The purpose of this Esmagamus guide is to enable you to diagnose and hopefully fix idle issues in most petrol engined cars (these hardly ever happen in diesels and being so much simpler than petrol engines, diesel problems are usually easier to pinpoint). Most of us have been hunting for information from various sources while trying to fix a problem we can't even name. A substantial amount of time and money can be saved just by knowing how various systems work together and how specific parts can fail.
We will cover:
-the importance of engine vacuum;
-the difference between not having or having a lambda sensor;
-lean and rich burning conditions;
Once again, the instructions provided will be influenced by the cars I have. Although I may not be familiar with your particular vehicle, the same basic principles and reasoning processes still apply.
Sweat saves blood, blood saves lives and brains save both. - E. Rommel
I add that brains also save time, and time is money. Be smart.
Note: Annexed schematics are available on on RAR format. Download and extract if you can't read the schematics.
Step 1: Occam's Razor: Number One Principle
Also known as law of parsimony, Occam's razor basically means you should thing about simple, uncomplicated problems first.
As an example, last week I found myself stranded on the side of the road, my car had developed a bad misfire. What could it be? An injector? A valve? Then I applied Occam's razor and checked the distributor first: the rotor arm was broken and the carbon button and spring had been dislodged.
Any shop manual on car electrics will state that wiring fails more often than senders or control units. Once again, before thinking "my battery is dead and must be replaced", one should think "are the terminals properly bolted down?".
In his Summa Totius Logicae, i. 12, Ockham cites the principle of economy, Frustra fit per plura quod potest fieri per pauciora [It is futile to do with more things that which can be done with fewer].
—Thorburn, 1918, pp. 352–3; Kneale and Kneale, 1962, p. 243.
Being an abstract subject, I include the following link to those interested: http://en.wikipedia.org/wiki/Occam%27s_razor
Step 2: Knowing How Things Work
Have you propped the hood up yet? See all those wires and tubes? What are they for?
First things first you should know your engine. What electronic controls does it have? What sort of ignition setup does it have? What sort of injection system does it have? What emissions control is fitted to the car?
All this questions should have an answer, usually found on a shop manual. Make sure your engine is in there: I own a Alfa Romeo 164 2.0 Turbo. Most shop manuals only cover the V6 powered versions, but mine is 4-cylinder, how can I find specific engine related data, troubleshooting procedures, etc? Knowing this is the same engine as fitted on the Lancia Delta Turbo, I obtained a Lancia workshop manual to add to the Alfa Romeo ones.
Search the web for manuals and forums. Knowing other people that own the same car you have can be invaluable. Forums such as Alfabb.com have immense information on specific and common malfunctions and fixes.
Step 3: Thinking Straight: Order and Method
Making a diagram will always help you keep your thoughts organized. What you're looking for during a diagnosis is evidence. Once you have evidence that a certain part is working as it should, you can rule it out and simplify the process. Using this method step by step will eventually pinpoint the exact problem, but you might get there faster taking calculated guesses. Let's see an example of a diagram for a no-start issue.
This one follows a situation I had a few months ago. I've only included some of the most basic things that can go wrong for the sake of simplicity. Some of these items ("distributor condition" for example) may not apply to your car.
The second diagram shows what I've checked out during the diagnosis. I started with simple things and worked my way up the scale of complexity.
Step 4: Fuel Problems: Diagnostics
Idle issues aren't usually caused by fuel problems, but they do happen, and so it's good to do a basic overview.
If you do have a pressure gauge and know how much pressure should get to the fuel rail/throttlebody, the diagnosis is simple: either the pressure is as specified and fuel supply is OK or something is wrong and the pressure isn't what's supposed to be.
Injector problems usually cause a misfire in one cylinder, as it is improbable that more than one injector fails at the same time. The annexed diagram covers the most usual issues, though others (such as injector issues) can happen but are a lot less likely to happen.
Step 5: Ignition Problems: Diagnostics
This step covers issues found in a system consisting of a single coil with electronic ignition advance, a distributor with no breakers or vacuum advance and a set of leads connecting said distributor to sparkplugs. This is the last system with distributors coming on the market. Since then much more reliable systems with solid state coil packs and no distributor or or coil-on-plug setups completely eliminating both moving parts and high voltage wiring became standard.
Because this unreliable system is still found in many older cars, it is imperative to know how it works, how it fails and how it's fixed.
