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Audi MB, 1B and MC Engine Idle Settings
The MB and 1B engines are the only Audi turbo engines that use a pulse-width controlled idle stabilisation valve but do not have a lambda probe. For this reason, setting idle is a little complex.
The idle stabilisation controller (doube-width "relay" in the auxiliary relay panel - code "232") receives a number of inputs and attempts to control idle accordingly. For this reason it is important that the idle stabilisation valve is set to operate in the centre of a range that gives it control scope in both directions. Engine idle speed will be raised deliberately in a number of situations:
If the system is working properly, the engine's idle speed should fall noticeably as the vehicle slows to a halt. A curious corollary of this servo system is that an MB can be carefully accelerated to a good speed simply by engaging gear and releasing the clutch gently - the idle stabilisation controller will return the engine to the set idle speed - the process can be repeated into third or even fourth gear.
- Idle switch engaged (throttle closed) but vehicle still moving. Idle is raised to provide more power steering and brake assist
- Low battery voltage
- (If fitted) air conditioning magnetic clutch engaged
- Long 3mm Allen key (Audi tool P377).
- Multimeter with 5-cylinder tacho scale
- Multimeter with Amps scale reading to at least 1 Amp
This multimeter also needs a special lead to measure the current flowing through the idle stabilisation valve (ISV). This lead can be made up using connectors from any of the flywheel sensors, taken from a scrapped car. The outer pins on two of the connectors should be connected together, the other two outer pins sould be fitted with "banana" plugs for the multi-meter. The VAG part number for thi$ lead i$ VAG 1315 A/2.
- Cross-head screwdriver
- Flat-bladed screwdriver
- Gunson Colortune
A gas analyser can be used in place of the Colortune, but it must be a true CO measuring device and not the Gunson "CO meter", which actually infers CO indirectly from other gases
- Check that the throttle is properly closed
- Check that the idle switch (upper and centre contacts on throttle body switch) closes when the throttle is closed
- Run the on-board diagnostics to ensure that the fuel frequency valve is working
- Remove the ISV - cylindrical valve on the end of the inlet manifold - and attempt to blow through it. It should be airtight.
- Attach a 9V battery with a high internal resistance - e.g., PP9 - and check that the valve opens. DO NOT USE 12V - it lets the smoke out
- Check the ISV for a resistance between 3 ohms and 8 ohms
- Refit the valve with the ammeter in circuit
- Remove the cover from the low-tension end of the ignition output stage and slide it back along the wires
- Attach the tachometer to the rear terminal on the output stage
- Warm up the engine. The main radiator cooling fan must run at least once
- If any injector lines have ben detached, raise idle to 3,000 rpm several times and allow the engine to idle for a few minutes
- Pull the codes - code 2312 (thermoswitch) must NOT appear
- Switch off all electrical loads, including air conditioning
- Switch the engine off
- Install the Colortune
- Detach and plug the crankcase breathers
- Using the long 3mm Allen key (Audi tool P377) in the hole between the fuel distributor and air mass sensor housing, adjust the mixture to give a CO value of ca. 1.5% - visually one out of four or five firings should be yellowish, the rest distinctly blue. Always set from lean to rich - remove the Allen key each time before blipping the throttle to settle the reading.
- It may be difficult to insert the 3mm Allen key. There are three possible reasons for this:
- During previous adjustments, someone may have blipped the throttle with the key in place and bent the adjusting screw
- The sealing plug over the hole (Audi part 026 133 777) may have been left off. This causes air (and a quantity of dust) to enter the adjuster hole and clog the screw. A long 2mm ball-end Allen key can be used to clear out the screw, bringing up a small quantity of detritus each time
- The adjustment may be close to one of its end stops. This could represent a problem, since the adjustment is normally set in the middle of the range. Check for air leaks or anything else that could be seriously affecting the mixture.
- With the mixture correctly adjusted, set the idle to 800 rpm +/- 50 rpm using the screw on the front of the throttle body. If it is difficult to line a screwdriver up on this screw because the front transom fouls, schedule an inspection of the right engine mount as soon as possible
- The current passing through the idle stabiliser valve should now be ca. 430ma +/- 10ma, with light swings around this value. A value of precisely 470ma is an alarm signal from the idle stabilisation controller and the problem needs to be diagnosed
- Beware that the radiator cooling fan can and will run during these settings. Suspend measurement during these occasions
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