Basic Diagnostics

Testing Air/Fuel Ratio

Take out the air filter;
run engine at idle;
slowly blank off the carby throat with some stiff cardboard;
if engine rpm increases the mixture is lean, if it decreases it is rich, if it stays stable with throat almost blanked she's scmick.
Repeat for 1500 & 2500 rpm.

another test

put car in first gear and step revs up to 1500 rpm and hold for 10 seconds, then 2000 and hold, 2500 and finally 3000. If the revs hesitate or surge you probably are running too lean;

next, keep in first gear and step the revs up to about 1300 then accelerate to 3000 rpm, but only at half (or a tad) over throttle, backoff to 1300 rpm and accelerate back to 3000 rpm at half throttle again. Do this about 3 or 4 times. If there is an interruption of any kind from smooth acceleration you are running too lean;

if you don't have any problems with the previous tests, keep the car in first and step the revs up to 1300 rpm again and hold for 5 seconds, then accelerate with foot flat to the floor to redline. If the car plays up from 3500 rpm and higher you are probably running rich.

PCV Test

The postive crankcase ventilation valve syphons oil fumes into the carby for burn off. If it clags the engine no like it, so:

Start engine and idle;
remove hose from PCV and block the hose end with your thumb;
if rpm drops more than say 50 -60 rpm then its ok;
if it drops less than 50 rpm the PCV needs o/haul.

Air Filter Test

Take out filter;
drop one of the vaccuum hoses off and start engine and idle;
put filter back in while idling;
if rpm drops your filter not so good...replace.
Dropping a vacuum hose off leans the mixture so a dirty or restrictive filter will have a measurable impact.

Equalisation of Cylinder Power

Start engine and increase idle speed to about 1300 rpm with the fast idle screw;
using electrical pliers and or rubber gloves remove each spark plug ignition lead and ground the end to the engine;
replace each lead as you go before removing the successive lead;
note the rpm change each time;
check for the difference in rpms between cylinders;
if they are about 50 rpm apart she''s ok;
if there are one or two that noticeably didn't drop the rpms around the others they are the ones with bad leads, plugs, compression, etc.

Oxygen Sensor

Your voltage readings should fluctuate.

If it's a single wire sensor:

it should be fluctuating between 0.2 and 0.8 volts every second (14.3:1 mixture= lambda 0.97)

the usual range is 0.8 volt = 14.3:1 to 0.2 volt = 15.1:1 (lambda 1.03)

0.9 to 1.2 is very significant and indicates very rich mixture.

Finding that Pesky Vacuum Leak

Anyone who has ever cleaned their carby or throttle body with carby cleaner, while the engine is running, will know that the engine is quick to tell you that it disapproves of the consequent fuel contamination. You can exploit this symptom when trying to find a hose or gasket leak:- simply spray around those parts that may be a source of an air leak and listen for the idling engine to change tone or chug.

Noisy Mechanicals

There's no substitute for experience when it comes to diagnosing annoying, possibly prophetic noises from an engine. Here's a rough guide to common ticks, clunks and clicks:

Straight up you have to remember that a camshaft turns at half the speed of the crank. An easy way to determine the frequency of a noise is to connect a timing light and if it flashes in time it is at half engine = camshaft speed. Lets start with the easy ones:

That dull hollow sound at start up that occurs at camshaft speed is piston slap. It occurs on each power stroke as the piston (usually skirt) hits the cylinder wall. It's due to excessive piston/bore clearance and gradually dies away as the piston expands from combustion heat. To test, retard your ignition timing by about 5° and the noise should noticeably reduce.

Easing into the harder stuff is the knocking/pounding noise. If its a reasonable knock at idle it's probably excessive big end rod bearing to crank pin clearance; known as big end knock . A lighter knock indicates a gudgeon (wrist pin) clearance problem. A tell tale sign is that the noise dissapears under engine load and reappears under light load as in descending down a hill. The knock usually affects one piston first so you can isolate it quite simply, by disconnecting and grounding one spark lead at a time until the noise dissappears and you will have found your problem piston. Disconnecting the spark lead eliminates the power stroke and unloads the rod.

So that's it pretty much for the lower end noises. For the top end (head related) a screwdriver or stethescope comes in handy. If you are using a screwdriver you place the handle against your skull just below the ear and the tip of the screwdriver on a stud or solid part of the engine. Don't put the tip on a rocker/valve cover because the gasket and air chamber will muffle the noise.

A sharp rapping sound is usually associated with a collapsed hydraulic lifter, if you have them.

A ticking noise is usually tappet clearance or excessive lash. Solid lifters and followers tick anyway, because they need lash to allow for expansion of the valve stems and the rockers when heated or else the valves won't seat properly when the engine is up to temperature for one thing. If you suspect excessive noise you will need to check the lash and adjust. Common sense says that if you increase the gap the louder the tick, so if you want to isolate the tappets as a cause reduce the lash temporarily.

