Matt,

Something else I just thought of, I have had the chip for a couple of years and it has idled just fine until the last couple of months. Where is the proper place to connect the vacuum gauge to monitor the IAC vacuum you are talking about? On the passenger side of the throttle body is two vacuum hoses. The top one is larger. I show no vacuum on that hose at an idle. On the lower hose I read about 19. Do these sound right? Thanks again, I have a long weekend to play with this thing but I would rather take off the top and cruise around the island :cool:

Where is Lafayette? I use to party around Fremont, Angola area. Lot's of fun lakes in that area.

Aloha,
Larry


[Modified by Larry Mackey, 1:38 PM 11/22/2001]

 

Hey Larry, I didn't mean to mislead you. There is no such thing as separate IAC vacuum. What I was referring to was the fact that vacuum can drop if the IAC is opening up all the way.

To check this on my car, I just did the AB jumper and actually hopped out of the drivers seat and put my finger on the IAC valve. I could feel a small amount of pulsing, which is due to the factr that it was a stepper motor. I was satisfied that that meant my connections were good, so I waited 30 seconds and then pulled the plug.

I started up my car and measured the vacuum using a T fitting at the same line the MAP sensor uses. Note: I updated my computer to speed density so I have the same setup as you now. When I measured the vacuum there (realize the MAP still has to have vacuum so you have to use a T somehow), I got 16 in Hg. Then I plugged the IAC back in. Vacuum went WAAAY down to 7 in Hg, and at idle, this is just plain wrong. Mine did it because like i said I was way too rich. The computer tried to take out more fuel than it could, and therefore the idle was raised.

Now along the lines of what Dr J said...I had a bad connector on my ECM temp sensor. I would not always get a code for it, but I did end up finding an erroneous reading on Diacom with it like that.

Since you have had the chip for a while, I would sway that the chip is NOT to blame. The only other thing I can think of is to cut out a piece of cardboard and put it over half of the air intake opening for a few seconds. If the idle comes back to normal from choking off some of the air, I would suspect either the IAC wiring, the CTS (note, there are 2, one for the gage and one for the ECM. THe ECM is the one that matters). If worse comes to worse, I would check the resistance at the pigtail of the MAT (manifold air temp) sensor under the plenum. You can disconnect the pigtail and check the resistance there. If the car has been sitting for a couple of hours, the MAT and the CTS on the front edge of the lower plenum should have nearly identical resistances. THis is because they are both the same part number, and if the car has been sitting for a while, the coolant and the engine block have reached thermal equilibrium.

-Matt-

 

you can let the computer think that you closed the throttle plate manually by adjusting the TPS. On my 87 at idle it should read about .60-.62 to ilde 7-800 rpm, and 4.8 wot. use a voltmeter, top wire is +, bottom is - . Push a sewing pin through each wire so you can read them and with the car on, but not running, pivot the TPS unit and monkey with it until it meets the #s.

 

JSailor,

I have checked the voltage at the TPS and it checks good. Also note that the TPS on a 91 is not adjustable. No slots, just holes. :cool: Thanks for the help.

Aloha,
Larry

 

I added some of the new Prestone Octane Booster/Injector cleaner stuff to a full tank of gas the other day. Care was running fine with NO problem.

Next morning when I fired it up the idle speed went to 1800 and would not come down.

I did all the normal checks, to see if the butterflies were stuck, vacuum line off, on and on and everything was okay. I let the engine run at that speed for over ten minutes and Diacom reported that everything was fine. No code and all read-outs were normal: timing, IAC counts, 02 activity, coolant temp, injector mv on-time, etc..

After exhausting all the possible causes, I got out my torx head socket and manually lowered the idle speed and all was well again. I don't know what Prestone did, but the high idle never recurred at the lower TB setting.

Another one of those mysteries. Had to be the Prestone with MMT since no other change was made.

One way to ID a cause is to revert to the way the engine was BEFORE the problem occurred. That would mean reinstalling the original coil, plugs, etc.

Jake

 

You should not be setting your idle speed with the throttle body stop screw. The throttle body plates are supposed to be closed when the car is at idle. The IAC is supposed to set the idle speed.

Make sure the throttle body plates are closed when the engine is idling. The stop screw should just hold the throttle body plates closed. It purpose is to prevent the throttle body plates from be damaged when the throttle is closed quickly.

 

I agree and disagree with this. The throttle plates need to be opened enough to allow the engine to run when the IAC valve is fully extended (meaning it is fully seated and no air is bypassing the pintle). IOW, at 450 RPM (stock prom, etc), when AB is jumpered and IAC is fully extended, enough air for 450 RPMs is bypassing the throttle plates. This doesn't mean that the throttle is opened a bunch. So places say that every 20% increase in throttle opening is about 50% increase in airflow, but I haven't crunched the (pi*D^2)/4 to figure it out that much.

IAC's main intention is to keep the engine at the correct RPM once minimum RPM has been established with the set screw. In the winter, the stock programming will have the engine running at 1000RPM when cold, and it is the IAC opening up to allow for this. In the summertime, when called for RPM is only around 500, the IAC is nearly seated. IAC also raises engine RPM when the AC is on, because at lower RPMs, there is not enough HP to turn the engine and it would stall.

I am kinda from the old school that agrees that idle is controlled by the combination of air entering around the throttl plates and by air that bypasses the throttle plates by means of the IAC. A good idle relies upon both to keep idle correct and engine vacuum in line. -Matt-

 

I looked in the service manual and your right. The stop screw is for setting the minimum idle. :)