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Cold start mechanism.
Right after start revolutions rise approximately up to 2000 rev/min, but quickly fall up to 1800 rev/min, in a minute they are about 1500 rev/min and slowly fall until the cooling fluid will not get warm up to 80 degrees (at this temperature the idle stop solenoid valve should be completely closed, and the lambda probe is turned into operating mode).
Petrol motors, as it is known, have quantity gas level control. Optimum it is considered a mixture in which for 14,7 parts of air it is needed 1 part of fuel, however for cold engine start strongly enriched mixture is necessary. For mixture enrichment at injection engines start the so-called IAC - Idle Air Control Valve or as it is also called, By-Pass Air Control Valve/Solenoid is used. Its work point is - formation of an air flow at the closed throttle valve. In usual position this valve is closed and opens only at warming up of the engine for increase of air consumption (the air line of this valve goes into an intake manifold by-passing the throttle valve). And according to the increased consumption of air (according to data that come from a measuring instrument of an air flow or as the phrase also goes, consumption indicator, and to be exact MAF-sensor - Mass AirFlow Sensor) the control device (ECU) makes a decision on the increased portion of fuel that leads to increase of revolutions.
Thus, if revolutions at cold engine start "float", than most likely two "are guilty": got littered or failed valve IAC (and it is even possible - an air line) or a MAF-sensor control.
IAC valve examination:
1. IAC valve is located on an intake manifold on the right, under TPS - Trottle Position Sensor. The air hose and hoses of a cooling liquid come up accordingly to it (same abbreviation I.A.C. is usually engraved directly on the plastic case of the solenoid so it is impossible to mix it up!). If you do not have close at hand tester then you can check up this valve's work (very roughly!) only having pulled off at start its plug-and-socket and having convinced, that rotations have come down (and the engine, most likely, has faded!). Do not forget that you can put on a plug-and-socket back only after cutting off the ignition!
2. However, if XX valve is absolutely "dead" the car's system of computer self-diagnostics will show it. For correct check of working capacity of an electromagnetic part of this valve it is necessary:
- to begin with check up an input voltage. For this purpose on the cold engine we disconnect a plug-and-socket, switch starting (to not start the engine!) and convince that at a plug-and-socket there is a voltage not less than 10 Volt (it is necessary to look at the power wire - usually it's colored: yellow or red);
- after that we check resistance between contacts 1 and 2, and also between 2 and 3 of the valve. At the cooling fluid temperature from -20 up to +80 degrees the resistance on valve's contacts should be within 7,3 and 13 Ohm (as usual its value is approximately 9 Ohm); - check, whether there is no short circuit on the case - resistance between each contact of the valve and the car body should be "infinite" (more than 1 megaohm);
- and finally, it could have done with checking up this valve in work. First, you have to be convinced, that on the signal wire with ECU the correct command is received. For this purpose it is necessary to find the signal wire (usually it is black or white) and to be convinced that on the first minute after start there is 1 Volt on it, and after one minute it changes to 10 Volt. Otherwise ECU malfunction is most probable.
3. After the pressure of 10 Volt has come on the solenoid coil - the valve starts closing. In the further the pressure can varies in a small range, (slightly opening valve for alignment XX on the hot engine) and XX valve's behavior at warming up will be defined only by its mechanical part overlapping a bore of an air vent depending on brought to it cooling fluid's temperature - the reel in this case only creates necessary constant strain. After warming up to working temperature IAC valve is completely closed. Thus usual for Subaru idle rpm should be established - about 750-800 rev/min.
Checkup of valve's mechanical part can be made only after achievement by the engine of working temperature. After good warming up (the cooling fluid temperature's arrow has risen in a midposition) it will be necessary to switch off the engine, to remove the valve and to make sure that it is completely closed!
If you are sure that the electromagnetic part of the idle running valve works good and ECU gives out a necessary signal, and revolutions on the cold engine continue "to jump", it is possible to try to checkup/to clean the mechanical part of IAC valve from sludges and/or to try to adjust its solenoid coil, having slackened two fixation screws and cautiously rotating the reel - clockwise or contraclockwise on +/-1 degree.
For cleaning it is not obligatory to remove right away IAC valve, it is possible to try just to pull off the air hose and to fill in any solvent directly in the inlet opening (for example, a liquid for washing carburetors). After that wait till the liquid will dissolve sludge, and then blow an air pipe with the compressor. And at the same time it won't be superfluous to clean sludge around the throttle blade in throttle's body. But do not forget upon termination to blow an air pipe with the compressed air for additional cleaning and the accelerated evaporation of solvent.
However if such procedure will not be useful, this valve nevertheless should be removed - first of all to be sure that it was closed after warming up and also to examine closely the mechanical part for possible breakages.
