Engine Diagnosis
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These are some of the tools you'll need to do basic and not-so-basic engine diagnosis to find out what's going on when things are not working right. These are the tools that every professional shop has, and when you bring your car into the shop, they run through various tests with these to figure out what's wrong. If you have them and learn how to use them you don't need to pay the shop to do this work, and if you want to do "custom" work on your ride, then you better get used to figuring out stuff on your own. Most shops won't touch non-factory vehicles with a ten foot pole.

All of the things listed below are in my toolbox and get regular use. To steal a phrase from the Visa commercials - What's in your toolbox?


Basic Diagnosis Tools

These are the items that any home mechanic worth anything should have. You can get all of them for a bit over $100, and these are right up there with a torque wrench - you need them to move beyond anything but the most basic "unbolt the old/bad part, bolt on the new/good one" type of repairs to your vehicle. I've listed them in rough order of importance - start at the top and buy down towards the bottom if starting your toolbox from nothing.


EFI Trouble Code Reader

Old-skool carb guys can skip this, but if you have EFI on your ride like so many folks do today, then you have to have a way to get the trouble codes out and see what the computer may or may not be upset about. Basic units for older EFI setups are dirt cheap - some GM systems can even display the codes with something as simple as a paper clip jumpering the right terminals on the ALDL connector. ewer "OBD II" readers are way more complicated than the old school ones, but can still be had for cheap. Most of the basic ones just cause the "check engine" light to flash some codes at you, which you then look up in a book to find out that code 33 means "high voltage at MAP sensor" or whatever yours happens to be. More advanced units will display the codes right on the little screen on the code reader, some really high end units will display the problem in words instead of just giving you the code. Find one that fits your needs, and stash it in the toolbox. Since I work on so much stuff, I have OBD I units for Ford and GM, plus a basic OBD II unit for peeking at those as well when I have to.

Tach/Dwell/Voltmeter/Multimeter Combo

The most basic diagnostic tool for the old-skool carb guys, and it's still handy for newer stuff. This is especially critical if you have points and need to adjust the dwell. (but why the heck does anyone run points anymore anyway? Whatever...) The tach gets you basic things like idle rpm, and you'll use that a lot to see where things are at when doing a basic tune-up. Factory in-dash tachs are notoriously inaccurate, and you can't see them while under the hood twiddling the idle speed screw on the carb or the throttle body. The voltmeter is handy for lots of testing on the car - battery, alternator, switches, wires, connections, etc. Modern units are digital and look like a multimeter, older units will be more basic and be an analog unit. I've had several of these picked up at swap meets and from other sources. Craftsman sells one, so does just about every other automotive tool place. Get one you understand and use it. Some have more advanced features, and professional/shop-sized units may even have a scope to check firing patterns and the like - think of the big old roll-around cart with the scope as the extreme example of this type of tool. I have both analog and digital units. I prefer the old needle analog units for RPM adjustments - it's really easy to see fine changes in the speed at a glance. I prefer the digital units for precision and for voltage readings (they read down to the tenth of a volt at a glance). Most good digital multimeters have various other functions that come in very handy for testing out electrical systems - resistance (ohm) testing, audible continuity testers, etc. - so get a good one, learn how to use it, and toss it into the toolbox.

Hand-Held Vacuum Gauge

One of the best ways to diagnose how your engine is running. Hook it up to ported vacuum, and watch the needle. It should be nice and steady at idle or any other steady-state RPM situation unless you're running some insane race cam in your engine. You can tune a carb for "lean best idle" using a tach and a vacuum gauge. It also helps tell if things are changing in your engine - if you know the engine normally makes 16" of vacuum at idle, and now it's only making 12" - something is wrong and needs attention, even if the engine seems to be doing sort of OK. Many basic vacuum testers double as a fuel pressure gauge for low-pressure fuel pumps used on carb'ed engines - see below.

Timing Light

This allows you to check the timing on the engine, and make sure it's right, and adjust it as needed. It's a pretty basic unit with a power source and a hook up for the #1 spark plug wire. When the plug fires, a bright strobe style light on the end of the tool flashes. You aim it at the timing marks on the engine, and it will make them appear to "stand still" so you can see where the timing is. It's high speed flash photography with your eyes instead of film. Very cool the first time you see it, old hat after 30 seconds. More advanced ones have what's known as a "dial back", "adjustable", or "advance" feature so you can change the timing of the flash. This is really handy if you are testing a custom advance curve on your distributor via custom weights or stuff like that, though that's more of an advanced tool usage that is not for the newbie or the faint of heart. Revving an engine to 2000 RPM with your had under the hood watching the timing marks to see the advance change with RPM is startlingly loud and scary - that engine fan starts moving awfully fast just a few inches from your head...