For simplicity's sake we will take for granted that the leads from the distributor are fitted according to the specified firing order.
Distributor: the distributor itself will hardly ever fail. Most problems are in the distributor cap and rotor.
Distributor cap: usually made of bakelite, a fragile early plastic, very prone to cracking and breaking. On a moist day, a cracked cap can allow water into the distributor and thus shorting the high voltage pulse to ground. Result is a no-spark condition on all cylinders. For each specific model of cap there is a rated resistance between the outside and inside contacts. This can be checked with a multimeter and if resistance isn't as specified, the cap must be replaced.
Distributor cap carbon button: it is found in the center of the cap. It must be protruding and be spring loaded. Conducts high voltage from the main lead to the rotor. Eventually it might wear out or not even be there, causing misfires. If so, cap must be replaced.
Distributor rotor: also made of bakelite with a metal contact. Routes the pulse from the main lead to the correct lead when the cylinder has to fire. On systems with electronic advance, the rotor may have some slop as this won't interfere with ignition advance. On older systems it has to be tight. Once again, there is a rated resistance for rotor. If resistance exceeds what the shop manual specifies, the rotor must be replaced.
Leads: failing leads will have out of specification resistance or torn or discoloured wires and boots. To check if a lead is conducting energy, pull it from the plug with the engine running: a sparking sound should be heard as the high voltage pulse arches to ground. Be careful though, high voltage can be deadly.
Spark plugs: plugs with cracked insulators must be replaced. When taking a plug out, check if the gap between the electrodes is as specified. If it's too large, there may not be a spark. If it's too small, the spark will be weaker.
Step 6: Vacuum Leaks
In order to understand why petrol engines have throttle plates and are dependent on keeping a vacuum to idle, an understanding of basic chemestry is necessary.
Gasoline and air must combine at a 1:14 ratio so it burns optimally. This is called the stoichiometric ratio and is defined by the chemical composition of gasoline. At less than the that ratio, gasoline either "explodes" causing engine knocking or does not burn at all. So, petrol engines have to run with a bias to rich, except for those engines where gasoline is injected directly in the cylinder (just like it happens with diesels) and the area around the plug has a stoichiometric ratio of gasoline to air and the rest of the cylinder has almost no petrol mixed with the air.
If the engine were to admit the same amount of air (if it had no throttle plate) at all times, it would have to be fed with enough petrol to raise the air to fuel mixture ratio to 14, it becomes obvious why the air intake is limited: so that the engine gets just a little bit of air with the right amount of petrol. When the engine is running slow but plenty of torque is necessary, the mixture is enriched. When it's running fast with the throttle wide open, the mixture is lean.
The throttle plate does that regulation. At idle, it is completely or almost completely closed. This generates a vacuum in the intake manifold. In a simple engine, like that of a lawnmower, the vacuum serves no purpose and is contained by the throttle plate. On a car, it does various tasks:
-powers the brake booster servo;
-guarantees positive cranckcase ventilation;
-sucks hydrocarbon vapours from the fuel tank;
-might regulate ignition advance and turbocharger
-powers miscellaneous devices such as wipers on antique cars.
For these tasks, vacuum has to be routed through hoses, gaskets, all sorts of devices. Obviously, gaskets, hoses and other items can and do fail. When that happens, the engine will run lean. If the car is fitted with a lambda probe, it will detect the excess air and warn the driver with the "check engine light". It will also try to adjust the system so it isn't running lean automatically. If there is no lambda probe and no automatic correction, the leak will manifest itself as a lean-running condition: idle speed will usually be faster or the car just dies if the leak is big enough. Between those two, the engine can develop a irregular idle.
One way of knowing if there is a large vacuum leak is removing the oil filler cap with the engine running. It should stop completely. If a vacuum gauge is available, it should be connected to the brake booster hose and ideally it will show a steady vacuum. If it doesn't, there is a leak.
Testing can be done in various ways: smoke testing is safe and effective but unless you have a smoke machine you can adapt to that use or build one, it won't be cost effective to buy a professional machine costing hundreds of euros. The other method is called hydrocarbon enrichment. It consists of using a unlit propane torch around a suspected leak. When the engine sucks the propane in, the idle goes a bit higher. Some people do it with carb cleaner but that's a lot more dangerous. Think safe.