Let's do the procedure of IAC valve dismantling:
Actually, it is aimlessly to remove the solenoid (well, unless for checkup the valve stem for free play or in order to prevent damages at cleaning mechanics): first, there is nothing to clean there, and secondly, it is possible to "disturb" setting (the reel can turn on the stem). Therefore, if it hindered you somehow, and you by all means wish to remove it, do not forget to remember position relatively screws of fastening. You'll have to expose it anew very accurately (+/-1 degree, as it was spoken above, can break engine work at warming up).
The valve is necessary to be removed very cautiously to not damage its lining (by the way, during replacement of out-of-work valve do not forget to replace it too). First it is necessary to remove hoses (air and cooling liquid) to turn off four bolts with a box key, and then accurately disconnect the valve from the engine.
Now it is possible to clean it with everything: with the same aerosol solvent, liquid for washing carburetors or even with a detergent powder in a basin. Only do not forget to dry properly it after that.

Other possible reasons of bad start.
Actually, this is a hornbook - an elementary school of any motorist But as very frequently such elementary things are forgotten, we shall list:
1. The most banal reason of problems during starting engine operation, can be strongly soiled air filter. If an external surface of a filter cartridge is frankly dirty - immediately replace it (for simpler visual probe such filters are specially paint in radical colors!). If it is simply dusty - try to blow it from the inside.
2. Ignition plugs. The visual control of a working part of ignition plugs can show a the feed system's condition right away. If the insulator is clean and absolutely without scurf it points at too poor mixture. It also can point at that the plug is too hot, that is the heat from an electrode is transmitted too slowly. If this is true it is necessary to replace the plug or to adjust proportions of mixture. At presence on the plug of black (or very dark) sediments, the fuel mixture opposite, is too rich, and this means that your car's feed system is not working well. If sediments are black and oily it testifies to the engine deterioration and to the necessity of its checkup and repair. And, at last, if the insulator is covered with light brown sediments without signs of breakdown the mixture proportions are optimum and the engine is in a good condition. But red terry sediments with characteristic signs of breakdown point to the poor-quality petrol with the superfluous content of the additive compounds raising an octane number. More often in such cases it is enough to replace plugs and the starting engine operation will be normalized.
3. Another, not less simple reason can become a usual depressurization of an inlet tract. "Economical" Japanese use, usually, cheap clevises on all air tubes. And frequently hoses simply come off (less often burst). Therefore closely examine joints of all tubes (hoses, branch pipes, clevises, etc.) which go from an inlet tract to other car systems or elements (these are braking system, the adsorber canister, PCV valve, and other hoses of crankcase's compulsory ventilation). The special attention should be given to a tube which connects a fuel pressure regulator with an intake manifold.
4. If the air system is in good repair, it is necessary to pass on checking the fuel system. One of the reasons of bad start can be the fuel mixture leaning because of insufficient fuel pressure. Two reasons can be here: a "dying" fuel pump (which, however, usually sings a "farewell" song before the death), or a fuel pressure regulator. Sometimes fuel pressure in system can be built up if pressing temporarily a hose of tapping outlet to the fuel tank (it is necessary to press the waste hose of superfluous fuel to the tank cautiously and to not hold it in such condition after start more than 5-10 seconds, in order to prevent plug oiling- or fueling-up). If such operation helps, but the engine continues stalling there is no need in increasing time of pressing the hose, and it is better to repeat it several times till the engine will not get warm and will not cease to stop, when you renew plums of fuel. 5. The initiator of bad start problem can be also a cooling fluid temperature's detector. And keep in mind, that there are two of them: one serves for taking indications for temperature recorder on the gage dial, and another (ECT - Engine Coolant Temperature Sensor) takes readings for the engine control unit (ECU). Both of them are located from a right side under an intake manifold. If the first gauge "lies" you will see this only on the dash panel, but indications of the second one can lead to much more serious consequences. For check of the ECT-sensor on all Subaru models it is necessary to disconnect its socket and to measure resistance of this gauge at different cooling liquid temperature: at 20 degrees it should give out 3,0 kilohm, at 50 - 0,7-1,0 kilohm, and at 80 degrees (normal working cooling liquid's temperature) - 0,3-0,4 kilohm. If you sometime overheated the engine this heat-sensing device should be checked up carefully and, whenever possible, to replace it. Otherwise, you constantly will have problems with a starting engine operation, especially during cold weather. If you cannot check up this device's operability (the engine does not work!) I advise you to take the 3-4 kilohm variable resistor, to connect it to this device's socket and to try to adjust revolutions manually (being based, for example, on temperature indications of the gage dial and of the tachometer). Do not forget only after warming up to cut off the ignition and to connect regular cooling liquid thermode.

If no one of the above listed manipulations have helped - take away ECU diagnostics codes and start serial check of all injection system's elements:
The leads;
All linings on air holes;
The injection system relay;
Fuel injection nozzles;
Ignition coils;
The ignition output unit (switchboard);
The pressure control device;
The speed sensor;
The crankshaft sensor;
The camshaft sensor;
And, at last, the control device (ECU - Engine Control Unit).
In the presence of elements of injection system in good repair, problems with cold start on Subaru cars don't happen!