A slow tick is usually a sticky valve that is caused by excessive buildup on the valve stem or a broken valve spring/dampener. It can also be a broken ring.

If you have a case of a ticks and rattles and it's more at the front (No1 piston) or rear of the engine, it is probably not part of the valve train itself, but a worn bearing or belt. A cambelt idler will click and grind if the bearing is dry or worn out. An alternator belt will have a rythmatic slap each time a crack or notch is encountered. Likewise a dry clutch plate thrust bearing will grind and click inside the bell housing, the noise is much worse when the clutch pedal is out, and will diminish when you slightly depress the pedal.

And finally here's the ones you have been champing at the bit to diagnose: Pinking/Detonation/Det/Pinging/Knock and the other biggy Pre Ignition. Lets settle on the first one being named pinking and we'll stay with pre_ignition for the second sympton. Why? Because we can.

Pinking occurs when, instead of a single controlled flame there are multiple spontaneous ignitions, multiple explosive colliding irregular flame fronts, very high peak pressures and pressure fluctuations causing the piston to rapidly vibrate against the bore, the metal rods to resonate, the bearings to get a hammering, etc and the engine starts chiming like a set of christmas bells. The flame fronts resemble the seriation of spikes you get when you cut some material with pinking shears:- get the connection now? Most of us know the sound, but if you want to hear it try taking off in 4th gear without getting the revs up first, a lugging engine just loves to pink. Severe pinking will generally attack and melt the ringland area of a piston. So what causes pinking? Lots of things, so lets go through some of the most common:

Well if we are honest with ourselves, unless we just installed an untuned ECU or distributor, the first time we really experience prolonged pinking is when the engine is burning more oil than an arab oil field and seems to lug under even minor slopes. Oil vapour lowers the effective octane rating of the fuel. So worn rings are a major cause. Carry out a compression test to make sure.

Second most common is the ignition timing; either too advanced because of piston ring/bore wear or just too advanced for the volatility of the fuel. In general the lower the RON rating of the fuel the less advanced your timing. Consider also if you you have a wide open throttle as occurs under heavy load, your volumetric efficiency is up and you are compressing more fuel/air mixture, so if you aren't keeping the ignition event at the right time you will create some pretty hefty stresses and pounding on your engine internals

Third place getter is the good old coking of the piston crowns and combustion chamber. You've been driving the old girl around town, never giving it a decent highway cycle, so the carbon, resins and oil additive deposits have solidified into a nice crusty layer that has increased you compression ratio and thus your cylinder pressures and instead of a nice smooth flame, explosions occur. The deposits are a good heat insulator too so an undesirable heat localisation will occur. A "top end cleaner" or physical cleaning will be required to reduce the build up.

Coming in fourth is pre_ignition, which we will get to soon.

Lean mixtures are another goody and you can usually get some good hesitation in acceleration happening at the same time.

If you have an EGR fitted, check it's working, because the recirculated exhaust gas is supposed to cause a drop in combustion temps and your engine computer is tuned to this.

Similarly if your knock sensor is playing up the ECU can chuck a wobbly and not correct fueling and ignition problems.

If you put "hot" plugs in you have caused your own problem. To reduce pinking you put in "colder" (more porcelain around the electrode) plugs.

It's a bit of no brainer, but if for some reason your pump fuel inexplicably drops 5 points on the RON scale, expect problems, but really this is the 21st century and the old low octanes of 1970's thing is long gone.

If you have a turbocharger or supercharger, high supply air temperatures going into the engine will increase the volatility of the fuel mixture. So either decrease your boost or throw an intercooler in.

If you have milled your head excessively the increased compression ratio will contribute to pinking.

It isn't hard to decipher how Pre_Ignition gets it's name. The fuel mixture ignites before the spark igntion has a chance to do it's thing. You definitely have a problem when the engine continues to run after you turn the ignition off. Mind you most cars these days have a fuel cut off solenoid so it may not "diesel" for long. Symptoms are pinking, loud knocking, severe hesitation, really hot engine, general farting around and loss of power caused by the combustion process trying to send the piston back from where it came from. The intense localised heat on top of the piston is enough to burn a hole through the crown or melt the ringlands. Whats causes it?

Usually glowing carbon/coke build up holding heat that gets to an high enough to ignite the fuel mixture. If you are lucky a "top end clean" using a disolving agent will work, otherwise it's off with the head and some elbow grease to clean away the deposits.

Next cab off the rank is very lean fuel mixture coupled with sharp metal edges (valves, piston ringlands, etc). The sharp edges glow incandescently bright under high heat.

Spark plugs not seated properly;

Spark crossfire mainly from unsupressed copper leads that induce a voltage in another lead.

Check for loss of coolant, buggered water pump, knackered thermofan operation.

Spark plugs heat range too hot.

Pinking.

Excessive valve guide clearance and/or bad valve seating