up they usually don't pass enough air. You engine is getting too much. I vote for the ECT sensor, or a bad IAC housing or housing gasket. If the ECM isn't trying to open the IAC that (ECT sensor) much, then there is an air (vacuum) leak somewhere.

My other point was that ALDL jumper has to remain in position while the IAC was being unplugged.

RACE ON!!!

 

Just try the final step in all FSM's first. Depress the accelerator slightly and start it - let in run for 10 seconds - then off for 5 seconds - then restart and it might just be working right. That is, 50 +/- RPMs from targeted in Drive and 100 +/- RPM's in Neutral or Park. One final test to verify ECM control. Start and idle after the motor is warmed up. Note RPMS. Turn off and immediately restart. Idle RPM should now be higher, but return to the previous # after a few seconds.