Step 7: Throttle Plate and Housing
This is an example of a Weber throttle body fitted to the Lancia 2000 DOHC 8v engine. A brief overview of the throttle body parts:
1 is the throttle plate also known as a butterfly valve;
17 is a hot coolant inlet to warm the throttle body and thus avoid fuel condensation in cold weather;
18 is the coolant outlet;
19 brings the extra air from the idle speed adjustment valve (this enters downstream of the throttle plate);
20 serves the positive crankcase ventilation system;
21 is the actual idle air bypass channel;
22 is the base idle adjustment screw with locknut;
23 is the throttle plate factory stop.
Next to number 23 you can see a pin with a rounded head. This connects the throttle plate to the gas pedal spring and position sensor through a threaded bar. This bar is adjustable and removable. When the bar is removed with the engine running, the idle speed should not change or fluctuate. If it does, the bar is misadjusted. More on that later.
Step 8: Idle Air Solenoid Valve
So what is this valve needed for if the bypass channel on the throttle body can regulate the base idle?
Simply put, when you turn any electric device on, it puts a load on the engine through the alternator. With no load compensation system, the engine might stall just from turning too many things on at once!
So, the ECU senses the lowering of the engine speed and actuates the idle air valve to allow more air into the engine manifold, bringing RPM to its base level. This valve will work the most on cold starts. Once the engine is warm it hardly opens at all. Many cases of cold start issues that disappear on hot starts are caused by the defective functioning of this valve. Note this valve is commanded by the ECU, getting signals from coolant temperature sender.
An idle air valve typically consists of:
1 a solenoid;
2 an air passage for air coming from 4;
3 a passage leading to the intake throttle body intake (#19 on previous step);
4 a filtered air inlet;
5 a moving cylinder raised and lowered by the magnetic field of solenoid 1.
Basic checks in annexed diagram.
Step 9: Preliminary Checks for Idle Adjustment
For the sake of brevity and simplicity, the annexed diagram will show the basic steps. The way I like doing things consists in having a mental diagram of all the steps I must follow. When you're first doing this sort of work, do one. Think clear, be smart.
Step 10: Actual Idle Adjustment
Have all the conditions on the previous step been met? Now you're ready to adjust the base idle and bar linkage length.
Although the only pictures I managed to get have a very low image quality, the adjustable linkage 3 is still identifiable. On the ends, a spring retainer connects the linkage to the accelerator position sensor and to the pin on the throttle plate. To make sure the length is correct, remove the bar with the engine running. This will ensure the throttle plate rests against the factory stop.
Now that the plate is against its stop, undo the locknut on the idle adjustment screw (#22 on step 7) and turn the screw until the idle is at the speed recommended in the shop manual. For the Lancia 2000 engine, it's 750 RPM or 800 when fitted with automatic gearbox.
Once the idle is at correct speed, tighten the locknut carefully. Refit the linkage bar. If RPM gets higher, undo locknuts 6 and lengthen the bar slightly. Refit when correct.
This linkage commands a throttle position sensor with a "minimum opening" switch. Some idle issues are caused by a bad adjustment of this sensor: make sure it clicks when the throttle is opened just about a degree. This adjustment is made by simply undoing a screw on the sensor and rotating it slightly and tightening the screw again.
When the adjustment is done, reconnect the idle air valve. This should increase RPM for a while and then it goes back to base idle.
Step 11: Final Words
The reason I did this is that most mechanics I've found know nothing about the car I have: the Alfa Romeo 164. Because I have a Lancia engine in it, their ignorance is aggravated. Once a famous Alfa Romeo mechanic told my brother mechanics are for common cars, horologists are for watches and a special breed of qualified technicians is what Alfa Romeos need.
Both Alfa Romeo and Lancia have long and proud histories. On many markets, both have a reputation for unreliability. The fact common mechanics scratch their heads looking at an engine made twenty years ago attests their lack of knowledge. Those of the special breed I mentioned will tune a car just by ear while others can't do it at all.
As I mentioned on a previous guide, in today's hard times, any money you can keep on your pocket is welcome. Anyone can buy a good old car for a lot less than it costs to buy a new one. With skill and ingenuity it can be restored to its previous glory for almost no money. Even a new car will be significantly cheaper to own if you do your own maintenance and repairs. Today information is free, and I've just shared with you some knowledge I gathered and applied. Feel free to ask any questions.
Keep the knowledge flowing, it's improving mankind through individual growth.