TT Oversteer

I've got the new cat installed and still sorting things out before going back for another attempt at CA smog testing. My idle is a bit high at 1050 rpm. I understand the correct speed is 850 rpm for the later ('89) Motronic system?

I'm a little confused on how exactly to set the idle speed for this engine. The book says to disable the idle volumetric control by jumpering a wire across the test port socket then adjust the air bypass screw to achieve the desired idle speed. If everything is copacetic the idle isn't supposed to change when you disconnect the jumper wire.

So here's the source of my confusion:

1) The tech (with 30 years experience) I talked to said the computer controls the idle speed and you can't set it (I'm guessing he's wrong).

2) My air bypass screw is full clockwise which is the lowest idle setting and the rpm's are still too high by about 200rpm. If I turn the screw counterclockwise the idle speeds up immediately.

3) What about the screw adjustment on the throttle plate? Wouldn't the position of the butterfly in the throttle body change everything? How do you check/set that?

To all the esteemed experts on this board who know the correct procedure and sequence of steps to set idle speed on a Motronic engine: Please clue me in! Thanks!

DRACO A5OG

He is incorrect, the base idle can be set.

1st you need to jumper the ICV, then set the base idle to 880 RPMs (just below middle of the 1000 RPM mark and the 800 RPM mark)

You should not have to mess with the throttle plate but need to check if it does open all the way. You should hear a click when the throttle is opened all they way (with the Car Off, but ignition to on position)

Hope this helps,

Jim

Peter Zimmermann

Draco is exactly right, but didn't tell the whole story . Here is how you do it:

1. Open test port on catalytic converter.
2. Run engine until it's at operating temperature - this is usually ideal just after the front oil cooler becomes hot to the touch - about 90 degrees C.
3. Disconnect oxygen sensor plug - left side of engine bay.
4. Check CO% at test port. Use a flexible screwdriver with 1/4" drive Allen socket to change mixture as necessary (spec = .6 - 1.0%). Adjustment is done on the air flow meter.
NOTE: Work quickly, excessive heat in the intake will skew your CO% numbers!
5. Bridge test ports B & C on the electrical panel test box ( "B" is at 1:00, "C" is at 6:00) - this stops idle speed regulation.
6. Check & adjust idle speed (use a digital device for accuracy) by turning the control screw on the throttle valve housing.
7. After adjustments are made remove your test bridge, connect your oxygen sensor, rev the engine and see where it settles. You may have to do the whole process a second time if your settings aren't very close to begin with.

The above order is crucial in gaining a correct result.

TT Oversteer

Thanks Jim and Peter,

I haven't had much luck with the "professionals" in my local area so I'm going to attempt this myself. I don't have the required tools to hook up to the test port on the cat so I will reinstall my hollowed-out cat and just probe the tailpipe for CO readings. (I assume this is the same as testing upstream of the cat). I'll be looking around for a suitable gas analyzer in the meantime. I'm hoping the CO is set wrong and that gives me some adjustment in the air bypass circuit because right now it's fully closed (clockwise) and the idle is still too high. Service records show the CO was set previously but I'm skeptical as to the competance of the shop that did it. The CO was set in Boise which is around 3000 feet above sea level and now the car lives closer to sea level. Would that make a noticable difference?

dshepp806

Your guy is correct in that the ECU sets it.

You're simply establishing the starting range for the ECU to control ("base idle",..NOT "idle",.... ECU's got this right?)

..at least, that's my understanding.

sub'd...

OK,...where's JW?

Doyle

Amber Gramps

TT Could your cruise control cable be too tight making your idle too high? I had this problem when I got my car. The idle was set by the cruise cable. The cable should just be sitting there, but not too floppy.

KeithC2Turto

I am not an expert but I suspect you can adjust your mix w/o a gas anlilzer by using a digital volt meter and mesureing off the black wire of your O2 sensor.

I suspect the sensor must be hot and disconected to work.

.8 v is about 11.8 AFR (rich), .9v is about 13.2 (max torque) , and 1.0v is 14.7 (mpg/smog).

I would shoot for .9v and let the Lamba function using the O2 input modify your AFR back to near 14.7.

Could be worth trying.

TT Oversteer

Doug, the only cruise control on this car is my right foot.

Keith, interesting idea, sounds like it has merit. Does the sensor need any input voltage to function? What are the other two wires for? I'll play around with it and see what happens.

KeithC2Turto

I suspect the wires are: out put voltage from the o2, a hot wire for faster warm up, and a ground.

I do not know what AFR equals the factory CO. For me, if it was performance I was after I would try being close to the .9v, for economy I would try almost 1v.

