take car to autozone and pull some codes off of it for free.  There are vacuum hoses that are hard to see under the intake that can be leaking, as well as at the manifold block on the firewall. If you have a vacuum gauge you can measure the vacuum to see if you got a leak.  Clean the throttle body with the appropriate cleaner, that wouldn't hurt to do nor cost much.  

diagnostic plug is under the hood, drivers side, under the wiper motor, near the brake booster.  Its an odb1 plug connector, so if you can get hold of a obd1 scanner that would be great. Or rent one from a local aut store, otherwise you need to jump some connections and count the blink codes which is a pain.   I didn't realize you were in Denmark, my apologies for making the assumption you were close by. You can use a vacuum guage to check for vacuum leaks.  Theres a "Spare" vacuum port on the vacuum distribution block mounted on the firewall.  It has 5 ports on it. One is the supply, other ports go out to brake booster, cruise control, ac. 

The car should run and idle just fine without the IAC plugged in. Just to be clear, the IAC is the long cylinder that is canted and on the forward side of the throtle body. The EGR valve is on the back. Take both of them off and clean them out with carb or brake cleaner. Make sure you can't just blow through the IAC - that would be a bad valve.

Put it back on and unplug the iac, you should adjust the TPS sensor, mounted on top of the with the KOEO (key on engine off) so that it is below 1V with the throtle closed.

Now start your engine and use the speed screw on the throtle body to set the minimum idle speed with the screw. ThE IAC will only add more air to keep the idle above this speed. The stock computer will try to keep the idle at 750rpms, so I would shoot for right around there.

Check the normal vacuum hoses for leaks. There are a lot of hoses under the upper intake that may leak. I had problems with the plastic vacuum manifold breaking that is located just under the passenger hood hinge. Just check all around there and make sure nothing is contributing to the idle speed. Spraying a can of ether may help (or explode, so be careful). Spray the brake booster, the vacuum lines and the two vacuum manufolds, one under the pass hood hinge, and the other on the firewall just behind the upper intake manifold. If the engine speed increases, that is a vacuum leak!

Keep in mind that in failure mode, the car should idle fine without the air meter, IAC and O2 sensors. It will not be driveable, but should idle just fine. If yours doesn't, something is out of adjustment, or leaking.

I had a 347 with a huge cam and no emissions equipment at all that idled just fine with the stock computer.

Good Luck!
BobN

From what I have read about the computer on the fox body, is that several sensors/actuators/valves/solenoids can be bad and it will default to a faulty runing override status where it might not even throw a check engine light, but there will still be codes stored to tell you that something is wrong. So just because the car might run with several faulty items  its basically just running in a faulty override limp mode and not at optimum performance. This is why its important to pull the codes, irregardless whether or not the check engine light is on.

I am of the opinion, that missing cats can contribute to the idle issue. There are four cats on a fox, and four o2 sensors that feed info to the computer as to how rich or lean the exhaus smells and the computer then tries to regulate the air fuel ratio based on the info it gets from the o2 sensors..If its not getting the right information, then the computer can adjust the air fuel ration properly and can cause idle issues.  

  Usually there are sensors (mil) that are put in the exhaust pipes of cars with modified exhaust systems (no cats, long tube headers, x pipes, etc) that basically trick the computer into thinking that the cats are still there and hooked up and feeding valueable info to the computer. . 

Fox has many things that can cause idle issues. You will have a hard time troubleshooting this without pulling codes.  If you have a paper clip and can count to ten, you can pull the codes without the proper obd1 scanner tool, provided that the cel light works of course.   
 

The MAP sensor will be a small flat thing mounted on the firewall directly behind the upper intake.....if you have a Mass Air sensor on your car, then it should have a vacuum hose connected to it from the intake manifold.....if you do not have Mass Air, then it should be open to the atmosphere to read Barametric pressure.

I am with BobN on this one.....remove both the EGR valve and the IAC valve and clean them THOROUGHLY with a carb or throttle body cleaner.

To revisit....re-check all vac lines especially those lines UNDER the upper intake.  Time and heat take their toll and there's more than one line under the upper intake.  

Your '88 is a Speed Density unit and if I remember correctly, one port under the upper intake is actually capped off with a rubber cap........it rots!!!  
I speak from experience because those lines on my 90GT were bad......but, not for long......

Remove the IAC again and clean the two holes thoroughly with a can of carb cleaner; keep the spray away from your paint.........and your eyes.....

Check your throttle plate (butterfly)......maually open it and see if there's any carbon build up in there....if there is......clean it thorougly.....

Did this idle issue just occur??
Did you do any other work on the car before experiencing the idle problem?

Good luck and keep us updated......


 

I have good news today about my Foxbody`s idling issue. I was lucky to get a little help from an "old" mechanic and he help me locate the problem. We tried to sort it out part by part. And we found out that it was the IAC that is causing the problem. The plunger inside it was not closed of when it was suppose to be so air could pass by even when closed. BobN wrote that if you could blow air from one hole to the other the valve would probably be broken.The tap from the Solenoid that goes into the plunger kept it open even when closed. So to try an eliminate the problem I made a gasket off some nylon, which was mounted in between the Solenoid and the mechanical part of the IAC and then BRAVO . I live in North/West of Jutland close to Thisted.
 A very nice weekend to all off you and again thanks for the help.

Last edited by Olling (8/08/2014 2:15 PM)

66fastback wrote:

The MAP sensor will be a small flat thing mounted on the firewall directly behind the upper intake.....if you have a Mass Air sensor on your car, then it should have a vacuum hose connected to it from the intake manifold.....if you do not have Mass Air, then it should be open to the atmosphere to read Barametric pressure.

I am with BobN on this one.....remove both the EGR valve and the IAC valve and clean them THOROUGHLY with a carb or throttle body cleaner.

That info is backwards , 66FB.  The sensor is purely baro pressure in a Mass air system and the  port is open to atmosphere.  In a speed density system the port is connected to the manifold so it can measure absolute manifold pressure (vacuum) to allow the PCM to calculate the mass of the intake air.

BB