Idle Speed Adjustment

[johnssi]

This may be a dumb question but.... how do I adjust the idle speed? I've looked through both service manuals and there's actually no mention of it. The engine was idling fine, around 800 rpm's, until I found that the Lamda sensor wire was unplugged early on when I first got the car. Since I plugged it back in, it's my feeling that the engine runs faster after a good warm up period, around 1300 rpm's. I'm not even sure if the two are related, hence the question. I know about the idle speed screw on my 81 spider, I messed with it and all I got was much slower, maybe 3 to 400 rpm. Any suggestions. At speed and load it runs just great, just seems to be idling to fast. Hmmmm?

[So Cal Mark]

I'll assume you have an FI engine. Don't use the idle stop screw for speed adjustment! That is only for setting the throttle closed position. Use the air bypass screw on the throttle body, just inside of the tps. It faces up, towards the right fender. Turning the screw clockwise lowers the idle speed by closing off the air bypass

[johnssi]

Thanks Mark, I tried that and it worked great. I've got it running at about 800 rpm's after warm up. I'm glad I asked cause I was thinking that it was a dumb question, but it gives credence to the old saying that "the only dumb question is the one you didn't ask"! By the way what does TPS stand for... throttle position sensor? Thanks again, John

[So Cal Mark]

throttle position switch on your car, throttle position sensor on cars with a potentiometer type tps

[jimincalif]

My car also idles at about 1300 when it is good and warm. I tried adjusting the idle down using the air bypass screw, worked fine when the car was warm. but then when it was cold it would not start, I put the screw back to its original position and it started fine. Any ideas?

Jim

[So Cal Mark]

you need enough air flowing through the afm to move the flap. It's possible if the aar is not working, and the bypass screw was closed down that there is too little air flow to move the flap. How's the cold idle speed? Is it higher than the warm idle speed?

[jimincalif]

the cold idle speed is low, just a few hundred initially, then increases as temp comes up.

[So Cal Mark]

sounds like the aar isn't working and the idle speed is set a little high. It should be high when started cold, then gradually slow down as the aar closes upon warmup

[jimincalif]

OK, I will try some tests. Based on some searching here it looks like I should try pinching off the aar hose when the engine is running cold. If no difference in how it runs, probably not working. Also check for voltage to it.

[jimincalif]

I confirmed that I do have voltage to the AAR. The car started normally (very slow idle when cold) with the AAR disconnected. How do these things work, do it sense the temp of the engine as it warms, or does it have a heating element in it, or something else? Do they default to closed, or do they just get stuck? Are the repairable, or just get replaced?

Thanks.

[rlux4]

It's an electrovalve, Jim. Its normal position is closed. When cold it gets a signal to open which lets a metered amount of air directly into the intake plenum. This raises idle speed as more fuel is called for by the ECU to compensate for the increase in air intake. Until recently there were only two ways to replace them; find a good used one, or buy a new replacement, which are very pricey. Now however, good ole' Mark has rebuilt units available.
Brad Artigue has a very good Bosch L-Jetronic system explanation that includes all of the components and how they work in his maintenance and modification manual. You should download it if you haven't yet;
http://www.artigue.com/?attachment_id=5
Ron

[jimincalif]

Thanks Ron, yes, I have Brad's FI manual. He does explain that the AAR lets in more air and then closes as the engine warms, but it doesn't seem to say how it works. Is it an electrovalve controlled by voltage or by heat? I disconnected mine and put my meter on it. When I started the car cold it registers full voltage, I let the car come up to operating temperature it still registers full voltage. If it is controlled electronically, then at some point the ecu should turn it off, right?

Jim

[So Cal Mark]

when the aar is cold the disc inside is wide open. It has a heating element inside, so as soon as it gets power it starts to close, thus the need for constant power to it. Otherwise it would open again raising the idle speed. The default position is open. They can fail with the movable disc in any position. I've seen them everywhere from shut to wide open. I do have rebuilt units in stock, $100 exchange with a 1 yr warranty

[jimincalif]

OK I may take you up on that. If I replace the aar I also need to replace the hose, as it is pretty old looking. Figure if I buy a bit of extra hose I can then temporarily bypass the aar and confirm that it is not opening as I assume with just a hose there it would be wide open and I would see a higher idle speed.

The hose looks like the same stuff as used for the brake boost, is it? That's old too, so I should replace it as well. Needs to be the sort that won't collapse under vacuum, right? Any recommendations on what to get? At the local parts stores around here I might as well be speaking Italian when I ask for something.

Thanks.

[So Cal Mark]

brake booster hose is reinforced. I do have the fabric covered hose in stock. Once the aar is off, merely look inside to see how large the hole is. It sounds like yours is closed completely