1988 Sierra Adjusting Idle???

[stuneis]

Why is there no set screws for the idle on the carb? Obviously I'm no mechanic, but I wanted to turn up the idle only to find that the carb is connected to a number of wires, but no set screws.

Is there a way to adjust the idle on these things , or do I need to take it in?

Any help would be greatly appreciated.

Stu

[pcv]

That is a fuel injected vehicle and as such you are not supposed to adjust it.
However, first thing you gotta do is to scan the system to look at the sensor readings. If one of the sensor(tps, coolant, iac) is out of spec. it will affect your idle.
Also check the timing to make sure it is within specs.
There is an idle adjustment screw but it is covered by an inverted cap. Remove it and with a t20 torx scewdriver turn it clocwise a little. Too much and your IAC reading will be affected.
If it is not stalling, I'd leave it the way it is.

[stuneis]

Well, it does stall out when I come to a stop. These sensors you mentioned, can they be puchased at a parts store? Are they difficult to install. I was thinking I would end up having to buy a Chilton and reading for a while.

[V-8Fan]

Stuneis, DO read before you start twisting screws and trying to adjust things. The TBI system is not very difficult to understand, but I think you'd have to have some pretty sophisticated equipment to actually adjust anything correctly. PCV is absolutely correct! There may be a sensor that is bad, or you could even have an EGR problem.

In a few words, the TBI system works like this: the fuel pump keeps constant pressure available to the TBI unit. The computer reads all the sensors (coolant temperature, oxygen, throttle position, air flow, etc.) and determines how much fuel to allow the TBI to deliver in order to keep the mixture correct. There are two injectors in the TBI unit that receive electronic pulses from the computer. The injectors are either on all the way or off all the way---there is no "in between". The amount of fuel delivered by the injectors is governed by the pulse width of the injector signal. Another way to state it is that the fuel quantity is governed by the amount of time the injectors are turned on. As long as the sensors are operating correctly, and everything else is right, the computer will cause the injection to be right on the money.

I learned all this trying to cure a problem similar to yours on an '87 pickup with the 305/TBI. In my case, the EGR Valve was opening waaaay too soon, causing the thing to be on the verge of stalling when the throttle was barely cracked open. If I was you, I would take a close look at that EGR valve. You can see it operating, and even manually open it by pushing on the backside of it. I would assume that you can force it to close manually also. If you force it closed and the idle smooths out, there's the problem. There is a solenoid that operates the valve, and it could fail also.

Whatever you do, DO NOT try to adjust the idle speed. You'll only complicate the situation!

Hope this helps!

-Andrew-

edited to add: Sometimes I don't think first...you can close the EGR by simply pulling the vaccuum hose off of it (be sure and plug the hose so you get an accurate picture of how things are with the EGR closed and WITHOUT a huge vaccuum leak). It SHOULD close unless it's seriously BROKEN or fouled by carbon deposits.

[V-8Fan]

BTW, you can purchase the sensors at the local parts store. However, I was told that O2 Sensors are best purchased at the dealership. I don't KNOW that this is absolutely true, but thought I'd pass it along. Again, DO look over the EGR system before buying any sensors!

[stuneis]

Al right then. I have taken your advise and unplugged the hose to the ERG. I was rather surprised that the truck all of the sudden began to idle properly. I also took the IAC out and cleaned it up with some carb cleaner. I don't know if that helped since I had already unplugged the hose and had it running smoother.

The IAC in the truck is not an original. I was told that the originals had a thin thread wall and that I was not careful, they have been known to break off inside the throttle body. The one I removed was not all varnished and gummed up so I con only assume that my next step would be to look at the ERG.

Is this a reasonable deduction?

STU

[V-8Fan]

Stuneis, you're on the right track! In fact, I had forgotten all about the IAC and you were right to look at it too. I think now all you need to do is figure out whether it's your EGR Valve itself or the EGR Solenoid screwing up. My money's on the EGR Valve itself. Let us know how it comes out!

-Andrew-