yayfortrees

I decided to make a write-up for anybody that is not experienced in diagnosing general running problems. This thread is not for any specific problem, but will outline the basics of what a trained technician might do to diagnose a problem.

While it is impossible to fit every detail in this write-up, the goal is to give a guideline. For more information about a specific procedure or component, search here on YT, or on the web. All of this is explained in fuller detail in a service manual, but I know how hesitant some people are in using the manual.

Warning: Working on and around motors can be potentially dangerous. I probably don't have to tell you this, but since this thread is made for people not experienced with auto mechanics, PLEASE BE CAREFUL! There is highly combustible fuel, hot temperatures, rotating parts, potentially dangerous voltages, and more that can cause severe injury or death.

Quickly, about me: I graduated from Motorcycle Mechanics Institute back in 2002. I am fully aware that Toyota does not make motorcycles (too bad ), but engine theory is engine theory. Most of the systems are close enough for comparison. In addition to the formal training, I have various automotive experience from different repair shops, as well as personal vehicles.

I welcome any and all additions from any experienced technicians out there. I'm sure I will forget some things, so feel free to patch up my work.


Engine theory:

An internal combustion engine needs 4 things to run. (Vehicles with turbos or superchargers still need these things, but some specs will change. This is outlined for normally aspirated engines.)

1. Air (Usually a given, as long as the intake is not obstructed by anything, like a rag.)

2. Fuel (In proper air/fuel ratio. This is not easily checked. More on that later.)

3. Compression (~125 PSI minimum; looking for 150 PSI up to ~180 PSI. Max 10% difference from cylinder to cylinder)

4. Timed spark (Not just spark, but properly TIMED spark)

General Engine Components (terminology): Starting from the bottom end and working up and out:

Motor block - the biggest part of a motor. This houses the crankshaft, rods, pistons, and cylinder walls. The oil pan bolts to the bottom of the block, and the transmission bolts to the back of the block.

Crankshaft - primary rotating component of a motor. Connecting rods bolt to the journals of the crankshaft

Pistons - What moves up and down inside the cylinders. The connecting rod is connected to the piston, and transfers motion to the crankshaft

Cylinder wall - part of the block, these are machined to match the diameter of the piston with proper clearance

Cylinder head - This component houses the spark plugs, valves, valve guides and seals, and in our motors, the camshaft

Valves - Intake valves open to let air/fuel mix in, while Exhaust valves open to let spent air charge out

Camshaft - lobes on the camshaft open the valves, either by shim and bucket, or rocker arms

Cam chain - connects the camshaft to the crankshaft. Camshaft spins only once per every 2 revolutions of the crankshaft

Valve cover - self explanatory

Oil pump / water pump - also self explanatory

There are other components, such as chain tensioner, chain guides, etc, but will not be covered in detail here.

Also involved with diagnosing running conditions, is the ignition system, intake / fuel injection or carburetion, depending on your vehicle, as well as exhaust.

Common tools that may be required:
Factory Service Manual
Compression gauge
Leakdown tester
Timing light
Tachometer
Multi-meter capable of at least Volts AC; Volts DC; ohms
Mechanics stethoscope or long screwdriver
Feeler gauges
Torque wrench
Breaker Bar
General tool set such as Ratchet/sockets/extensions; wrenches; etc.


Diagnosing any automotive problem is typically a process of elimination.

Some problems are very common, and have a known, easy fix. I.E. Idle surge caused by idle set too high

Some problems are unique, and might be caused by any single component, or a combination of components, either mechanical (valves, rings, etc.) or electrical (sensors, coil, distributor, etc.)

A word about 'tune-ups': A tune-up is NOT designed to REPAIR any running conditions. A tune-up is MAINTENANCE! Sometimes, a running problem is caused by a lack of maintenance, which could be remedied by a tune-up as long as no damage occured as a result of the lack of maintenance.

Do not ASSUME anything when diagnosing a problem. (You know what happens when you assume, right? You make an ASS out of U and ME) (Oldie but a goodie.)

Now, on to the steps to the diagnosis. Depending on what you already know about the vehicle, will determine where you start. Assuming nothing has yet been checked, start by ruling out the worst-case scenario.

Step 1: In this case, CHECK THE COMPRESSION! There is nothing worse than spending hours checking other components, or trying band-aid repairs, only to finally find out your motor doesn't have compression.

NOTE: You must be careful when checking compression. Follow the exact procedure given in the FSM to prevent damage to electronic components, or to human components.

