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Head Gasket

A head gasket is installed between the cylinder head and the engine block. It's not a very expensive part, but it has a very important function: it seals the combustion chambers as well as the oil and coolant passages that run between the engine block and cylinder head. 

Head gasket failures are often caused by overheating, for example, when the engine is low on coolant (antifreeze) or when the radiator fans don't work. Other reasons include: a detonation (pinging, spark knock), lean air-fuel mixture, design flaws, etc. 
Symptoms of a blown head gasket include: Coolant present in engine oil (engine oil has a "coffee with milk" color ) see the photo, white steam (smoke) with a strong smell of coolant from the exhaust, bubbling in the cooling system, coolant boiling over in the overflow tank, loss of coolant with no visible leaks, overheating, no-start after the engine has been overheated. 

The head gasket repair is quite expensive because of the amount of labor involved. If the engine hasn't been overheated and there is no other damage, the repair could cost $150-$300 for parts, another $150-$300 to have the cylinder head checked and resurfaced, plus $400-$700 for labor. A new head gasket itself is not very expensive, however many other parts like cam seals, valve cover gasket, intake gaskets often need to be replaced too. If the engine was severely overheated, it might be cheaper to replace the whole engine with a used or rebuilt unit. How is a blown head gasket diagnosed? Usually a diagnostic involves performing a cooling system pressure test or checking for exhaust gases in the cooling system with a gas analyzer. The compression and cylinder leak-down tests can also tell a lot. 

Replacing a head gasket is a big job, as the cylinder head with manifolds has to come off. Once removed, the cylinder head needs to be tested in a machine shop and re-surfaced if needed. The cylinder block mating surface needs to be checked and cleaned. When a rebuilt cylinder head is installed, the timing may need to be re-set and valves re-adjusted.

IAC valve

The engine computer (ECM) constantly adjusts the idle speed according to the conditions. For example, when you start the engine cold, the ECM increases the idle speed to help warm up the engine faster. In some cars there is a separate idle air control (IAC) valve or solenoid that controls the idle speed. In other cars the electronic throttle is used to control the idle speed. 

The idle air control (IAC) valve works by allowing metered amount of air past the closed throttle. The ECM adjusts the engine idle speed by slightly opening or closing the idle air control valve (IAC). For example, when you turn on the air conditioner, the air conditioner compressor adds more load to the engine. To keep the idle speed steady, the ECM slightly opens the idle air control (IAC) valve to add more air to the engine. 
Typical problems with the idle air control (IAC) are often caused by carbon build-up inside the valve. Sometimes this could cause the valve to stick, other times, the carbon build-up blocks the IAC valve passages. In either case the engine may have rough or unstable idle. It also could be too high or too low. The carbon build-up can be cleaned with a carb cleaner or another similar product. If the cleaning doesn't help or the idle air control valve has an internal problem, it may need to be replaced.

Intercooler

Many turbocharged and supercharged engines have an intercooler, installed between the turbo- or supercharger and the engine intake. 

The purpose of the turbocharger or supercharger is to boost the intake air pressure. Boosting the pressure increases the temperature of the intake air. To improve the engine performance and prevent the engine knock (detonation), the intake air temperature must be reduced. This is the job of the intercooler. The intercooler is essentially a radiator for the intake air. An intercooler is installed in front or on top of the engine. In either setup, the passing air is directed through the intercooler fins, dissipating the heat and reducing the intake air temperature.

Misfiring

The term 'misfiring' refers to the engine with one or more cylinders not working properly; meaning there is no full combustion in the affected cylinder. An engine cylinder can misfire for many reasons, including a bad spark plug or ignition wire, faulty fuel injector, vacuum leak, etc. A misfiring engine might shake and exhibit lack of power. When one of the engine cylinders misfires, the unburned fuel entering the exhaust can overheat the catalytic converter. The car computer (ECM) detects that the engine misfires and informs the driver by flashing the Check Engine light on the instrument panel. To find what can cause the engine to misfire,

P0301 - Cylinder 1 Misfire Detected

The term engine misfires means that there is no combustion in one or more of the engine cylinders. If the engine misfires at certain conditions but not all the time, it's called intermittent misfire. The engine computer or PCM can detect a misfiring cylinder by monitoring the engine speed. Once misfire is detected, the PCM sets the trouble code and turns on the Check Engine light to alert the driver. The OBDII trouble code P0301 means that the cylinder number 1 misfires. The code P0302 stands for cylinder 2 misfires, P0303 is the cylinder number 3 and so on. 
The code P0300 - random misfire means that multiple engine cylinders misfire at random. The codes P0300-P0308 are called misfire codes.

Symptoms of a misfiring engine

When misfiring, the engine will shake and run unevenly. This is especially noticeable at idle or when accelerating from a stop. A car will lack power and the "Check Engine" light on the dash may blink repeatedly or stay on solid. You could also notice smell of unburned fuel from the exhaust.

