How do I kill this automatic function?
 

I have an 87 TBI 350 silverado. Which seems to pick and choose at random, when its going to engage that weird, higher RPM engine warming function. Its when the engine will idle higher, in some kind of seemingly unnecessary attempt to warm up... better? Faster?

I'm told that it has something to do with the cold, but it doesn't always do it when its really cold outside. It does it when there is... like an average temperature change. Here in TX I do not need the function. And its malfunctioning.

It won't do it in the summer, unless we get one of those rare cooler nights. Then it may do it. But as a general rule in the summer the truck won't do it. Its in the spring and the fall when its the worst. In the winter it doesn't really do it much either, although there are times when it will. Today I was sitting in the drive thru, and it was idling at 1500 rpm. I've seen it go to 1700 when its doing this. And its hard to get it to stop. It just decides all on its own when its going to stop, but this is usually after I've been driving for 10-20 minutes. Its really annoying and I'm sure people around me are wondering why my truck is making so much noise.

So how do you kill this ill conceived function? I don't need it, and I don't ever want to see it again. I just want the truck to always start up and idle the same.

Re: How do I kill this automatic function?
 

There is a temp sender that the computer sees to know when to enrich and bring the idle up . This sender may be out of calibration in those temp zones you are talking about .

Also you may want to look into the air idle control . It could be dirty or malfunctioning .

I don't think you can do away with this function . It has to work together as a whole .

If you get the problem fixed it will work as you want it to .

Re: How do I kill this automatic function?
 

Could be your a/c circuit controlling the idle.
My 84 did the same thing. I simply unplugged the compressor on the a/c so it wouldn't cycle on/off and the idle never changed anymore.

Re: How do I kill this automatic function?
 

Do you happen to know where this sensor is located? Perhaps just replacing it would help. Its probably from 1987 so it could have gone lousy by now. The truck does have new coolant temp sensors. Well newer. IIRC I replaced them about 5 years ago.

Re: How do I kill this automatic function?
 

"New" doesn't always mean functioning correctly. Coolant sensor should be in the front of the intake near the thermostat housing.

Coolant temp sensor is separate from this sensor.

Make sure your cooling system is working normally, low coolant temps will effect the sensor making the computer think the engine is still cold.

Re: How do I kill this automatic function?
 

It may even have a code stored to tell you what is out of spec.

Re: How do I kill this automatic function?
 

As Desert said , its right in front of the throttle body . This is the one the computer sees . The one in the rear of the head on the pass side is for the gauge .

Which one did you replace .


Cadillac has a good point . Check to see if any codes are set . There is a way to check with out a scanner with the flashes of SES light . May give us a direction to go .

Re: How do I kill this automatic function?
 

Now I'm questioning if what I replaced was on another truck I used to have. An 81 lol. So I guess I'll scan the codes (I have scanner) and if nothing I'll replace...whichever one it is.

So to be clear, the one that governs the function I'm complaining about, is right in front of the throttle body? No wait, I went back an reread and desert said its a separate. Anyone know the exact location of this sensor, or otherwise how the computer gets the information to turn up the idle?

Re: How do I kill this automatic function?
 

As Desert said, its right in front of the throttle body ( this is the one for the computer ). This is the one the computer sees and you are concerned about . The one in the rear of the head ( by the exhaust manifold ) on the pass side is for the gauge only ( only makes the gauge work , not concerned about this one ) .

Re: How do I kill this automatic function?
 

Download WINALDL free of charge. just buy or make a cable and then you can data log in real time and take the guessing out of it.

Re: How do I kill this automatic function?
 

My 93 model was doing the same thing. Changing the sensor didn't help me. what did help was cutting the ends offa the grounds at the thermostat housing & crimping new ends on. Then get all the grounds on the manifold stud that has the neg bat cable on it. Fixed mine.

Re: How do I kill this automatic function?
 

most of the new temp sending units come with a new wiring pigtail to use and most up the time its just the wiring connection that causes problems

Re: How do I kill this automatic function?
 

Benny,

Have you found the problem? I can say that without data logging you may never find it.

Here are a few things that will cause high idle. without reading the computer you will have to guess.

plastic shift indicator at the base of the steering column is broke or cracked and the truck thinks it is in gear when it is not

02 sensor to far from the manifold and not going into closed loop ,needs heated 02

temperature sender for the computer is off and reading cold.

possible vacuum leak.


TPS set wrong.

here are some screen shots of the program. you will find your problem in one minute. If you want to PM me i can walk you through it.

My TBI idled high and i wasted a lot of time and money guessing. My issue was the shift indicator and 02 sensor.



sticking IAC.

calling for AC by default.

Re: How do I kill this automatic function?
 

Hey! I didn't know that TBI went to closed loop! Learn new stuff ever day!;)

Re: How do I kill this automatic function?
 

On mine the 02 sensor was at the y pipe ~2' from the manifold. It would only get hot enough to operate at highway speeds. If you look at the next generation manifold, the 02 is mounted right at the #7 cylinder.