Since your pulling the spark plugs anyway, inspect the condition and gap before re-installing the spark plug. If the motor has recently been cranked over, look for a small amount of fuel on the plug. If you have good, even compression on all cylinders, move on to step 2. Good compression usually indicates there are no obstructions with the air intake, and that cam timing is accurate.

If you find a cylinder with low or no compression, you know you likely have internal problems. You can perform a leakdown test, or you can squirt ~1 tbsp. of motor oil down the spark plug hole, and recheck compression. If it goes up, you likely have ring issues. If there is no change, it's likely a bent/burnt/tight valve. In this case, pull the valve cover and inspect/adjust valve clearance to spec. A valve with zero clearance may not be closing, which will cause a no compression condition. Try adjusting the valve(s) back to spec., reassemble, and re-check compression and running.

If you still have low compression, your cam timing may be off.

A leakdown test is a test to determine where exactly you are losing your compression, and what percentage is being lost. This can be a difficult test to perform, and can also be dangerous. An air compressor is required, and for a technical test, a proper gauge is also necessary.

A poorman's leakdown test can be performed without the proper test equipment, but will not give the percentage leaking. To make a poorman's leakdown tester, remove the valve core from your compression gauge hose. Also remove the hose from the gauge, and securely connect it to your air hose. Set your compressor to around 80 to 100 PSI.

Using either type of leakdown tester, the cylinder being tested must be held at EXACTLY top dead center compression (TDCC, or TDC compression). With all spark plugs removed, rotate the motor to TDCC on that cylinder, using a breaker bar and appropriate socket on the crank pulley. (Do not use a ratchet unless it can be set to not ratchet, I.E. it will hold in either direction.) Remove the breaker bar. Thread the tester hose into the spark plug hole. Carefully, apply the air pressure into the tester hose. The motor will want to violently push the piston down if that cylinder is not at EXACT TDCC. For this reason, do not leave the breaker bar on the crank pulley bolt. The motor can rotate quickly, and will violently spin the breaker bar as well. This can break other components and/or fingers/hands/arms.

If the piston holds TDCC, the air will leak out either: past the piston rings; past the intake valve; or past the exhaust valve. Check for air coming out from the exhaust (EX valve leak), intake (IN valve leak), or crankcase vent, (PCV valve in valve cover, which would indicate it's leaking past the piston rings.)

Step 2: Check ignition timing. You've already determined that your spark plugs are in good condition, or have been replaced. Now verify that the ignition timing is accurate, which will also verify that you have spark at the same time. Don't forget to jump the two terminals in your diagnosis connection under the hood. If you have verified correct ignition timing, continue to step 3.

If necessary, adjust the timing by moving the distributor. If your timing light will not fire, you likely don't have spark. Look into your ignition system for faulty components or wiring. Components include: coil/ignitor; distributor; spark plugs; spark plug cables; ECU

Step 3: Check to make sure you're getting fuel delivery. This can be kind of tricky, since you don't have a window to look in your throttle body or cylinder. Hopefully, you already determined you are getting fuel by inspecting the spark plug. You can also check fuel pressure per the FSM. If you have fuel delivery, proceed to step 4.

If not, look into your fuel injection system or carburetor. Components for carburetor vehicles include the fuel pump; fuel filter; and carburetor/choke/etc. Components for fuel injected models include fuel pump; fuel filter; fuel injectors; cold start injector; throttle position sensor (TPS); air flow sensor; ECU; temp. sensor; O2 sensor; throttle body

Follow the FSM procedures for testing all electronic sensors.

Here's a crash course in using a multi-meter. A very common check is ohms, or resistance. This is exactly what it sounds like, checking the resistance of a component or wire. A perfect connection or wire will have nearly 0 resistance, or 0.01 to 0.10 ohms of resistance. This is also known as having continuity. If a wire or connection has more than .1 ohms, it may have too much resistance, and could be faulty. Ofl, or offline, means infinite resistance, or no connection. This does not have continuity, and can represent a broken wire or connection.

Many sensors work by changing the resistance, which the ECU can read and interpret as data. The higher the number of ohms, the more resistance it has. To test your meter, touch your leads together. It should read between 0.01 and 0.10 ohms.

Volts AC and Volts DC are similar, but have differences. DC has polarity (+ and -), while AC doesn't. (Technically, it can, but that's not really relevant here) You test battery voltage with DC. Generator current is tested in AC, but is really not used often unless diagnosing charging problems.

If testing for battery voltage, touch the red lead to the wire or connection, and touch the black lead to the best known ground available, such as the frame or a clean spot on the body, (no rust or paint).

There will be more added to this later, but I have to take a break for now.

I hope this information helps some people that want to learn more about repairing their own vehicles.

rlordjr

Good stuff! Keep it coming.