November 2001 Articles & Features
By Ken Layne | November 2001
Evaluating engine operation and pinpointing specific problems requires a comprehensive testing routine. Here's how to do it.
Previous installments of Mastering the Basics have dealt with ignition testing, engine vacuum testing, fuel delivery and compression. These are the basics of engine operation and performance troubleshooting. This month, we're going to put them all together in a comprehensive testing routine to evaluate overall engine operation and pinpoint specific problems.
Some "big-box" engine analyzers have test sequences programmed into them. These can be particularly helpful to inexperienced technicians, and can also help the old pros get through a test routine faster. With the trend toward hand-held analyzers in the past decade, similar programmed tests have been built into these new-generation testers that are even more powerful than the big boxes of the past. You also can perform these comprehensive tests with an assortment of basic meters and gauges. Regardless of your equipment choices, the following checks should be part of your troubleshooting routine.
Take a Good Look
Engine testing starts when you open the hood. The visible condition of the engine can tell you a lot about its performance. Is the battery growing white fuzz on the terminals? Are the drivebelts cracked, glazed or worn? Are vacuum hoses loose, damaged or connected improperly? Are wires broken or frayed? Are nuts, bolts and studs on manifolds missing or broken?
Before you even think about cranking the engine, look for signs of fuel leakage. Remove the air cleaner and check the condition of the filter. Examine the PCV valve and hoses for damage and wear, then inspect the coil, distributor (if the engine has one), plug leads and terminals for carbon tracking and broken insulation.
Don't forget to pull the dipstick and check the oil condition. Also look for oil leaks at valve covers and other seals. Check the inside of the radiator for dirt, rust or other crud. And don't overlook all the coolant hoses.
Crank It Up
Noises, odors and smoke can tell you a lot when you drive a car or simply run the engine in the shop. Listen and look for:
*Uneven cranking due to uneven compression or starting system problems.
*Rattling from loose brackets or a broken catalytic converter.
*Squealing from loose or glazed belts.
*Rumbling from bad bearings in a water pump, an alternator or other accessory.
*Black, blue or white exhaust smoke.
*Hissing due to a vacuum leak.
Several engine tests, such as those done during cranking, require that the ignition be disabled. The specific ways to do this vary from car to car. We can't list all the details here, but we can remind you that you'll need to do it early in the test sequence. This also might be a good time to connect a remote starter switch, if you plan to use one.
Test equipment connections also will vary widely. A big-box analyzer usually has a harness that may have eight or ten individual leads, while hand-held equipment may have just two test leads or a single probe. We'll leave the connection details to your equipment's instruction manual.
Many engine analyzers, large and small, are designed to step through preprogrammed tests in an automatic or semiautomatic sequence. If you're going to check tailpipe emissions, install the analyzer probe in the tailpipe now.
Battery, Starter & Alternator Tests
Several basic tests should be done before starting the engine or while it's cranking. Grouping them together makes for fast and accurate testing. Begin by recording the battery open-circuit (no-load) voltage. Next, disable the ignition system so the engine won't start and crank the engine for about 15 seconds. Record starter current draw, battery cranking voltage and cranking rpm. Note that some analyzers also provide relative compression readings based on rpm variations during the cranking procedure.
These quick steps give you a general view of starter condition and battery capacity, and the combined ability of both to crank the engine at a satisfactory speed. If any of these general indications seems to be out of limits, perform detailed battery or starter tests right away to pinpoint the problem.
Along with testing battery and starter condition, you can use the engine cranking period to check vacuum. Cranking vacuum specs can vary widely from one engine to another-from 3 to 15 in.-Hg, in fact. What you're looking for, however, is steady vacuum and cranking speed. If cranking speed is steady at about 200 rpm and vacuum is steady at around 5 inches (or whatever the spec may be), the engine most likely is in good mechanical shape. Remember, if rpm and vacuum are uneven, the cylinders most likely aren't pumping equally.
This early stage of testing also is a good time to check fuel pump pressure and volume. Woefully inadequate fuel delivery will result in a no-start condition. Below-spec pressure and/or volume may let the engine run, but very poorly.
After these cranking quick checks, the battery will need recharging, so let's continue with some basic alternator tests. Start the engine and run it at 2500 rpm. Record the alternator recovery voltage and amperage output, then the regulated charging voltage. If your test equipment records alternator ripple and it exceeds .4 volt, make a note that detailed tests of the alternator diodes may be in order.
While the engine is running at 2500 rpm for alternator testing, perform a quick check of emissions by recording the readings for HC, CO, O2 and CO2. Then let the engine return to idle and check the idle emissions readings. Note that emissions test specifications vary from state to state (in some places, from town to town), but an engine in good shape should produce the following general readings at both 2500 rpm and idle:
HC-150 ppm or less
CO-1.5% or less
O2-4% or less
CO2-10% or more
Although emissions are checked most often for official smog tests, remember that the combined four-gas readings are excellent indicators of combustion efficiency and important factors in general engine troubleshooting.
