Saab Central - Technical Help

Classic Saab 900: Guide to curing Variable and high idling

This article kindly written and supplied by Fred Lane

The correct idle speed is 800 to 900 rpm. High and variable idle is a fairly common problem on the 900 engine; this article pulls together the experience of the bulletin board. Thanks to everyone who posted and contributed, I've added, checked with the manuals and edited. There may be a bias in favour of Lucas Fuel injection equipped cars, either because I've got one or because the fault afflicts them more. If you've anything to add e-mail me and I'll do an update.

Note that if the engine is running like a pig there are many other thing to check (like the ignition system!) this write up assumes everything is running well except for the idle problem.

There are a number of possible causes:-

1. Vacuum leaks

Air leaks into the vacuum side of the induction system are just about the most common cause of high idle speed. Check all the small-bore rubber pipes around the manifold are connected at both ends and have no leaks or splits. Check also the rubber bushes into the inlet manifold where these lines are tapped in, they go very hard with oil and age. Check the larger bore pipes that connect the Automatic Idle Control (AIC) valve to the throttle body and the large bore pipe to the brake booster.

With the engine warm and idling squash each small bore pipe flat between your fingers, if the engine speed changes something is leaking.

To find vacuum leaks you can also spray carb cleaner around various places on the engine and notice a change in rpm. You can also use propane, acetylene, butane etc. Without lighting of course. Be cautious of fire hazard.

Other places that can leak vacuum are:-
Could be the brake booster diaphragm, these unfortunately have been known to deteriorate and leak vacuum. On a Turbo the Dump or Bypass Valve (Classic symptom "When I come to a quick stop, usually in traffic or when someone cuts me off or at a red light, the idle drops from just under 1000 to almost zero and the car stalls.") The diaphragm in these plastic valves fails and leaks vacuum. If you can suck air through the small-bore pipe to the valve (which is down front of engine above turbo) it's broken; they are reasonably cheap (£20) and quick to replace. Sometimes failed hose clamps on these valves cause the same stalling problem.

For the record this is how the vacuum pipes run on a 91 16valve UK market LPT.

· top of throttle body to distributor
· front T on top of inlet manifold to camcover breather via plastic non return valve.
· front T on top of inlet manifold to carbon canister on front wing, connects to a solenoid valve.
· rear T on top of inlet manifold to Turbo dump valve down front of engine.
· rear T on top of inlet manifold to Fuel pressure regulator.
· 3 way on side of manifold to brake servo, 1 inch pipe.
· 3 way on side of manifold to heater control reservoir.
· 3 way on side of manifold to instruments (overpressure switch and pressure sensor/ turbo gauge on full blown).
· Two 1 inch pipes from AIC valve to either side of throttle body.
· If you have full pressure turbo then 3 lines from APC solenoid, one to wastegate, one to manifold(?) one to inlet before turbo. But exact details required from someone please.
· If you have EGR then many variations on USA market, some with solenoid valves. My 89 UK has a simple EGR system, one pipe from top of throttle body to a temperature switch on side of head between ports for 1&2 cyls. Then a pipe down to the EGR valve under Inlet manifold.

2. Leaks sorted and Idle is still high?

For the engine to run fast air has to be getting in somewhere. Having sorted out the vacuum leaks the next place to look is either the throttle body or the AIC.

Try squashing one of the big hoses leading to the AIC. If Idle is too high then the ECU should have already closed the AIC valve fully and squashing hose shouldn't make any difference. If air is flowing through valve it either needs cleaning or its broken. If squashing hose doesn't lower idle then the air is going past throttle plate and the throttle housing needs cleaning.

3. Cleaning throttle body

Carbon build up in throttle body can prevent throttle plate closing properly, so that even with idle valve shut the idle is too high. Very little dirt on the edge or where edge bears in body makes a difference. Clean the whole lot as explained here.