Re: How do I kill this automatic function?
 

Yeah I thought about using that WINADL thing but I don't have a cable.

Is anyone familiar with what its supposed to be doing? As in, when its cold outside, is it supposed to idle higher? I really just don't like the function. I don't ever need this thing to idle higher. And its definitely related to the cold. But I guess you'd have to flash the computer to kill it. I do have a scanner that may be able to read the codes though. I haven't had time to work on the truck lately. But when I do I'll post up the results of the scanner in this thread.

Its got several other much more pressing problems as well that I have to fix first though. :lol: Like a starter that doesn't engage the flywheel and a power window that won't go up. When something breaks, I've got to stop saying "Eh I'll fix that later" cause they all add up and cascade on you.

Re: How do I kill this automatic function?
 

Benny,

The idle would have nothing to do with ambient temperate. Only the engine coolant temp.


here is the cable
http://www.moates.net/cabl1-obd1styl....html?cPath=64

there are others likely cheaper but moates stuff works.

odds are good you do not have a heated o2,, Also
there is a white plastic tab at the base of the steering column that crudely operates a switch that tells the computer is you are in gear or not. It adds IAC steps under certain parameters. This may be the cause of your problem but it is is not as much an issue when the motor is warm because there are other parameters that limit the idle.

There is a myriad of code the work hand in hand. When one is flagged it will cause other problem.


I can promise you the cable will be your smallest expense if you try to guess instead.

Re: How do I kill this automatic function?
 

Well yes but the ambient temp will of course lower the coolant temp if its colder outside. And that is when I notice the issue. Once the truck is warmed up, it won't do it anymore. Say for example if I go somewhere: I go inside for a bit, and come back out, and the engine is still warm enough (on a cold day). It won't do it then.

Re: How do I kill this automatic function?
 

I have the same issue occasionally with my R30. Plus keep getting a EGR code and been chasing that with new parts. Just bought an ALDL cable. I'm done going at this blindly. Ordered it from here - http://www.aldlcable.com/

Re: How do I kill this automatic function?
 

My truck also has a problem with the EGR. Wonder if its related in some way to the issue...

Re: How do I kill this automatic function?
 

Once the engine is warmed up and idling normally, unhook the wires hooked up to the IAC valve. Now the idle will be set. Yes, you'll have to apply some gas pedal pressure during cold start ups but that's not hard to do and get used to. Plus, it prevents others from driving it.

Re: How do I kill this automatic function?
 

Thanks Ed I'll give it a try. In your experience does this somehow... kill the problem I was talking about?

In my experience it idles fine when its cold. Again I'm in TX, and we don't really have cold winters. When it doesn't go into high idle mode on a cold day, it still idles fine. Maybe a bit lower than normal, but its a fuel injected truck and I usually just drive off anyway and its well on its way to warming up.

Re: How do I kill this automatic function?
 

Just give it a try and report back. Make sure you disconnect the wires to the IAC while the engine is idling.

Re: How do I kill this automatic function?
 

Benny,

Listen to the wisdom of Dieselsj, You will spend nothing but money and time throwing darts at it.

If you want to stop all IAC functions do as edahall suggested or set the IAC to zero this way.

1) With the IAC valve connected, ground the diagnostic (ALDL) terminal (same as you would do to flash trouble codes thru the check engine light).

2) Turn ON the ignition, but do NOT start the engine. Wait at least 30 seconds. The IAC will go to zero.

3) With the ignition still on, disconnect the IAC electrical connector. This will leave the IAC at zero and with no connection it will remain there.

4) Remove grounding of the diagnostic (ALDL) connector and start the engine.

5) Adjust the idle stop screw on the TBI.


If your IAC is working right it is nice to have. It adds counts for power steering, ac engaged, cold starts, when in gear.

I can't urge you enough to take advantage of the free (donation) software. There is most likely more going on than you will know and a little more MPG will pay for the cable.

good luck.

Re: How do I kill this automatic function?
 

Got my cable yesterday. It came with a CD that had more than a dozen different scanning/logging programs. I loaded up winaldl and went for a quick drive.

- I have CEL flags that weren't registering when I grounded the pins on the connector and read the flashes. The only code that showed up was for EGR. I also have codes for TPS low voltage and ESC.

- My knock counts are out of control. I'm seeing counts of several hundred on each cycle. Actually it cycles between just a few counts and several hundred.

- Confirmed that my temp sensor is reading correctly.

- IAC operation seemed fine

- I am slightly lean everywhere. This was expected due to the high-flow cat and 3" exhaust that I added. It seems to be within the parameters that the computer can handle though.

Hooked up the timing light and found that my initial timing (with plug disconnected) is sitting at about 10 degrees and is bouncing around. Didn't have time to adjust it. While I wouldn't think 6 degrees of extra timing would really hurt on a sub 8:1 compression engine, the combustion chamber might just suck enough to detonate anyway. Need to investigate further to determine if timing scatter is due to bad/failing ESC or timing chain.