Re: stock 1989 TPI automatic.

The car is idling too fast and I'm looking for any suggestions before heading over and simply adjusting the idle via the throttle body linkage.

Would a dirty IAC have these symptoms?

What is the normal fuel pressure and idle rpm in drive?

thanks

 

There is nothing to adjust. That screw isn't going to move unless it's gets played with there's no reason to touch it. If the throttle body is so gunked up that it's restricting air flow, then clean it - but the ECM can compensate for most conditions.

Idle Air Control Motor is controled/pulsed by the ECM and specs are 50 +/- rpms from Targeted Rpm in Drive; 100 +/- rpms in Park/Neutral. You'll need a Scanner to view the Targeted and Actual RPMs. If within spec, capture the Coolant Temp Sensor. It's the same as a choke meaning the colder it is, the higher the commanded RPM's. If the signal is off, commanded idle can be too high/low for operating temp.

 

Does the idle adjustment screw still have a plug in the bore it's in? If not then you may need to do a minimum air idle procedure, someone may have played with it but as stated, you can adjust it all you want and it will only mess everything else up. If the IAC is dirty it could be a factor, or if it is not working it would definitely be a factor.
First I would look for a vacuum leak, that will cause high idle,,, these cars are notorious for vacuum leaks.
Second, I would check the setting of the tps, if it is set more than .54 that will cause a high idle also.
If none of this helps then I would do the minimum air idle procedure, if you don't have an fsm then do a search to find out how to do it.

 

Also, check for any linkage hang ups.
Here is a write up on min idle and TPS idle volatge.
http://www.lbfun.com/warehouse/tech_...ch%20Paper.pdf

 

Just had a chance to look at the car! It's actually a 1987...

When it pulled in idle was at 900-1000 rpm range. First thing I did was check for codes and found:

32 EGR
36 Burn Off Diagnostics

Then checked the stream..., closed loop, coolant temp at 192 and rpm 1025.

Checked for vacuum leaks with my water bottle spray and feel confident it does not have vacuum leaks. TPS at .54 volts

Then removed the IAC (came out hard) ..it was very dirty, pintle carboned up..cleaned it along with the very dirty throttle body plates/passage. Reset the IAC, installed it, did the A-B connector jump, allowed it to reset, removed IAC plug and A-B jumper and started it.

Adjusted idle to 450 in nuetral!!! then shut engine off and restarted engine. After a short time later it adjusted itself to 600-700 RPM range. Runs good, but idle is too fast again, no codes yet.

Any ideas?

 

If the "high idle again" condition only occurs after you drive it:

Ensure the throttle linkage is totally back against the stop screw @ idle....physically push it back to see if there is movement and/or see if th idle drops back down. Check for sloppy bushing (driver's side) on the throttle body. Grab the throttle linkage where the cables connect to the throttle body and see if you can feel any "slop" up, down, forward and backward. The thought is the throttle shaft bushings are worn and the blades are hanging up inside the bores.

 

eagle1147

I did try that...wondering if the IAC could just be bad?

 

Could be jamming in the bore so if you want to throw some Bucks at it, replacing it might work. When you do, reset/verify miminum idle at 450 rpms and then, start it by depressing the accelerator slightly and let it run for 10 seconds. Then off for 5 seconds and when you restart it normally, it should be within specs. Otherwise, you need to scan it to compare actual vs. targeted and I'd clear those trouble codes too.

 

SunCr

Thanks for the suggestion, I picked up a 1-1/4 deep socket this morning and tuned down it's diameter, I'll be able to remove the IAC without removing the throttle body. I have a few working IAC valves on the shelf.

Hoping that the IAC R&R will correct the idle problem.

I'll move to the EGR and Burn Off problem next...

Question: on the 87 Vette where are the relays located (ie burn off relay).

Thanks

 

Is the timing spot on? Fuel pressure?

If those are good, borrow a scanner and set the IAC to about 20 counts, give or take then set the TPS.

 

Besides IAC operation and OEM positioning of the idle screw, those are the other two things I thought of. Having too much of either timing or FP would raise the idle.

Custom chipping could too.

 

Thanks...the idle can be adjusted right on at 450 RPM and the car runs great - ie after IAC is reset & reinstalled (but unplugged), it (idle)only jumps back up when the IAC is plugged back in.

 

Sounds about right. What does it jump back to?

 

700 to 800 RPM range, the reason for the large range is that I 'm reading the single digit digital dash tach which alternates between 7 & 8. The actual RPM is verified with my OTC scanner approx 785 RPM.

Ed

FYI: The car is my neighbors and may it may be over here again in the next couple of days. Where are the relays located on the car? I have a C3 (82) so I'm not that informed related to relay locations.