Vacuum, Power Balance & Compression Testing
You can pinpoint several basic mechanical problems by looking at manifold vacuum at idle, low cruise (1800 to 2200 rpm) and high cruise (2500 to 3000 rpm). Note the vacuum readings and any fluctuations at each speed. Our previous Mastering the Basics article on vacuum testing (Aug. 2001) has all the details on vacuum readings and interpretations. It's important to point out, however, that vacuum checks are a basic part of any thorough comprehensive engine test.
Power balance testing and compression testing provide similar information about engine condition. Either test can identify a weak cylinder, but a compression test that follows a power balance test can help differentiate between a fuel or ignition problem on one hand and a mechanical problem on the other.
The principle of power balance testing is to short-circuit the ignition for each cylinder and observe changes in engine speed and vacuum. Using an infrared exhaust analyzer during power balance testing, you also can observe changes in exhaust emissions, as an additional diagnostic aid.
Some engine analyzers have built-in power balance test programs that automatically short-circuit individual spark plugs for fast and accurate testing. You also can do a power balance test with just a tachometer and a vacuum gauge, however. Just ground each plug lead and watch the changes in rpm and vacuum.
A modern variation on power balance testing is built into many Ford engines. The engine control system has a special test mode that lets a scan tool command the vehicle's computer to cut off fuel injection to individual cylinders. The computer reports the rpm drop to the scan tool, which displays the results.
The lack of a speed drop or vacuum change when one cylinder is killed identifies a weak cylinder, but it doesn't tell you if the cause is a fuel, ignition or mechanical problem. Power balance testing can lead you to an engine compression test to verify cylinder mechanical condition. Compression specs for a particular engine may be given as a minimum pressure with an allowable variation between cylinders. Some manufacturers simply may say that the lowest cylinder pressure must be within 75% of the highest. Either way, good compression with little variation among cylinders indicates an engine in sound mechanical condition.
Power balance and compression testing used to be routine practice for top-notch engine diagnosticians. These tests are performed far less often today, however, primarily because of time constraints and complications caused by some electronic systems.
For example, a power balance test must be run quickly to avoid overheating the catalytic converter with unburned fuel. Also, some EGR systems must be disabled for accurate testing because fluctuating manifold vacuum may cause the EGR valve to cycle and create inaccurate test results. Additionally, idle speed control systems may fight the rpm drop caused by power balance testing.
On many engines with distributorless ignition systems (DIS), it's simply impractical to spend the time required to hook up the special test harnesses of many engine analyzers. Waste-spark DIS installations also can lead to inconclusive power balance results if you try to short one cylinder of a waste-spark pair.
Note that engines with OBD II diagnostic systems will set trouble codes for misfires. Power balance testing may set such codes, which may invalidate the test and add extra service time to clear the codes and reset the OBD II monitors.
Just as crowded, convoluted DIS installations make power balance testing impractical, crowded engine bays and "bunch-o-bananas" intake manifolds can make compression testing almost impossible. As a practical matter, if book time to change the plugs on a given engine is two or three hours, you're not likely to check compression as part of routine testing. All of these qualifications notwithstanding, power balance and compression tests are important tools in a troubleshooter's arsenal, to be used when appropriate.
Cylinder Leakage & Ignition Testing
Cylinder leakage testing supplements power balance and compression testing. Leakage tests are used most often when one or both of the other tests indicate a weak cylinder. Pressurizing a cylinder with compressed air lets you pinpoint the source of a leak by listening for the sound of air escaping or looking for bubbles in the radiator. If the test indicates that leakage is present, engine teardown is often the next step. On the other hand, if the test does not indicate an internal mechanical problem, you save your customer unnecessary expense and stay on track to pinpoint other causes of performance problems.
Engine testing wouldn't be comprehensive without a check of the ignition system. Secondary scope patterns pinpoint the condition of the coil secondary windings, the high-voltage lead to the distributor, the distributor cap and rotor, the plug wires and the spark plugs. Also, because mechanical conditions inside cylinders affect secondary resistance, secondary ignition patterns are useful for analyzing air/fuel ratios, stuck or burned valves and cylinder compression.
Whether you're using a big engine analyzer or a hand-held unit, check secondary patterns in both the raster and parade modes. Note the spark plug firing voltage and spark line (burn time). Stab the accelerator to check coil output and the condition of all spark plug leads. Go into raster mode to check for excessive dwell variation, which may indicate distributor pickup coil or crankshaft position sensor problems. If you spot something abnormal, switch to an individual cylinder display for a closer look.
If the engine has a base timing specification, check it with the engine running at idle or at the rpm specified by the manufacturer. Also check idle speed. If applicable, use a timing light to check base timing and advance and adjust the timing, if necessary.
Wrapping It All Up
We haven't mentioned the cooling system very much in this survey of engine testing. Besides inspecting the condition of the radiator, the hoses and the coolant, it's always a good idea to pressure-test the system. It takes only a few minutes to pump the system up to 12 to 15 psi to see if it holds pressure. Remember, an otherwise sound engine can overheat and suffer poor mileage and performance, all because of a pinhole leak in a hose or radiator tank that won't let the system hold pressure.
That's it for this general survey of engine testing. All operating factors listed here are examples of internal combustion basics, and comprehensive testing highlights their relationships to each other and to total engine performance.