Can you drive with the misfiring engine?

Driving with misfiring engine can damage the catalytic converter, which is an expensive component of the vehicle emission control system. When one of the engine cylinders misfires, unburned gasoline entering the exhaust can overheat and melt the catalytic converter. If the ECM detects that the misfire rate is high enough to damage the catalytic converter, the Check Engine light will start blinking on the instrument panel. Some manufacturers advise not to drive with a misfiring engine; others recommend driving only in a very moderate fashion and, of course, have your vehicle checked out as soon as possible. See your owner's manual for details.

What can cause the engine to misfire:

- problems with the ignition components: spark plugs, ignition wires, coils, distributor, ignition module. 

- fouled spark plugs 
- faulty fuel injectors
- lean air/fuel mixture due to bad airflow sensor, vacuum leaks, etc.
- vacuum leaks; examples: cracked vacuum hose, disconnected vacuum line, cracked vacuum line, cracked intake snorkel
- low fuel pressure
- leaking cylinder head gasket
- low compression
- misadjusted valves
- worn valve train components
- improperly set timing
- Stuck open EGR valve or purge valve (solenoid) 
- clogged exhaust 
- problems with the crankshaft position sensor (CKP) or camshaft position sensor (CMP)

Common problems causing misfire: 

- In some Honda vehicles the misfiring often caused by valves being out of adjustment. Misfiring could be more noticeable when the engine idles after a cold start. As the valve train components wear, the gaps change; to compensate, the valves in many Honda engines need to be adjusted in recommended intervals. 
- A technical service bulletin (TSB) for some Buick Rainier, Chevrolet TrailBlazer, GMC Envoy and other models with a 4.2L in-line 6 cylinder engine describes a condition where the codes from P0300 to P0306 could be set after a heavy rain, especially if the vehicle is parked nose down on an incline, as the dripping water can collect on the ignition coils and spark plugs. As a solution, the bulletin recommends checking and replacing the ignition components if needed, as well as installing a revised seal for the engine compartment. 
- In many older cars, especially if a tune-up hasn't been done for a while, washing the engine compartment or driving through a rain can cause the engine to misfire, as water might get into the ignition components and short them out. The fix usually involves a tune up with new spark plugs, wires, distributor cap and rotor.
- In some GM vehicles with V8 engine from late 90's to early 00's, a corrosion at the ECM ground terminal can cause misfiring codes. 
- In early 00's Nissan Sentra, a bad cylinder head gasket could cause the engine to misfire. A head gasket is quite expensive to replace. Read more about a head gasket.
- In many older cars, worn and cracked ignition wires often cause misfire; the longer the ignition wire, the more chances the spark will jump off the wire. The solution again is a tune up.
- Faulty on-plug ignition coils are very common reason for misfiring in some Ford, Mazda, Nissan, Volkswagen and other cars. 
- Cracks or corrosion spots on the coils are known to cause misfiring in the rotary engine of Mazda RX-8. Whenever you have a missfire in the Mazda RX-8, coils, spark plugs and spark plug wires are recommended to be replaced first. 
- In many high-mileage cars, oil leaking into a combustion chamber from worn valve seals can foul the spark plugs causing misfire; often this happens after the engine runs at idle for a while. If this issue is suspected, spark plugs need to be checked for oil contamination. 
- Vacuum leaks, as well as a stuck open EGR valve or purge valve (solenoid) can cause misfire that mostly happens at idle. 
- Sometimes, the engine could misfire if during the timing belt or chain replacement, the timing hasn't been set properly. If the problem started after replacing a timing belt or chain, checking the timing is usually the first thing to do.

Overdrive

Generally speaking, overdrive (O/D) is the highest gear in the transmission. On most cars the automatic transmission has three or four speeds and overdrive (which is fourth or fifth speed accordingly). Overdrive allows the engine to have less rpm with higher speed for better fuel efficiency. When you switch it ON, you allow the transmission to shift into overdrive mode after a certain speed has been reached (usually 30-40 mph depending on the load). When it's OFF, you limit transmission shifting to third or fourth speed, not allowing the transmission to shift into overdrive.

In normal driving conditions the overdrive should be always ON. You may need to switch it OFF if you drive in a mountainous area.
[The automatic transmission automatically shifts from O/D to the lower gear when it feels more load. When it feels less load it shifts back to O/D, but under certain conditions, e.g. driving uphill or towing a trailer, the transmission cannot decide whether to stay in O/D or shift into 3rd speed and it starts shifting back and forth. That's the time when you may switch the O/D off and help the transmission to decide].
You also may need to switch the Overdrive OFF when you want to slow down using the engine braking, for example, when driving downhill. For more details, check your owner's manual.

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Junior's Auto Repairs