Varying idle speed

[Squidders]

Hi,

Looking for some advice on my 1980 FI spider. I have been following and using Brads A's guide to effectively tune the car. Getting the car to operating temp (2 x the fan initiating). I have set the idle speed as instructed, 800 -900 RPM via the accelerator linkage stop screw and then 850 -900 RPM via the idle speed screw. I have removed the screw cleaned the screw thread and orifice. However after doing a test drive around the block the car seems to idle at 1300 -1400 RPM.

Tried multiple times to correct this by starting again and by dropping the RPM slightly each time in the hope for it to not reach the high RPM's, however same results. So I just dial back the revs and the the car seems to be too low at idle. It seems the car doesn't hold the same idle speed and when I connect a multi meter with a RPM reading/ setting it does vary and you can hear the slight change in revs.

The TPS is fine and can be heard when the engine is running and also when the engine is off and you move the accelerator cable/assembly by hand. I have also checked the aux air reg and it works fine. Air flow sensor and fuel pump work fine.

Should I replace all of the 7.5 mm hose in case there is an air leak somewhere? Any help or suggestions would be appreciated?

[wizard124]

Air leaks may be the cause. Also, make sure the hood liner is not drooping insuch a way that it is making contact with the throttle cable and pivot up on the intake plenum. This could be preventing the plate from returning to idle during your test drives.

[Kevin1]

Replace the o-ring for the idle speed screw and confirm there are no air leaks between the AFM and the throttle body.
If you changed the throttle stop screw to obtain 800 rpm the throttle position sensor MUST be readjusted using a multimeter or test light - listening to it will not tell you much.
Wandering idle speed (FI) is almost always related to an intake tract air leak. Remove the plastic duct between the throttle body and AFM, inspect it carefully for cracks. They get brittle as they age.

[Squidders]

Thanks Wizard. I will check the hood liner, never thought of that.

Kevin, The o ring has been replaced. I'll revisit the sensor again. The main air hose between the throttle body and AFM is a near new replacement.

[Squidders]

OK, update, the throttle positioning sensor is working correctly and set via a multi-meter. You can even hear an audible click when you adjust and set it up. So that's not the problem. The hood liner is also not causing an issue.

I guess the next step is to look for any air leaks with all hoses. Might take me a while as I will need to do it systematically as time is a bit limited.

[Kevin1]

The easiest (but a little messy) way I have used to find an air leak is to give a short spray of WD40 around each count in the intake one at a time with the engine running. If there is a leak the WD40 will be drawn into the air stream. You will know when this happens because it will make the engine slow down and/or stumble.

Whatever you do, don't use anything flammable - like starter fluid.

[4uall]

This might help as well and you get to celebrate with a cigar

https://www.youtube.com/watch?v=zMok2y05jNE

[Squidders]

Thanks Kevin will try the WD40. Failing that!

4uall I will give "Scotty Kilmer" video a try. and smoke the cigar.

[micbrody]

I had same problem with my '81. I had a cracked hose for axillary air valve; fixing that helped ; but I still had variable idle issues. I then discovered that the brake booster, though functioning fairly well, had a slow leak that would occasionally "pop off" into larger leak. I changed booster, and idle very good; along with easy starting of engine

[KevAndAndi]

The only thing that solved my wandering idle problem was to get the AFM refurbished. It ran like a new car after that. Air leaks are the more common cause, though.