4. Cleaning out the AIC

The AIC builds up all sorts of sticky black stuff inside, which stop it controlling properly. Remove the AIC valve and flush it out with carb cleaner, follow up the carb cleaner with WD40. You should remove a lot of sticky carbon deposits. On the Lucas equipped car take the AIC valve apart. The plastic actuator screws into the aluminium body with a very large hex nut but you can sometimes remove it with some long nosed pliers. There is a conical plug that moves in and out to control the idle - it is usually coked with hard deposits and you need some carb cleaner and an old toothbrush to shift it from all the moving surfaces. Some WD40 along the shaft towards the electric parts to finish off. To check Lucas AIC operation connect the plug up before putting the body back on - bung up the 2 throttle to AIC hoses and start the engine. It should idle low or stall with a warm engine and the hoses blocked - if not you have an air leak somewhere past the throttle plate. Also observe the conical plug as you increase and decrease the idle speed by opening and closing the blocked pipe on the inlet side of the throttle, it should move in and out in small steps as the ECU senses the wrong idle speed and tries to compensate. Don't vary engine speed by opening the throttle, as the ECU will only respond when it senses a shut throttle. If you get no movement from the conical plug your AIC may be faulty or you may have an ECU problem or the throttle position sensor may be wrong. Refit to the AIC body with the soft gasket and a dab of loctite on the threads.

The Bosch AIC has a rotary valve rather than the conical plug and seat of the Lucas. I'm told it's not possible to dismantle the Bosch valve.

Other checks on the AIC are the resistance of the two windings, and also that it's getting volts, details vary with AIC type:

Bosch LH2.2 from middle pin to either outer pin, 15 to 25 ohms
Bosch LH2.4 from middle pin to either outer pin, 2 to 12 ohms
Bosch LH2.4.2 from middle pin to either outer pin, 9 to 15 ohms
Lucas CU14 pins 1 to 26 and pins 28 to 29 both, 40 to 60 ohms

Also check the Ground wiring at Cylinder Head.

5. Check the throttle position sensor, and others

If the ECU doesn't realise the throttle is closed, it won't start controlling the idle. So correct setting of the throttle position sensor, either switches (Bosch LH2.2. and 2.4) or potentiometer (Lucas and Bosch 2.4.2) is essential. The instructions for the Lucas system are:-
a) Connect a voltmeter between terminal 2 and 3 on the throttle pot.
b) Slacken the screws securing the throttle pot to the inlet manifold.
c) Turn on the Ignition, and rotate the throttle pot so that the reading on the voltmeter is maximum of 400 mV,or 0,4 Volts. If this is not possible, you might need to enlarge the holes (for screws) on the T. pot.
d) Retighten the securing screws.
Note that the voltmeter should read between 4.2 and 4.9 volt at full throttle.

The Bosch settings for idle switches (LH2.2 and 2.4) are in the Bentley manual. The idle switch (pins 1&2) should open as the throttle just comes off idle. The full throttle switch (pins 2&3) should close at 72 degrees of opening. Bentley notes that the switch assembly, if wrongly adjusted, can hold the throttle open. For Bosch LH 2.4.2, the potentiometer, pins 2&3, should give 0.25volts at idle and 4.0volts at full throttle. The ECU also checks road speed before running the AIC, there's a sensor in the speedometer. On Lucas jack up one front wheel and spin it, you can then measure a signal at pin 6 of the ECU that varies between 0.5 and 5.0volts. On automatic cars the ECU also detects when you move to D or R. If you have air con the Bosch system detects whether it's on.

6. Cleaning done, Now set the idle speed

If you've cleaned the throttle body and AIC out you will probably need to set the idle speed back down, as some previous owner has been winding it up. The method varies a bit depending on fuel system.

Idle speed has to be set with the engine at normal operating temperature. If the thermostat's bad and the engines not reaching normal temperature expect idle problems until you've replaced the thermostat. The ECU detects engine temperature with a sensor in the flange of the inlet manifold, check the resistance of this NTC sensor at normal engine temperature, it should be 320ohms for Bosch and 270 to 390ohms for Lucas. Engine cold about 2500ohms.

Basic setting of throttle butterfly; Undo the lock nut, slacken the tiny stop screw, wind it in until it just touches the lever, then wind a further 1/3 turn. Lock it. This should set a clearance of .002" between butterfly and body.