 

Question. How do you KNOW what the rpm you set it for is and what it is idling at? Did you use a scanner or an inductive timing light with the tach feature?

 

How are you suppose to set the RPM to exactly 450 using the digital dash that only displays single digits.

Are you or can you using scan software like Datamaster.

Also, can you use the smae software to adjust and set the TPS voltage to 0.54v ?

 

I wouldn't trust the dash tach that far. I use a scantool to set the min idle via the IAC and also the TPS.

 

As I mentioned in my post...my OTC Scanner was used to verify the exact RPM. Once started there was no need to hook up the scanner again..I could see and hear the car was idling too fast. The dash tach was alternating between 7 and 8 , previously the scanner indicated +/-785 RPM ..consistent with the dash figures.

My scan tool is the OTC Nemisys...I used it to set the TPS voltage, monitor the coolant sensor and make sure it's in closed loop...What count should I be targeting for the IAC?

One odd thing I noticed was as I entered the "stream mode" with the scanner I could hear the RPM drop very slightly, exit the data stream I could hear it pick up again.

Found this on another site:

Snap-on says the IAC counts should be between 60 and 150 when the engine is on but not running. When at idle with a warmed up engine, IAC counts should be between 9 and 20. If they are above 20, cleaning the idle air control circuits is recommended. Be careful with solvents around rubber and electrical parts. Snap-on recommends GM top engine cleaner (105262), AC/Delco cleaner (X66-A) or an equivalent cleaner that does not contain methylethylketone. The service manual explains this in more detail and also gives the torque values for reassembly. Use new gaskets when reassembling otherwise you risk air leaks. (I think the throttle assembly gasket kit was $10 at my local AC/Delco distributor.)

Be careful you don't force the pintel in; you can ruin an IAC motor this way.

There were design problems with early idle air control motors. If yours is original, it might be easiest to just replace it (for about $65). According to Snap-on, the updated IAC motors have a black pintel spring and part number 17112351.

There's a simple test for a stuck IAC motor; there should be an RPM "flare up" when you first start the engine. If you have a scan tool, you can check to see if the IAC counts follow this.

That post was in response to an F Body with 3.4 but the IAC is the same part number for the 1987 Y Body. I'll make an effort to change the IAC. I just turned down a 1-1/4 socket that will allow me to change the IAC without removing the throttle body.

 

Wait a minute...I thought it was supposed to idle at 450 w/o the timing connector plugged in. (And, I believe that "deactivates the IAC.) So, the 450 idle point is ONLY for reference -- NOT normal operation.

Maybe you're unaware that the ECM has settings in the 700-750 range for idle. That's where it's supposed to idle. I have a stick, but I'm guessing an auto is supposed to idle slightly higher, probably between 750-800 -- which is right where you reported it!!!

Because the ECM/IAC has the ability to "learn" where it's supposed to be -- whatever you do -- the idle WILL return to the computerized settings -- unless you really screw up. The car is only supposed to idle at the 450 number while setting the idle screw and distributor. Once that's set, connecting the timing connector creates the connection to the computer (ECM) and the IAC varies air flow thru the lower port in the TB. It learns where the IAC stepper motor needs to be positioned to induce the necessary air flow to hit the programmed setting inside the ECM. The only way to change that is by altering the "Idle by Coolant Temp" table. You need a computer connection and chip to accomplish that.

 

You disconnect the timing connector to prevent the ECM from advancing timing which would raise the idle and pretty much make it impossible to set the correct minimum idle speed.

Autos and Sticks do not target the same idle rpm, but all most all GM's have a minimum idle speed of 425 +/-25 rpms as the Minimum spec (and the throttle bodys were set to whatever the spec is/was during production and you don't accomplish anything by changing it).

If you've extended the IAC pintle fully into the bore or replaced it, you get it working right again by depressing the accelerator slightly when you start it and then turning it off after it runs for 10 seconds. Not doing this isn't fatal as it will eventually get back to it's correct position; but you may have an idle of 1200 to 1500 rpms for a day or two.

The IAC acts like a dashpot to prevent stalling AND is retracted to allow plenty of air for a quick restart when the engine is turned off. Simple test of the IAC is to note rpm's; turn it off; restart and it should have higher rpms, but return to what you noted initially. If that works, there's nothing wrong with it.

IAC counts, for the most part, are meaningless as a diagnostic tool. There are no specs and there can't be since engines can vary from car to car and there is a range for Minimum Idle which is built into the tuning - ditto the TPS and some other things that are sometimes fiddled with when they should really be left alone.