NO control of Idle at all

[huntman58]

Okay see if rewording this will help.

I have a 93 XLT 4 door and I can not set a base idle on it at all nor make it slow down when unplugging the AIC! (I even made a gasget to block it off fully with no change.)

With AIC unplugged TP stop screw out and not touching the stop the truck idles at 850 to 920 RPMS.
Plug in the AIC and it jumps up about 1500 RPMs then comes back down to the same 850 to 920 RPMs in P or N.
If you put it in gear R or D it drops to about 750 RPMs it’s a bit ruff and or vibrating.
Turn the A/C on with it in R or D and it drops to 550 to 600 RPMs and very smooth.

MAFs cleaned and computer has been reset many times
Vac lines all check out so one would tend to believe an intake leak. I can not find it using WD 40, propane, carb cleaner or TB cleaner.

Is it more likely to be the
Upper intake gasket?
The one below the fuel rail?
Or the intake on the very bottom next to the heads?

It has a very slight miss also that seams to move around.

KOEO is all pass and KOER I get 411 and 412 can not control RPM normaly from to high of an idle. I have also gotten on and off O2 always lean for both banks. they show together or they are both gone never only one showing that also leads me to think intake leak before injectors.

It is not using any antifreeze so do not think it’s the heads at all.
I do not have much money to throw at it so I want to do it right the first time .I know if I go below the fuel rail I do plan to change the injectors also .

As a side note any one notice the dash TAC off by about 250 to 300 RPMs besides my self?

 

[maniac_3_2000]

I would actually say the computer isn't getting good readings. And unfortunately getting obd1 systems to actually give you codes that are worth a damn is hard. Honestly it does sound like your AIC is bad. If your seeing a drop in idle, the computer should see this as well, and adjust the AIC accordingly.

 

[huntman58]

I would actually say the computer isn't getting good readings. And unfortunately getting obd1 systems to actually give you codes that are worth a damn is hard. Honestly it does sound like your AIC is bad. If your seeing a drop in idle, the computer should see this as well, and adjust the AIC accordingly.

ya know thats what I was thinking but changing out the IAC dose nothing . cleaning it the same thing . even tried a new one and again the same thing no control at all.

 

[TedJ]

There has to be an airleak somewhere. The computer can't make the engine run faster unless it is also letting in air to mix with the fuel. With the IAC disconnected it can't do that, so its not a computer problem. I had a problem exactly like this except on on a 5.0l. Did all kinds of playing with the IAC, found a vacuum leak in a reserve can, but the actual problem was the upper intake manifold gasket had a 2in section missing.

Those 5.0l intake gaskets are more of a known problem, but I would suspect you have some kind of vacuum leak. Check the external stuff first and then look at all of the gaskets.

If you can do the KOER tests, you might get a code like unable to control rpm low, but it probably wont give you anything else useful.

Also, you should probably just try to return the idle set screw on the throttle body to the original setting. It's not the problem and won't fix the problem, and you can cause problems with the throttle body sticking.

 

[huntman58]

Tedj I here ya. I did put the screw back to were it was just for that very reason. I also have done the KOER test and did get the no control idle high and low codes. I do believe it’s in the intake system but was hoping some one would be able to say if it was a certain gasket over the others that seam to go out more likely then the others. If not have to run it this way tell at lest Jan. as that’s when I will have some cash to thaw at it. need to also do a compression check as I believe it also now has added to my problems a possible bad head gasket as cold starts are now missing a bit more and I am starting to use antifreeze . Just one thing after another but in Jan I will have the funds for heads gaskets and injectors if there all needed Heck may be that would be a good time for headers also. But then again maybe its time to say good by and go back to a ranger.

 

[TedJ]

I'd guess at the upper intake gasket to be the most likely to fail. The upper intake gasket set is probably <$20, so I would start there. It's just paper so it has a higher chance of blowing out. Use only FEL-PRO gaskets....

Its not that big of a deal to change it, and then you know if it's not that you need to really start tearing into the motor.

I would check / do the following things:
1. PCV hose + PCV valve - connected and PCV valve good
2. EVAP Solenoid / Hose - solenoid should be closed and hose good
3. EGR - apply vacuum to EGR, engine should run worse/stall
4. All the vacuum lines - plug each port one at a time and see if it makes a difference
5. Anything else visible - look around, use a mirror and a good light
6. Change upper intake gasket