Setting base idle speed if no AIC. Adjust with the bolt poking out of the throttle body, which controls a throttle bypass passage.

Setting base idle on Bosch system with AIC. This requires a dwell angle meter; details are in the Bentley manual. If anyone has a simpler way let us know. You might try isolating the AIC pipes and setting it to 650rpm as below. This leaves the AIC just open (to give the other 200 rpm) at normal engine temp and load.

Setting base idle speed on Lucas CU14 (thanks to Sturla)

a) Warm engine and let it run at idle speed, block both air hoses fitted to the AIC-valve
b) Adjust throttle angle/speed (by loosening the 8mm nut) the turn the throttle stop screw clockwise until idle speed is approx. 600-650 RPM
c) Stop engine and remove clamps on the AIC valve.

7. What about the throttle dashpot?

If you have a dashpot which slows the closing of the throttle set it as follows:-
As for the almost stalling when you let off the gas: I have just recently come across that problem myself. There is a small diaphragm-like actuator that touches the throttle linkage called the deceleration dashpot. The purpose of this is to allow the rpm of the engine to come down before slamming the throttle plate shut and causing that "near stalling behavior." Once the engine has settled, the throttle will then fully close. If you twist the throttle with your hand then let it spring back, you should see this actuator stop it just before the idle screw and hold it for a second or two. If it doesn't hold it, it could either be 1.) Ruptured and in need of replacement or 2.) Out of adjustment. The way to adjust it is to adjust it in or out until when you let go of the throttle just as described above, there is a 2-3 second delay before the throttle completely closes. It is some times hard to see such little movement to make sure its working so try listening closely and you will hear the throttle positioning sensor click 2-3 seconds after releasing the throttle. This all of course while the car is not running.

8. What if you have the auxiliary air valve rather than an AIC?

If there is no AIC, an auxiliary air valve connected to hoses across the throttle body provides a controlled higher idle speed during warm up. The valve is a plate that moves as the bimetal strip in the long part of the body is electrically heated. So gives more air on cold start and, with some regard to ambient temps, drops it off over a few minutes the book gives the numbers for the aux. air valves on the LH2.2 system.
-153 automatic turbo
-107 manual turbo and I
-122 auto I
Suspect the auto ones gives higher cold idle.

A basic test for the Auxiliary Air Valve is to remove the top hose on the Auxiliary Air Valve and shine a flashlight in it to check that it is open with the engine cold. Then do the same when the engine is warm to check that the Auxiliary Air Valve is closed.

If you have too slow an idle during cold start then:- The answer is to adjust the amount that the AIC allows in when cold. If you remove the AIC you will see a tiny nut on it which, in some cases is painted yellow (6mm or 7mm spanner will be needed to undo it). Once the nut is undone you can move it about a small amount. I can't remember which way you need to move it, but try different positions. Obviously tighten the nut. Start the car and see how much the idle has changed. (Credits to Alan B)

The valve winding should have a resistance of 40 to 60 ohms.

9. Finally the O2 sensor

If you've done all of the above, you may still be left with a slightly varying idle (say 50rpm swing). This is probably due to the O2 sensor. Varying idle is one of the early signs of a tired O2 sensor, but change it after all of the above have been ruled out. At idle the ECU swings the mixture back and fore between slightly rich and slightly lean, searching for the spot on mixture. Connect a digital multimeter to the volts out of the O2 sensor you'll see the idle speed changes in time with the mixture variation. Bad O2 sensors are slow to respond so the mixture swings more and there is more idle variation. One of the indicators of O2 sensor condition is the number of swings the system can achieve in a given time (zero crossings) but I've no idea what an acceptable value is.


It's probably one of the vacuum hoses been knocked off, unless it's a turbo when it'll be the bypass valve leaking.

Recent Bulletin Board threads suggest two further things to check:

The ignition timing, check with a timing light that this is constant at idle. If ignition timing varies, but is constant when engine speed is increased, the Hall effect sensor may be getting old and tired. Search on posts from Profz for more on this.

Valve timing, One tooth out on the cams can make a big difference to the way the engine idles, Search on post from A.Nijhof for more.

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