You have to disconect the O2 to set the idle any way. I would disconect it, take a drive around the block to get the 02 up to temp, then plug in your jumper, unplug the O2, and hook up the digital volt meter after seting it to DC and a two volt range.

Then you can set your AFR where you want it. I would probably go .9v or just a bit more.

Again, as long as you are close I think the Lambda functin takes over when you plug the O2 back in and readjustes the AFM at idle for your idle and cruse smog test.

This should only affect your AFM's low in the rpm range. For adjustments through out the range the fuel quality switch on the side of the CPU would have to be used.

I am not sure but if your two lean or rich at idle with the O2 functining you can play with the brown Calif jumper under the drivers seat.

Let me know how this works for you.

TT Oversteer

I played around with the air/fuel mixture and idle today and got nowhere. Using the O2 sensor voltage testing technique I got a range of adjustment from .5v with the CO adjustment screw a couple of turns out to .83v with the screw fully closed. There was not a lot of sensitivity in the adjustment in that a lot of turning of the screw = not much voltage change on the meter.

Then I played with the idle. With the engine warmed up, O2 sensor unplugged, and the idle volumetric control test port jumpered I got about 1900 rpm with the air bypass screw on the throttle body fully clockwise (closed).

So......obviously there is something wrong here! High idle and abnormal response to both air bypass and mixture adjustment means what? Do I have a vacuum leak somewhere? If so what is the likely source of such a leak? The rubber sleeve on the manifold? The gaskets between the intake ports and the manifold? Or could it be an incorrectly set throttle plate? I'll spray some carb cleaner around the potential leak areas and see if I can find anything. I know I'm shooting in the dark here without a proper gas analyzer but I should be able to get close or at least set the idle properly. With the O2 sensor connected and DME controlling the idle it runs about 1050 rpm.

KeithC2Turto

Did you use a digital Volt meter, on DC ?

Also, I did not think an O2 senser could generate 8v.

Somthings up.

Next best diy is a wide band afm or just take it to a shop that has a CO.

PS, my best friend has a Guzzi -- Rosa with pipes or some such. Lots of carbon fiber w a hint of red trim.

Peter Zimmermann

TT: I'm thinking that you've got an AFM that's causing extreme rich running. I've heard stories of people tampering with the AFM by popping off the plastic cover and messing with the spring setting. Have you opened yours to see if it looks like someone was in there at one point? I've never used the voltage method to check CO% (only a CO machine), but .8v seems to be very rich - but you said it went from rich to leaner (.5v to .83v) as you screwed the adjustment in (clockwise), which is the opposite of what should happen. Richer running is achieved by turning the screw in (clockwise), so you have a problem with that.

In addition, rich running can be caused by a cylinder head temp sensor that has gone open internally, and I'm sure that is sufficient to cause a fast idle. With Motronic you can rarely point at one single item and say that's the problem, wire connections all have to be tight and clean, sensor depth has to be correct, air leaks are important, throttle linkage must be set right, etc. Personally, I would not change the throttle body setting - back in the day people would do that with MFI cars (alter butterfly stops), and would it make a mess of things!

TT Oversteer

Keith, that was a typo that I edited; it was (point) eight volts. I used a digital meter set to 2V DC but maybe I was using it incorrectly? I probed the male pin on the O2 sensor with the positive lead and grounded the negative lead of the multi-tester to the engine. The other two female pins didn't yield any voltage indications so I figured the male pin was the black wire.

Peter, I didn't realize that tightening the CO screw richened up the mixture (opposite of a carb). I'll take a look at the AFM and see if it's been tampered with. I do have a record of the cyl head temp sensor being replaced recently on this car. I haven't messed with the throttle plate setting but it does close firmly against it's stop under positive spring force. What should I look for inside the AFM that could have been modified and how to I return it to stock? Thanks.

Peter Zimmermann

I don't know, I've never tried to change one. I guess that you can check to see how the glue on the cover was applied, or if there is a broken paint mark on a fastener - the usual, "this doesn't look right" kind of stuff... Sorry that I'm not more helpful with this one.

ned monaghan

TT Oversteer,
When a carrera idles at 1900 or so it means that the idle control switch is not closed. With the engine off, move the throttle from closed to just open and if the idle microswitch is positioned correctly you will hear it click. If it does not, adjust the mounting bracket so that it clicks just when the throttle closes.

You can see the mounting bracket with with a mirror on the front drivers side of the throttle body

I changed out the phillips mounting screws for a couple of hex screws, easier to tighten.

Ned