Compression Tester

A simple and basic gauge with a hose attached that screws into the engine in place of the spark plug. You disconnect the coil so the engine won't start, then crank the engine over a few times to get a max reading on the gauge. record the number, and repeat this for all cylinders. If you have a weak cylinder (down on compression), then it will be really obvious which one it is. For any low cylinders, you squirt oil into the spark plug hole and re-do the test to see if the reading changes. The general wisdom is that if the oil increases the reading, then your rings are worn. If the reading stays the same, then you have worn valve guide seals, a blown head gasket, or something else other than worn rings.


Advanced Diagnosis Tools

These are the items that a serious gearhead should have. You can get all of them for about $100, and these are right up there with a torque wrench - you need them to move beyond anything but the most basic "unbolt the old/bad part, bolt on the new/good one" type of repairs to your vehicle. These require some more money, and sometimes more skill to use, but for most true gearheads, it's all part of the tool box you have, or the one you want to have.


Mechanics Stethoscope

This is one of those "you used a what?" tools until you try it. You all know what a doctor's stethoscope is - it's the thing with the rubber tubes that they put in their ears and then put the cold metal round thing on your chest and ask you to breath/cough/etc. It's basically a really sensitive sound amplifier that does an amazingly good job. A mechanic's version of one of these is nearly identical to a doctors stethoscope, except that the round part is in a little housing mid way down the tubes and it has a long thing solid metal rod attached to it. You place the metal rod on something and the sound transmits up to the round part in the middle, and then the sound is transmitted up to your ears. It is amazingly good - you will learn this the first time you have the ear tubes in place and you drag the tip across a metal surface or bump it into something - it's so loud it'll make your eyes water and make you want to cry in pain. OK, maybe not that bad, but it is a shock the first time it happens. You can hear a surprising amount of detail from the engine while it's running - valves, lifters, bearings, injectors, pumps, etc. all make distinct noises when they are working right - and very different noises when they are not working right. Placing the metal rod on different parts of the engine will help isolate different sounds. You can even listen to alternator, power steering pump, and water pump bearings to hear if they are wearing out. Listen to a known good example and then a bad example - the differences should quickly become obvious. Oh, and it helps if you haven't blown out your hearing from one too many concerts...

Hand-Held Vacuum Pump

A variation on the vacuum gauge, this has a gauge with a hand-held pump attached. It's perfect for hooking up to a vacuum component and seeing if it actually holds vacuum or not. Vacuum diaphragms get old, stiff, and rupture. And various components these days are vacuum operated - everything from the AC/heater door controls, down to emissions stuff on the engine, to the MAP sensor or fuel pressure regulator on an EFI system. These double as a vacuum gauge, and they often come in a kit with a vacuum brake bleeder, which can be very handy for doing one-man brake bleeding jobs. A good addition

Cylinder Leak Down Tester

A variation on the compression tester that is more accurate and helps you learn more about the engine, but it more expensive. This one has two gauges (line pressure, cylinder pressure), a hookup to your air compressor, and is used to measure the amount of pressure "leak down" in a cylinder over time. Ideally, when the valves are closed and the spark plug is ready to fire, each cylinder is a sealed chamber with very few places for pressure to escape - small gaps around the ring ends should be about it. Any place else indicates problems. You pressurize each cylinder and see how fast it the pressure goes away. It helps you figure out what's wrong - bad head gasket? Cracked head/block? Bad piston? Bad rings? The instruction manual with the tool will help you learn how to use it.

Fuel Pressure Gauge

If you have fuel feed problems, this is how you sort it out. For a carb'ed engine with a low pressure fuel pump (under 10 PSI), many basic compression testers double as a fuel pressure testers - a handy two-for-one deal. For EFI systems, you need a more expensive tool with the right fittings to connect to your fuel system - EFI systems run from 40 PSI on up, so the fittings need to be much better, as does the hose to connect to the gauge. Either way, you use them the same basic way - hook them up to the system, then run the engine to see what the fuel pressure is. Too high or too low is bad. If the fuel system has a regulator, you need to test that, usually by applying vacuum to it with your trusty hand held vacuum pump, see above.

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Page last updated 01/02/2009 01:51:39 PM